50 dB GSM PA Controller
AD8315
Rev. C
Information furnished by Analog Devices is believed to be accurate and reliable. However, no
responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other
rights of third parties that may result from its use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
Trademarks and registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700 www.analog.com
Fax: 781.461.3113 ©2006 Analog Devices, Inc. All rights reserved.
FEATURES
Complete RF detector/controller function
>50 dB range at 0.9 GHz (−49 dBm to +2 dBm, re 50 Ω)
Accurate scaling from 0.1 GHz to 2.5 GHz
Temperature-stable linear-in-dB response
Log slope of 23 mV/dB, intercept at −60 dBm at 0.9 GHz
True integration function in control loop
Low power: 20 mW at 2.7 V, 38 mW at 5 V
Power-down to 10.8 μW
APPLICATIONS
Single, dual, and triple band mobile handset (GSM, DCS, EDGE)
Transmitter power control
GENERAL DESCRIPTION
The AD8315 is a complete low cost subsystem for the precise
control of RF power amplifiers operating in the frequency range
0.1 GHz to 2.5 GHz and over a typical dynamic range of 50 dB.
It is intended for use in cellular handsets and other battery-
operated wireless devices. The log amp technique provides a
much wider measurement range and better accuracy than
controllers using diode detectors. In particular, its temperature
stability is excellent over a specified range of −30°C to +85°C.
Its high sensitivity allows control at low signal levels, thus
reducing the amount of power that needs to be coupled to
the detector.
For convenience, the signal is internally ac-coupled. This
high-pass coupling, with a corner at approximately 0.016 GHz,
determines the lowest operating frequency. Therefore, the
source can be dc grounded.
The AD8315 provides a voltage output, VAPC, that has the
voltage range and current drive to directly connect to most
handset power amplifiers’ gain control pin. VAPC can swing
from 250 mV above ground to within 200 mV below the supply
voltage. Load currents of up to 6 mA can be supported.
The setpoint control input is applied to the VSET pin and has
an operating range of 0.25 V to 1.4 V. The associated circuit
determines the slope and intercept of the linear-in-dB
measurement system; these are nominally 23 mV/dB and
−60 dBm for a 50 Ω termination (−73 dBV) at 0.9 GHz.
Further simplifying the application of the AD8315, the input
resistance of the setpoint interface is over 100 MΩ, and the bias
current is typically 0.5 µA.
The AD8315 is available in MSOP and LFCSP packages and
consumes 8.5 mA from a 2.7 V to 5.5 V supply. When powered
down, the sleep current is 4 µA.
FUNCTIONAL BLOCK DIAGRAM
VPOS
ENBL
RFIN
C
OMM
DET
10dB
DET
10dB
DET DET DET
10dB 10dB
OFFSET
COMP’N
INTERCEPT
POSITIONING
LOW NOISE
GAIN BIAS
LOW NOISE
BAND GAP
REFERENCE
OUTPUT
ENABLE
DELAY
×1.35
VAPC
HI-Z
LOW NOISE (25nV/√Hz)
RAIL-TO-RAIL BUFFER
FLTR
VSET
V-I 23mV/dB
250mV TO
1.4V = 50dB
01520-001
Figure 1.
AD8315
Rev. C | Page 2 of 24
TABLE OF CONTENTS
Features .............................................................................................. 1
Applications....................................................................................... 1
General Description ......................................................................... 1
Functional Block Diagram .............................................................. 1
Revision History ............................................................................... 2
Specifications..................................................................................... 3
Absolute Maximum Ratings............................................................ 5
ESD Caution.................................................................................. 5
Pin Configuration and Function Descriptions............................. 6
Typical Performance Characteristics ............................................. 7
Theory of Operation ...................................................................... 12
Basic Theory................................................................................ 12
Controller-Mode Log Amps ..................................................... 13
Control Loop Dynamics............................................................ 13
Practical Loop............................................................................. 15
A Note About Power Equivalency ........................................... 16
Basic Connections...................................................................... 16
Range on VSET and RFIN ........................................................ 17
Transient Response .................................................................... 17
Mobile Handset Power Control Example ............................... 18
Enable and Power-On................................................................ 19
Input Coupling Options ............................................................ 19
Using the Chip Scale Package................................................... 20
Evaluation Board ........................................................................ 20
Outline Dimensions ....................................................................... 22
Ordering Guide .......................................................................... 23
REVISION HISTORY
6/06—Rev. B to Rev. C
Updated Format..................................................................Universal
Changes to Ordering Guide .......................................................... 23
1/03—Rev. 0 to Rev. B
Edits to Product Description Section ............................................ 1
Edit to Functional Block Diagram ................................................. 1
Edits to Specifications ...................................................................... 2
Edits to Absolute Maximum Ratings ............................................. 3
Ordering Guide Updated................................................................. 3
TPC 9 Replaced with New Figure .................................................. 5
Edits to TPC 27................................................................................. 8
Edit to Figure 1.................................................................................. 9
Edit to Figure 3................................................................................ 10
Edit to Equation 9........................................................................... 10
Edit to Equation 10......................................................................... 10
Edit to Equation 11......................................................................... 10
Edits to Example section ............................................................... 10
Edit to Basic Connections Section ............................................... 12
Edits to Input Coupling Options Section.................................... 14
Table III Becomes Table II............................................................. 15
Table II Recommended Components Deleted ........................... 15
Using the Chip-Scale Package Section Added............................ 15
Edits to Evaluation Board Section................................................ 15
Figure 12 Title Edited..................................................................... 16
Figure 13 Title Edited..................................................................... 16
8-Lead Chip Scale Package (CP-8) Added.................................. 17
Updated Outline Dimensions....................................................... 17
10/99—Revision 0: Initial Version
AD8315
Rev. C | Page 3 of 24
SPECIFICATIONS
VS = 2.7 V, T = 25°C, 52.3 Ω termination on RFIN, unless otherwise noted.
Table 1.
Parameter Conditions Min Typ Max Unit
OVERALL FUNCTION
Frequency Range1To meet all specifications 0.1 2.5 GHz
Input Voltage Range ±1 dB log conformance, 0.1 GHz −57 −11 dBV
Equivalent dBm Range −44 +2 dBm
Logarithmic Slope20.1 GH 21.5 24 25.5 mV/dB
Logarithmic Intercept20.1 GHz −79 −70 −64 dBV
Equivalent dBm Level −66 −57 −51 dBm
RF INPUT INTERFACE Pin RFIN
Input Resistance30.1 GHz 2.8 kΩ
Input Capacitance30.1 GHz 0.9 pF
OUTPUT Pin VAPC
Minimum Output Voltage VSET ≤ 200 mV, ENBL high 0.25 0.27 0.3 V
ENBL low 0.02 V
Maximum Output Voltage RL ≥ 800 Ω 2.45 2.6 V
vs. Temperature485°C, VPOS = 3 V, IOUT = 6 mA 2.54 V
General Limit 2.7 V ≤ VPOS ≤ 5.5 V, RL = ∞ VPOS − 0.1 V
Output Current Drive Source/Sink 5/200 mA/μA
Output Buffer Noise 25 nV√Hz
Output Noise RF input = 2 GHz, 0 dBm, fNOISE = 100 kHz, CFLT = 220 pF 130 nV/√Hz
Small Signal Bandwidth 0.2 V to 2.6 V swing 30 MHz
Slew Rate 10% to 90%, 1.2 V step (VSET), open loop5 13 V/μs
Response Time FLTR = open, see Figure 26 150 ns
SETPOINT INTERFACE Pin VSET
Nominal Input Range Corresponding to central 50 dB 0.25 1.4 V
Logarithmic Scale Factor 43.5 dB/V
Input Resistance 100 kΩ
Slew Rate 16 V/μs
ENABLE INTERFACE Pin ENBL
Logic Level to Enable Power 1.8 VPOS V
Input Current when Enable
High
20 μA
Logic Level to Disable Power 0.8 V
Enable Time Time from ENBL high to VAPC within 1% of final value,
VSET ≤ 200 mV, refer to Figure 23
4 5 μs
Disable Time Time from ENBL low to VAPC within 1% of final value,
VSET ≤ 200 mV, refer to Figure 23
8 9 μs
Power-On/Enable Time Time from VPOS/ENBL high to VAPC within 1% of final value,
VSET ≤ 200 mV, refer to Figure 28
2 3 μs
Time from VPOS/ENBL low to VAPC within 1% of final value,
VSET ≤ 200 mV, refer to Figure 28
100 200 ns
AD8315
Rev. C | Page 4 of 24
Parameter Conditions Min Typ Max Unit
POWER INTERFACE Pin VPOS
Supply Voltage 2.7 5.5 V
Quiescent Current ENBL high 8.5 10.7 mA
Over Temperature −30°C ≤ TA ≤ +85°C 12.9 mA
Disable Current6ENBL low 4 10 μA
Over Temperature −30°C ≤ TA ≤ +85°C 13 μA
1 Operation down to 0.02 GHz is possible.
2 Mean and standard deviation specifications are available in Table 2
3 See Figure 11 for plot of input impedance vs. frequency.
4 This parameter is guaranteed but not tested in production. Limit is −3 sigma from the mean.
5 Response time in a closed-loop system depends on the filter capacitor (CFLT) used and the response of the variable gain element.
6 This parameter is guaranteed but not tested in production. Maximum specified limit on this parameter is the 6 sigma value.
Table 2. Typical Specifications at Selected Frequencies at 25°C (Mean and Sigma)
±1 dB Dynamic Range
Slope (mV/dB) Intercept (dBV) Low Point (dBV) High Point (dBV)
Frequency (GHz) Mean Sigma Mean Sigma Mean Sigma Mean Sigma
0.1 23.8 0.3 −70.1 1.8 −57.7 1.3 −10.6 0.8
0.9 23.2 0.4 −72.6 1.8 −61.0 1.3 −11.2 0.8
1.9 22.2 0.3 −73.8 1.6 −62.9 0.9 −18.5 1.7
2.5 22.3 0.4 −75.6 1.5 −64.0 1.1 −20.0 1.7
AD8315
Rev. C | Page 5 of 24
ABSOLUTE MAXIMUM RATINGS
Table 3.
Parameter Rating
Supply Voltage VPOS 5.5 V
Temporary Overvoltage VPOS
(100 cycles, 2 sec duration, ENBL Low) 6.3 V
VAPC, VSET, ENBL 0 V, VPOS
RFIN 17 dBm
Equivalent Voltage 1.6 V rms
Internal Power Dissipation 60 mW
θJA (MSOP) 200°C/W
θJA (LFCSP, Paddle Soldered) 80°C/W
θJA (LFCSP, Paddle Not Soldered) 200°C/W
Maximum Junction Temperature 125°C
Operating Temperature Range −40°C to +85°C
Storage Temperature Range −65°C to +150°C
Lead Temperature (Soldering 60 sec)
MSOP 300°C
LFCSP 240°C
Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
ESD CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the
human body and test equipment and can discharge without detection. Although this product features
proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy
electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance
degradation or loss of functionality.
AD8315
Rev. C | Page 6 of 24
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
8
7
6
5
1
2
3
4
AD8315
TOP VIEW
(Not to Scale)
RFIN
ENBL
VSET
FLTR
NC = NO CONNECT
VPOS
VAPC
NC
COMM
01520-002
Figure 2. Pin Configuration
Table 4. Pin Function Descriptions
Pin No. Mnemonic Description
1 RFIN RF Input.
2 ENBL Connect to VPOS for Normal Operation Connect Pin to Ground for Disable Mode.
3 VSET Setpoint Input. Nominal input range 0.25 V to 1.4 V.
4 FLTR Integrator Capacitor. Connect between FLTR and COMM.
5 COMM Device Common (Ground).
6 NC No Connection.
7 VAPC Output. Control voltage for gain control element.
8 VPOS Positive Supply Voltage: 2.7 V to 5.5 V.
AD8315
Rev. C | Page 7 of 24
TYPICAL PERFORMANCE CHARACTERISTICS
RF INPUT AMPLITUDE (dBV)
0.2
0
–10
–20
–30
–40
–50
–60
–70
–80
0.1GHz
0.9GHz
1.9GHz
2.5GHz
23
13
3
–7
–17
–27
–37
–47
–57
–67
RF INPUT AMPLITUDE (dBm)
10
0.4 0.6 0.8 1.0 1.2 1.4
V
SET
(V)
0
1520-003
Figure 3. Input Amplitude vs. VSET
0.1
10
0
–10
–20
–30
–40
–50
–60
–70
4
3
2
1
0
–1
–2
–3
–4
ERROR (dB)
–30°C
+85°C
+25°C
–30°C
+25°C
+85°C
(+3dBm)
(–47dBm)
0.3 0.5 0.7 0.9 1.1 1.3 1.5
V
SET
(V)
RF INPUT
AMPLITUDE (dBV)
ERROR AT +85°C AND –30°C
BASED ON DEVIATION FROM
SLOPE AND INTERCEPT AT +25°C
01520-004
Figure 4. Input Amplitude and Log Conformance vs. VSET at 0.1 GHz
0.1
10
0
–10
–20
–30
–40
–50
–60
–70
4
3
2
1
0
–1
–2
–3
–4
ERROR (dB)
–30°C
+85°C
+25°C
–30°C
+25°C
+85°C
(+3dBm)
(–47dBm)
0.3 0.5 0.7 0.9 1.1 1.3 1.5
V
SET
(V)
RF INPUT
AMPLITUDE (dBV)
ERROR AT +85°C AND –30°C
BASED ON DEVIATION FROM
SLOPE AND INTERCEPT AT +25°C
01520-005
Figure 5. Input Amplitude and Log Conformance vs. VSET at 0.9 GHz
0.2
4
E
R
R
OR (dB)
3
2
1
0
–1
–2
–3
–4
0.1GHz 0.9GHz
1.9GHz
2.5GHz
1.60.4 0.6 0.8 1.0 1.2 1.4
V
SET
(V)
01520-006
Figure 6. Log Conformance vs. VSET
0.1
10
0
–10
–20
–30
–40
–50
–60
–70
4
3
2
1
0
–1
–2
–3
–4
ERROR (dB)
–30°C
+85°C
+25°C
+25°C
+85°C
–30°C
(+3dBm)
(–47dBm)
0.30.50.70.91.11.3 1.5
V
SET
(V)
RF INPUT
AMPLITUDE (dBV)
ERROR AT +85°C AND –30°C
BASED ON DEVIATION FROM
SLOPE AND INTERCEPT AT +25°C
01520-007
Figure 7. Input Amplitude and Log Conformance vs. VSET at 1.9 GHz
0.1
10
0
–10
–20
–30
–40
–50
–60
–70
4
3
2
1
0
–1
–2
–3
–4
ERROR (dB)
–30°C +85°C
+25°C
–30°C
+85°C
+25°C
(+3dBm)
(–47dBm)
0.3 0.5 0.7 0.9 1.1 1.3 1.5
V
SET
(V)
RF INPUT
AMPLITUDE (dBV)
ERROR AT +85°C AND –30°C
BASED ON DEVIATION FROM
SLOPE AND INTERCEPT AT +25°C
01520-008
Figure 8. Input Amplitude and Log Conformance vs. VSET at 2.5 GHz
AD8315
Rev. C | Page 8 of 24
RF INPUT AMPLITUDE (dBV)
4
–4
0–70
ERROR (dB)
2
0
–1
–2
–3
3
1
–60
(+3dBm)(–47dBm)
–30°C
+85°C
ERROR AT +85°C AND –30°C
BASED ON DEVIATION FROM
SLOPE AND INTERCEPT AT +25°C
–80 –50 –40 –30 –20 –10
01520-009
Figure 9. Distribution of Error at Temperature After Ambient Normalization vs.
Input Amplitude, 3 Sigma to Either Side of Mean, 0.1 GHz
RF INPUT AMPLITUDE (dBV)
4
–4
–70
ERROR (dB)
2
0
–1
–2
–3
3
1
(+3dBm)(–47dBm)
0–60
+85°C
–30°C
ERROR AT +85°C AND –30°C
BASED ON DEVIATION FROM
SLOPE AND INTERCEPT AT +25°C
–80 –50 –40 –30 –20 –10
01520-010
Figure 10. Distribution of Error at Temperature After Ambient Normalization vs.
Input Amplitude, 3 Sigma to Either Side of Mean, 0.9 GHz
FREQUENCY (GHz)
3000
2700
002
.5
1800
900
600
300
2400
2100
1200
1500
RESISTANCE (Ω)
0
–200
–2000
–800
–1400
–1600
–1800
–400
–600
–1200
–1000
REACTANCE (Ω)
RX
FREQUENCY
(GHz)
0.1
0.9
1.9
2.5
R –
2700 –
730 –
460 –
440 –
jXΩ
j1500
j220
j130
j110
R –
2900 –
700 –
130 –
170 –
jXΩ
j1900
j240
j80
j70
MSOP CHIP SCALE (LFCSP)
R (LFCSP)
R (MSOP)
X (LFCSP)
X (MSOP)
0.5 1.0 1.5 2.0
0
1520-011
Figure 11. Input Impedance
RF INPUT AMPLITUDE (dBV)
4
–4
–70
ERROR (dB)
2
0
–1
–2
–3
3
1
(+3dBm)(–47dBm)
0–60
–30°C
+85°C
ERROR AT +85°C AND –30°C
BASED ON DEVIATION FROM
SLOPE AND INTERCEPT AT +25°C
–80 –50 –40 –30 –20 –10
01520-012
Figure 12. Distribution of Error at Temperature After Ambient Normalization vs.
Input Amplitude, 3 Sigma to Either Side of Mean, 1.9 GHz
4
–4
–70
ERROR (dB)
2
0
–1
–2
–3
3
1
RF INPUT AMPLITUDE (dBV)
0–60
(+3dBm)(–47dBm)
–30°C
+85°C
ERROR AT +85°C AND –30°C
BASED ON DEVIATION FROM
SLOPE AND INTERCEPT AT +25°C
–80 –50 –40 –30 –20 –10
01520-013
Figure 13. Distribution of Error at Temperature After Ambient Normalization vs.
Input Amplitude, 3 Sigma to Either Side of Mean, 2.5 GHz
10
8
01.3
SUPPLY CURRENT
(
m
A)
6
4
2
INCREASING
VENBL
DECREASING
VENBL
VENBL (V)
1.4 1.5 1.6 1.7
01520-014
Figure 14. Supply Current vs. VENBL
AD8315
Rev. C | Page 9 of 24
FREQUENCY (GHz)
25
0
SLOPE (mV/dB)
24
23
22
21
20 0.5 1.0 1.5 2.0 2.5
+85°C
+25°C
–30°C
01520-015
Figure 15. Slope vs. Frequency; −30°C, +25°C, and +85°C
24
2.5
SLOPE (mV/dB)
23
22
21
0.1GHz
0.9GHz
1.9GHz
2.5GHz
V
S
(V)
3.0 3.5 4.0 4.5 5.0 5.5
01520-016
Figure 16. Slope vs. Supply Voltage
FREQUENCY (Hz)
45
10
AMPLITUDE (dB)
35
25
15
5
–5
–15
–25
–35
0
–20
–40
–60
–80
–100
–120
40
20
0
–20
30
10
–10
–30
–40
–10
–30
–50
–70
–90
–110
–130
PHASE (Degrees)
C
FLT
= 0pF
C
FLT
= 220pF
100 1k 10k 100k 1M 10M
01520-017
Figure 17. AC Response from VSET to VAPC
FREQUENCY (GHz)
–66
0
INTERCEPT (dBV)
–68
–70
–72
–74
–76
–78
–80
+25°C
+85°C
–30°C
0.5 1.0 1.5 2.0 2.5
01520-018
Figure 18. Intercept vs. Frequency; −30°C, +25°C, and +85°C
2.5
INTERCEPT (dBV)
–
68
–70
–72
–74
–76
–78
–80
0.9GHz
1.9GHz
0.1GHz
2.5GHz
V
S
(V)
3.0 3.5 4.0 4.5 5.0 5.5
0
1520-019
Figure 19. Intercept vs. Supply Voltage
FREQUENCY (Hz)
10000
100
NOISE SPECTRAL DENSITY (nV/
√
Hz)
1000
100
10
1k 10k 100k 1M 10M
RF INPUT
–51dBV
–48dBV
–13dBV
–23dBV
–43dBV
–33dBV
–53dBV AND
–63dBV
CFLT = 220pF, RF INPUT = 2GHz
01520-020
Figure 20. VAPC Noise Spectral Density
AD8315
Rev. C | Page 10 of 24
SUPPLY VOLTAGE (V)
3.5
2.7
3.3
3.1
2.9
2.7
2.5
2.3
4mA
6mA
0mA
2mA
V
APC
(V)
2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5
0
1520-021
Figure 21. Maximum VAPC Voltage vs. Supply Voltage by Load Current
AVERAGE = 16 SAMPLES
GND
GND
200mV PER VERTICAL
DIVISION
VAPC
1V PER
VERTICAL
DIVISION
2µs PER
HORIZONTAL
DIVISION
VENBL
0
1520-022
Figure 22. ENBL Response Time
NC = NO CONNECT
RFIN
ENBL
VSET
VPOS
VAPC
NC
COMMFLTR
AD8315
1
2
3
45
6
7
8
220pF
2.7V
52.3Ω
0.1µF
TRIG
STANFORD DS345
PULSE
GENERATOR
PULSE OUT
RF OUT
TIMEBASE TRIG
OUT
TEK TDS694C
SCOPE
TEK P6205
FET PROBE
TEK P6205
FET PROBE
10MHz REF
OUTPUT
R AND S
SMT03
SIGNAL
GENERATOR
01520-023
Figure 23. Test Setup for ENBL Response Time
SUPPLY VOLTAGE (V)
2.8
2.7
2.7
2.6
2.5
2.4
2.9
V
APC
(V)
SHADING INDICATES
±3 SIGMA
2.8 3.0
01520-024
Figure 24. Maximum VAPC Voltage vs. Supply Voltage with 4 mA Load Current
GND
GND
AVERAGE = 16 SAMPLES
1V PER
VERTICAL
DIVISION
PULSED RF
0.1GHz, –13dBV
100ns PER
HORIZONTAL
DIVISION
RF
INPUT
V
APC
0
1520-025
Figure 25. VAPC Response Time, Full-Scale Amplitude Change, Open-Loop
NC = NO CONNECT
RFIN
ENBL
VSET
VPOS
VAP C
NC
COMMFLTR
AD8315
1
2
3
45
6
7
8
2.7V
0.1µF
TRIG
PULSE OUT
EXT TRIG
NC
PULSE MODE IN OUT
52.3Ω
RF OUT
–3dB
–3dB
2.7V
0.3V
R AND S SMT03
SIGNAL
GENERATOR
PULSE
MODULATION
MODE
RF
SPLITTER
PICOSECOND
PULSE LABS
PULSE
GENERATOR
TRIG
OUT
TEK TDS694C
SCOPE
TEK P6205
FET PROBE
10MHz REF
OUTPUT
0
1520-026
Figure 26. Test Setup for VAPC Response Time
AD8315
Rev. C | Page 11 of 24
GND
GND
1V PER
VERTICAL
DIVISION
200mV PER
VERTICAL
DIVISION
2µs PER
HORIZONTAL
DIVISION
V
S
AND
V
ENBL
V
APC
AVERAGE = 16 SAMPLES
01520-027
GND
GND
AVERAGE = 16 SAMPLES
500mV PER
VERTICAL
DIVISION
V
APC
V
S
1V PER
VERTICAL
DIVISION
2µs PER
HORIZONTAL
DIVISION
0
1520-029
Figure 29. Power-On and Power-Off Response with VSET and ENBL Grounded
Figure 27. Power-On and Power-Off Response with VSET Grounded
NC = NO CONNECT
RFIN
ENBL
VSET
VPOS
VA P C
NC
COMMFLTR
AD8315
1
2
3
45
6
7
8
220pF
TRIG
PULSE OUT
RF OUT
EXT TRIG
AD811
49.9Ω
52.3Ω
TRIG
OUT
732Ω
10MHz REF
OUTPUT
R AND S
SMT03
SIGNAL
GENERATOR
STANFORD DS345
PULSE
GENERATOR
TEK TDS694C
SCOPE
TEK P6205
FET PROBE
TEK P6205
FET PROBE
01520-030
NC = NO CONNECT
RFIN
ENBL
VSET
VPOS
VA P C
NC
COMMFLTR
AD8315
1
2
3
45
6
7
8
220pF
52.3Ω
TRIG
PULSE OUT
RF OUT
EXT TRIG
AD811
732Ω
49.9Ω
10MHz REF
OUTPUT
R AND S
SMT03
SIGNAL
GENERATOR
STANFORD DS345
PULSE
GENERATOR
TEK TDS694C
SCOPE
TEK P6205
FET PROBE
TEK P6205
FET PROBE
TRIG
OUT
01520-028
Figure 30. Test Setup for Power-On and Power-Off Response with VSET
and ENBL Grounded
Figure 28. Test Setup for Power-On and Power-Off Response with VSET Grounded
AD8315
Rev. C | Page 12 of 24
THEORY OF OPERATION
The AD8315 is a wideband logarithmic amplifier (log amp)
similar in design to the AD8313 and AD8314. However, it is
strictly optimized for use in power control applications rather
than as a measurement device. Figure 31 shows the main
features in block schematic form. The output (Pin 7, VAPC) is
intended to be applied directly to the automatic power-control
(APC) pin of a power amplifier module.
BASIC THEORY
Logarithmic amplifiers provide a type of compression in which
a signal having a large range of amplitudes is converted to one
of smaller range. The use of the logarithmic function uniquely
results in the output representing the decibel value of the input.
The fundamental mathematical form is:
Z
IN
SLP
OUT V
V
VV 10
log= (1)
Here VIN is the input voltage, VZ is called the intercept (voltage)
because when VIN = VZ the argument of the logarithm is unity
and thus the result is zero, and VSLP is called the slope (voltage),
which is the amount by which the output changes for a certain
change in the ratio (VIN/VZ). When BASE-10 logarithms are
used, denoted by the function log10, VSLP represents the volts/
decade, and since a decade corresponds to 20 dB, VSLP/20
represents the volts/dB. For the AD8315, a nominal (low
frequency) slope of 24 mV/dB was chosen, and the intercept VZ
was placed at the equivalent of −70 dBV for a sine wave input
(316 µV rms). This corresponds to a power level of −57 dBm
when the net resistive part of the input impedance of the log
amp is 50 Ω. However, both the slope and the intercept are
dependent on frequency (see Figure 15 and Figure 18).
Keeping in mind that log amps do not respond to power but
only to voltages and that the calibration of the intercept is
waveform dependent and is only quoted for a sine wave signal,
the equivalent power response can be written as
VOUT = VDB (PIN − PZ) (2)
where:
PIN, the input power, and PZ, the equivalent intercept, are both
expressed in dBm (thus, the quantity in parentheses is simply a
number of decibels).
VDB is the slope expressed as so many mV/dB.
For a log amp having a slope VDB of 24 mV/dB and an intercept
at −57 dBm, the output voltage for an input power of –30 dBm
is 0.024 [−30 − (−57)] = 0.648 V.
Further details about the structure and function of log amps can
be found in data sheets for other log amps produced by Analog
Devices, Inc. Refer to the AD640 data sheet and AD8307 data
sheet, both of which include a detailed discussion of the basic
principles of operation and explain why the intercept depends
on waveform, an important consideration when complex
modulation is imposed on an RF carrier.
VPOS
ENBL
RFIN
COMM
(PADDLE)
DET
10dB
DET
10dB
DET DET DET
10dB 10dB
OFFSET
COMP’N
INTERCEPT
POSITIONING
LOW NOISE
GAIN BIAS
LOW NOISE
BAND GAP
REFERENCE
OUTPUT
ENABLE
DELAY
×1.35
VAPC
HI-Z
LOW NOISE (25nV/√Hz)
RAIL-TO-RAIL BUFFER
FLTR
VSET
V-I 23mV/dB
250mV TO
1.4V = 50dB
(CURRENT-
NULLING
MODE)
(WEAK GM STAGE)
(CURRENT-MODE SIGNAL)
(CURRENT-MODE
FEEDBACK)
(SMALL INTERNAL
FILTER CAPACITOR
FOR GHz RIPPLE)
(ELIMIN
A
TES
GLITCH)
(PRECISE SLOPE
CONTROL)
(PRECISE GAIN
CONTROL)
0
1520-031
Figure 31. Block Schematic
AD8315
Rev. C | Page 13 of 24
The intercept need not correspond to a physically realizable
part of the signal range for the log amp. Therefore, the specified
intercept is −70 dBV, at 0.1 GHz, whereas the smallest input for
accurate measurement (a +1 dB error, see Table 2) at this
frequency is higher, being about −58 dBV. At 2.5 GHz, the
+1 dB error point shifts to −64 dBV. This positioning of the
intercept is deliberate and ensures that the VSET voltage is within
the capabilities of certain DACs, whose outputs cannot swing
below 200 mV. Figure 32 shows the 100 MHz response of the
AD8315; the vertical axis does not represent the output (at pin
VAPC) but the value required at the power control pin, VSET,
to null the control loop.
1.5
1.0
0.5
0
–70dBV
1.416V @ –11dBV
0.288V @ –58dBV
ACTUAL
IDEAL
SLOPE = 24mV/dB
V
SET
100µV
–80dBV
–67dBm
1mV
–60dBV
–47dBm
10mV
–40dBV
–27dBm
100mV
–20dBV
–7dBm
1V (RMS)
0dBV
+13dBm (RE 50Ω)
V
IN
, dBV
IN
, P
IN
01520-032
Figure 32. Basic Calibration of the AD8315 at 0.1 GHz
CONTROLLER-MODE LOG AMPS
The AD8315 combines the two key functions required for the
measurement and control of the power level over a moderately
wide dynamic range. First, it provides the amplification needed
to respond to small signals in a chain of four amplifier/limiter
cells (see Figure 31), each having a small signal gain of 10 dB
and a bandwidth of approximately 3.5 GHz. At the output of
each of these amplifier stages is a full-wave rectifier, essentially a
square law detector cell that converts the RF signal voltages to a
fluctuating current having an average value that increases with
signal level. A further passive detector stage is added before the
first stage. These five detectors are separated by 10 dB, spanning
some 50 dB of dynamic range. Their outputs are each in the
form of a differential current, making summation a simple
matter. It is readily shown that the summed output can closely
approximate a logarithmic function. The overall accuracy at the
extremes of this total range, viewed as the deviation from an
ideal logarithmic response, that is, the log conformance error,
can be judged by referring to Figure 6, which shows that errors
across the central 40 dB are moderate. Other performance
curves show how conformance to an ideal logarithmic function
varies with supply voltage, temperature, and frequency.
In a device intended for measurement applications, this current
would then be converted to an equivalent voltage, to provide the
log (VIN) function shown in Equation 1. However, the design of
the AD8315 differs from standard practice in that its output
needs to be a low noise control voltage for an RF power amplifier
not a direct measure of the input level. Furthermore, it is highly
desirable that this voltage be proportional to the time integral of
the error between the actual input VIN and the dc voltage VSET
(applied to Pin 3, VSET) that defines the setpoint, that is, a
target value for the power level, typically generated by a DAC.
This is achieved by converting the difference between the sum
of the detector outputs (still in current form) and an internally
generated current proportional to VSET to a single-sided,
current-mode signal. This, in turn, is converted to a voltage (at
Pin 4, FLTR, the low-pass filter capacitor node) to provide a
close approximation to an exact integration of the error between
the power present in the termination at the input of the AD8315
and the setpoint voltage. Finally, the voltage developed across
the ground-referenced filter capacitor CFLT is buffered by a
special low noise amplifier of low voltage gain (×1.35) and
presented at Pin 7 (VAPC) for use as the control voltage for the
RF power amplifier. This buffer can provide rail-to-rail swings
and can drive a substantial load current, including large
capacitors. Note that the RF power amplifier is assumed to have
a positive slope with RF power increasing monotonically with
an increasing APC control voltage.
CONTROL LOOP DYNAMICS
To understand how the AD8315 behaves in a complete control
loop, an expression for the current in the integration capacitor
as a function of the input VIN and the setpoint voltage VSET must
be developed (see Figure 33).
3
1
RFIN
4
FLTR
7
VAPC
×1.35
I
SET
= V
SET
/4.15kΩ
V
SET
V
SET
V
IN
I
DET
= I
SLP
log
10
(V
IN
/V
Z
)
I
ERR
I
DET
C
FLT
SETPOINT
INTERFACE
LOGARITHMIC
RF DETECTION
SUBSYSTEM
01520-033
Figure 33. Behavioral Model of the AD8315
AD8315
Rev. C | Page 14 of 24
First, the summed detector currents are written as a function of
the input
IDET = ISLP log10 (VIN/VZ) (3)
where:
IDET is the partially filtered demodulated signal, whose
exact average value is extracted through the subsequent
integration step.
ISLP is the current-mode slope and has a value of 115 µA per
decade (that is, 5.75 µA/dB).
VIN is the input in V rms.
VZ is the effective intercept voltage, which, as previously noted,
is dependent on waveform but is 316 µV rms (−70 dBV) for a
sine wave input.
Now the current generated by the setpoint interface is simply
ISET(4) = VSET/415 kΩ (4)
The difference between this current and IDET is applied to the
loop filter capacitor CFLT. It follows that the voltage appearing
on this capacitor, VFLT, is the time integral of the difference
current:
VFLT(s) = (ISET − IDET)/sCFLT (5)
(
)
FLT
Z
IN
SLP
SET
sC
VVIV 10
logk4.15 −
= (6)
The control output VAPC is slightly greater than this, because the
gain of the output buffer is ×1.35. In addition, an offset voltage
is deliberately introduced in this stage; this is inconsequential
because the integration function implicitly allows for an
arbitrary constant to be added to the form of Equation 6. The
polarity is such that VAPC rises to its maximum value for any
value of VSET greater than the equivalent value of VIN. In
practice, the VAPC output rails to the positive supply under this
condition unless the control loop through the power amplifier
is present. In other words, the AD8315 seeks to drive the RF
power to its maximum value whenever it falls below the
setpoint. The use of exact integration results in a final error that
is theoretically 0, and the logarithmic detection law would
ideally result in a constant response time following a step
change of either the setpoint or the power level, if the power-
amplifier control function were likewise linear in dB. However,
this latter condition is rarely true, and it follows that in practice,
the loop response time depends on the power level, and this
effect can strongly influence the design of the control loop.
Equation 6 can be restated as
()
(
)
sT
VVVV
sV Z
IN
SLP
SET
APC
10
log
−
= (7)
where VSLP is the volts-per-decade slope from Equation 1,
having a value of 480 mV/decade, and T is an effective time
constant for the integration, being equal to 4.15 kΩ × CFLT/1.35;
the resistor value comes from the setpoint interface scaling
Equation 4 and the factor 1.35 arises because of the voltage gain
of the buffer. Therefore, the integration time constant can be
written as
T = 3.07 CFLT in µs, when C is expressed in nF (8)
To simplify our understanding of the control loop dynamics,
begin by assuming that the power amplifier gain function is
actually linear in dB, and for the moment, use voltages to
express the signals at the power amplifier input and output.
Let the RF output voltage be VPA and let its input be VCW.
Furthermore, to characterize the gain control function, this
form is used
(
)
GBCAPC VV
CWOPA VGV 10= (9)
where:
GO is the gain of the power amplifier when VAPC = 0.
VGBC is the gain scaling.
While few amplifiers conform so conveniently to this law, it
provides a clearer starting point for understanding the more
complex situation that arises when the gain control law is less ideal.
This idealized control loop is shown in Figure 34. With some
manipulation, it is found that the characteristic equation of this
system is
()
(
)
(
)
O
Z
CWOGBCSLPGBC
SET
APC sT
VVkGVVVV
sV +
−
=1
log10 (10)
where:
k is the coupling factor from the output of the power amplifier
to the input of the AD8315 (for example, ×0.1 for a 20 dB coupler).
TO is a modified time constant (VGBC/VSLP)T.
This is quite easy to interpret. First, it shows that a system of
this sort exhibits a simple single-pole response, for any power
level, with the customary exponential time domain form for
either increasing or decreasing step polarities in the demand
level VSET or the carrier input VCW. Second, it reveals that the
final value of the control voltage VAPC is determined by several
fixed factors:
(
)
(
)
(
)
Z
CWOSLPGBC
SET
APC VVkGVVVV 10
logτ −
=
∞
=
(11)
AD8315
Rev. C | Page 15 of 24
Example
Assume that the gain magnitude of the power amplifier runs
from a minimum value of ×0.316 (−10 dB) at VAPC = 0 to ×100
(40 dB) at VAPC = 2.5 V. Applying Equation 9, GO = 0.316 and
VGBC = 1 V. Using a coupling factor of k = 0.0316 (that is, a
30 dB directional coupler) and recalling that the nominal value
of VSLP is 480 mV and VZ = 316 µV for the AD8315, first calculate
the range of values needed for VSET to control an output range of
+33 dBm to −17 dBm. This can be found by noting that, in the
steady state, the numerator of Equation 7 must be 0, that is:
VSET = VSLP log10 (kVPA/VZ) (12)
where VIN is expanded to kVPA, the fractional voltage sample of
the power amplifier output. For 33 dBm, VPA = 10 V rms, which
evaluates to
VSET (max) = 0.48 log10 (316 mV/316 µV) = 1.44 V (13)
For a delivered power of −17 dBm, VPA = 31.6 mV rms
VSET (min) = 0.48 log10 (1 mV/316 µV) = 0.24 V (14)
Check that the power range is 50 dB, which should correspond
to a voltage change in VSET of 50 dB × 24 mV/dB = 1.2 V,
which agrees.
Now, the value of VAPC is of interest, although it is a dependent
parameter, inside the loop. It depends on the characteristics of
the power amplifier, and the value of the carrier amplitude VCW.
Using the control values previously derived, that is, GO = 0.316
and VGBC = 1 V, and assuming the applied power is fixed at
−7 dBm (so VCW = 100 mV rms), the following is true using
Equation 11
VAPC(max) = (VSETVGBC)/VSLP − log10 kGOVCW/VZ
= (1.44 × 1)/0.48 − log10(0.0316 × 0.316 × 0.1/316 µV)
= 3.0 − 0.5 = 2.5 V (15)
VAPC(min) = (VSETVGBC)/VSLP − log10 kGOVCW/VZ
= (0.24 × 1)/0.48 − log10(0.0316 × 0.316 × 0.1/316 µV)
= 0.5 − 0.5 = 0 (16)
both of which results are consistent with the assumptions made
about the amplifier control function. Note that the second term
is independent of the delivered power and a fixed function of
the drive power.
RF PA
DIRECTIONAL COUPLER
RF DRIVE: UP
TO 2.5GHz
AD8315
V
RF
V
CW
V
IN
= kV
RF
V
SET
V
APC
C
FLT
RESPONSE-SHAPING
OF OVERALL CONTROL-
LOOP (EXTERNAL CAP)
01520-034
Figure 34. Idealized Control Loop for Analysis
Finally, using the loop time constant for these parameters and
an illustrative value of 2 nF for the filter capacitor CFLT
TO = (VGBC/VSLP) T
= (1/0.48)3.07 µs × 2 (nF) = 12.8 µs (17)
PRACTICAL LOOP
At present time, power amplifiers, or VGAs preceding such
amplifiers, do not provide an exponential gain characteristic. It
follows that the loop dynamics (the effective time constant)
varies with the setpoint because the exponential function is
unique in providing constant dynamics. The procedure must
therefore be as follows. Beginning with the curve usually provided
for the power output vs. the APC voltage, draw a tangent at the
point on this curve where the slope is highest (see Figure 35).
Using this line, calculate the effective minimum value of the
variable VGBC and use it in Equation 17 to determine the time
constant. Note that the minimum in VGBC corresponds to the
maximum rate of change in the output power vs. VAPC.
For example, suppose it is found that, for a given drive power,
the amplifier generates an output power of P1 at VAPC = V1 and
P2 at VAPC = V2. Then, it is readily shown that
VGBC = 20 (V2 − V1)/(P2 − P1) (18)
This should be used to calculate the filter capacitance. The
response time at high and low power levels (on the shoulders
of the curve shown in Figure 35) is slower. Note also that it is
sometimes useful to add a 0 in the closed-loop response by
placing a resistor in series with CFLT. For more information on
this, see the Transient Response section.
AD8315
Rev. C | Page 16 of 24
33
0
23
13
3
–7
V
2
, P
2
V
1
, P
1
P
RF
(dBm)
V
APC
(V)
0.5 1.0 1.5 2.0 2.5
01520-035
Figure 35. Typical Power-Control Curve
A NOTE ABOUT POWER EQUIVALENCY
In using the AD8315, it must be understood that log amps do
not fundamentally respond to power. It is for this reason that
dBV (decibels above 1 V rms) are used rather than the commonly
used metric of dBm. The dBV scaling is fixed, independent of
termination impedance, while the corresponding power level is
not. For example, 224 mV rms is always −13 dBV (with one
further condition of an assumed sinusoidal waveform; see the
AD640 data sheet for more information about the effect of
waveform on logarithmic intercept), and this corresponds to a
power of 0 dBm when the net impedance at the input is 50 Ω.
When this impedance is altered to 200 Ω, however, the same
voltage corresponds to a power level that is four times smaller
(P = V2/R) or −6 dBm. A dBV level can be converted to dBm in
the special case of a 50 Ω system and a sinusoidal signal by
simply adding 13 dB (0 dBV is then, and only then, equivalent
to 13 dBm).
Therefore, the external termination added ahead of the AD8315
determines the effective power scaling. This often takes the
form of a simple resistor (52.3 Ω provides a net 50 Ω input), but
more elaborate matching networks can be used. The choice of
impedance determines the logarithmic intercept, that is, the
input power for which the VSET vs. PIN function would cross the
baseline if that relationship were continuous for all values of
VIN. This is never the case for a practical log amp; the intercept
(so many dBV) refers to the value obtained by the minimum
error straight line fit to the actual graph of VSET vs. PIN (more
generally, VIN). Where the modulation is complex, as in CDMA,
the calibration of the power response needs to be adjusted; the
intercept remains stable for any given arbitrary waveform.
When a true power (waveform independent) response is
needed, a mean-responding detector, such as the AD8361,
should be considered.
The logarithmic slope, VSLP in Equation 1, which is the amount
by which the setpoint voltage needs to be changed for each
decibel of input change (voltage or power), is, in principle,
independent of waveform or termination impedance. In
practice, it usually falls off somewhat at higher frequencies,
due to the declining gain of the amplifier stages and other
effects in the detector cells (see Figure 15).
BASIC CONNECTIONS
Figure 36 shows the basic connections for operating the
AD8315, and Figure 37 shows a block diagram of a typical
application. The AD8315 is typically used in the RF power
control loop of a mobile handset.
A supply voltage of 2.7 V to 5.5 V is required for the AD8315.
The supply to the VPOS pin should be decoupled with a low
inductance 0.1 µF surface-mount ceramic capacitor, close to the
device. The AD8315 has an internal input coupling capacitor.
This negates the need for external ac coupling. This capacitor,
along with the low frequency input impedance of the device of
approximately 2.8 kΩ, sets the minimum usable input frequency to
around 0.016 GHz. A broadband 50 Ω input match is achieved
in this example by connecting a 52.3 Ω resistor between RFIN
and ground. A plot of input impedance vs. frequency is shown
in Figure 11. Other coupling methods are also possible (see
Input Coupling Options section).
NC = NO CONNECT
RFIN
ENBL
VSET
VPOS
VAPC
NC
COMMFLTR
AD8315
1
2
3
54
6
7
8
RFIN
(2.7V TO 5.5V)
C1
0.1µF
R1
52.3Ω
C
FLT
V
SET
+V
S
+V
S
+V
APC
01520-036
Figure 36. Basic Connections
RFIN VSET
AD8315
VAPC
FLTR
DAC
RFIN
ATTENUATOR
52.3Ω
POWER
AMP
DIRECTIONAL
COUPLER
GAIN
CONTROL
VOLTAGE
C
FLT
01520-037
Figure 37. Typical Application
AD8315
Rev. C | Page 17 of 24
In a power control loop, the AD8315 provides both the detector
and controller functions. A sample of the power amplifier’s (PA)
output power is coupled to the RF input of the AD8315, usually
via a directional coupler. In dual-mode applications, where
there are two PAs and two directional couplers, the outputs of
the directional couplers can be passively combined (both PAs
will never be turned on simultaneously) before being applied to
the AD8315.
A setpoint voltage is applied to VSET from the controlling
source (generally, this is a DAC). Any imbalance between the
RF input level and the level corresponding to the setpoint
voltage is corrected by the AD8315’s VAPC output that drives
the gain control terminal of the PA. This restores a balance
between the actual power level sensed at the input of the
AD8315 and the value determined by the setpoint. This
assumes that the gain control sense of the variable gain
element is positive, that is, an increasing voltage from
VAPC tends to increase gain.
VAPC can swing from 250 mV to within 100 mV of the supply
rail and can source up to 6 mA. If the control input of the PA
needs to source current, a suitable load resistor can be connected
between VAPC and COMM. The output swing and current
sourcing capability of VAPC is shown in Figure 21.
RANGE ON VSET AND RFIN
The relationship between the RF input level and the setpoint
voltage follows from the nominal transfer function of the device
(see Figure 4, Figure 5, Figure 7, and Figure 8). At 0.9 GHz, for
example, a voltage of 1 V on VSET indicates a demand for
−30 dBV (−17 dBm, re 50 Ω) at RFIN. The corresponding power
level at the output of the power amplifier is greater than this
amount due to the attenuation through the directional coupler.
For setpoint voltages of less than approximately 250 mV, VAPC
remains unconditionally at its minimum level of approximately
250 mV. This feature can be used to prevent any spurious
emissions during power-up and power-down phases.
Above 250 mV, VSET has a linear control range up to 1.4 V,
corresponding to a dynamic range of 50 dB. This results in a
slope of 23 mV/dB or approximately 43.5 dB/V.
TRANSIENT RESPONSE
The time domain response of power amplifier control loops,
using any kind of controller, is only partially determined by the
choice of filter, which, in the case of the AD8315, has a true
integrator form 1/sT, as shown in Equation 7, with a time
constant given by Equation 8. The large signal step response is
also strongly dependent on the form of the gain-control law.
Nevertheless, some simple rules can be applied. When the filter
capacitor CFLT is very large, it dominates the time domain
response, but the incremental bandwidth of this loop still varies
as VAPC traverses the nonlinear gain-control function of the PA,
as shown in Figure 35. This bandwidth is highest at the point
where the slope of the tangent drawn on this curve is greatest,
that is, for power outputs near the center of the PA’s range, and
is much reduced at both the minimum and the maximum
power levels, where the slope of the gain control curve is lowest
due to its S-shaped form.
Using smaller values of CFLT, the loop bandwidth generally
increases in inverse proportion to its value. Eventually, however,
a secondary effect appears due to the inherent phase lag in the
power amplifier’s control path, some of which can be due to
parasitic or deliberately added capacitance at the VAPC pin.
This results in the characteristic poles in the ac loop equation
moving off the real axis and thus becoming complex (and
somewhat resonant). This is a classic aspect of control loop
design. The lowest permissible value of CFLT needs to be determined
experimentally for a particular amplifier. For GSM and DCS
power amplifiers, CFLT typically ranges from 150 pF to 300 pF.
In many cases, some improvement in the worst-case response
time can be achieved by including a small resistance in series
with CFLT; this generates an additional 0 in the closed-loop
transfer function, that serves to cancel some of the higher order
poles in the overall loop. A combination of main capacitor CFLT
shunted by a second capacitor and resistor in series is also
useful in minimizing the settling time of the loop.
AD8315
Rev. C | Page 18 of 24
NC = NO CONNECT
RFIN
ENBL
VSET
VPOS
VAP C
NC
COMMFLTR
1
2
3
45
6
7
8
ENABLE
0V/2.7V
3
4
1
5
8
7
LDC15D190A0007A
26
AT T N
20dB
PF08107B
VCTL
VAPC
1000pF
3.5
V
1000pF
0.1µF
1.5kΩ
150pF
R1
52.3Ω
49.9Ω
4.7µF 4.7µF
500Ω
AD8315
BAND
SELECT
0V/2V
P
OUT
GSM
35dBm MAX
P
OUT
DCS
32dBm MAX
P
IN
GSM
3dBm
P
IN
DCS
3dBm
(OPTIONAL,
SEE TEXT)
+V
S
2.7V
R3
1
1kΩ
R2
1
600Ω
1
R2, R3 OPTIONAL,
SEE TEXT
8-BIT
RAMP DAC
0V TO 2.55V
TO
ANTENNA
01520-038
Figure 38. Dual-Mode (GSM/DCS) PA Control Example
MOBILE HANDSET POWER CONTROL EXAMPLE
Figure 38 shows a complete power amplifier control circuit for a
dual-mode handset. The PF08107B (Hitachi), a dual mode
(GSM, DCS) PA, is driven by a nominal power level of 3 dBm.
The PA has a single gain control line; the band to be used is
selected by applying either 0 V or 2 V to the PA’s VCTL input.
Some of the output power from the PA is coupled off using a
dual-band directional coupler (Murata LDC15D190A0007A).
This has a coupling factor of approximately 19 dB for the GSM
band and 14 dB for DCS and an insertion loss of 0.38 dB and
0.45 dB, respectively. Because the PF08107B transmits a maximum
power level of 35 dBm for GSM and 32 dBm for DCS, additional
attenuation of 20 dB is required before the coupled signal is
applied to the AD8315. This results in peak input levels to the
AD8315 of −4 dBm (GSM) and −2 dBm (DCS). While the
AD8315 gives a linear response for input levels up to 2 dBm,
for highly temperature-stable performance at maximum PA
output power, the maximum input level should be limited to
approximately −2 dBm (see Figure 5 and Figure 7). This does,
however, reduce the sensitivity of the circuit at the low end.
The operational setpoint voltage, in the range 250 mV to 1.4 V,
is applied to the VSET pin of the AD8315. This is typically
supplied by a DAC. The AD8315’s VAPC output drives the
level control pin of the power amplifier directly. VAPC reaches a
maximum value of approximately 2.5 V on a 2.7 V supply while
delivering the 3 mA required by the level control input of the
PA. This is more than sufficient to exercise the gain control
range of the PA.
During initialization and completion of the transmit sequence,
VAPC should be held at its minimum level of 250 mV by keeping
VSET below 200 mV.
In this example, VSET is supplied by an 8-bit DAC that has an
output range from 0 V to 2.55 V or 10 mV per bit. This sets the
control resolution of VSET to 0.4 dB/bit (0.04 dB/mV times
10 mV). If finer resolution is required, the DAC’s output voltage
can be scaled using two resistors, as shown in Figure 38. This
converts the DAC’s maximum voltage of 2.55 V down to 1.6 V
and increases the control resolution to 0.25 dB/bit.
A filter capacitor (CFLT) must be used to stabilize the loop. The
choice of CFLT depends to a large degree on the gain control
dynamics of the power amplifier, something that is frequently
poorly characterized, so some trial and error can be necessary.
In this example, a 150 pF capacitor is used and a 1.5 kΩ series
resistor is included. This adds a zero to the control loop and
AD8315
Rev. C | Page 19 of 24
increases the phase margin, which helps to make the step response
of the circuit more stable when the PA output power is low and
the slope of the PA’s power control function is the steepest.
A smaller filter capacitor can be used by inserting a series
resistor between VAPC and the control input of the PA. A
series resistor works with the input impedance of the PA to
create a resistor divider and reduces the loop gain. The size of
the resistor divider ratio depends upon the available output
swing of VAPC and the required control voltage on the PA.
This technique can also be used to limit the control voltage in
situations where the PA cannot deliver the power level being
demanded by VAPC. Overdrive of the control input of some
PAs causes increased distortion. It should be noted, however,
that if the control loop opens (that is, VAPC goes to its maximum
value in an effort to balance the loop), the quiescent current of
the AD8315 increases somewhat, particularly at supply voltages
greater than 3 V.
Figure 39 shows the relationship between VSET and output
power (POUT) at 0.9 GHz . The overall gain control function is
linear in dB for a dynamic range of over 40 dB. Note that for
VSET voltages below 300 mV, the output power drops off steeply
as VAPC drops toward its minimum level of 250 mV.
0
40
30
20
10
0
–10
–20
–30
–40
4
3
2
1
0
–1
–2
–3
–4
ERROR (dB)
1.6
+85°C
+25°C
–30°C
+85°C
+25°C
–30°C
P
OUT
(dBm)
V
SET
(V)
0.2 0.4 0.6 0.8 1.0 1.2 1.4
01520-039
Figure 39. POUT vs. VSET at 0.9 GHz for Dual-Mode Handset
Power Amplifier Application, −30°C, +25°C, and +85°C
ENABLE AND POWER-ON
The AD8315 can be disabled by pulling the ENBL pin to
ground. This reduces the supply current from its nominal level
of 7.4 mA to 4 µA. The logic threshold for turning on the device
is at 1.5 V with 2.7 V supply voltage. A plot of the enable glitch
is shown in Figure 22. Alternatively, the device can be completely
disabled by pulling the supply voltage to ground. To minimize
glitch in this mode, ENBL and VPOS should be tied together. If
VPOS is applied before the device is enabled, a narrow 750 mV
glitch results (see Figure 29).
In both situations, the voltage on VSET should be kept below
200 mV during power-on and power-off to prevent any
unwanted transients on VAPC.
INPUT COUPLING OPTIONS
The internal 5 pF coupling capacitor of the AD8315, along with
the low frequency input impedance of 2.8 kΩ, give a high-pass
input corner frequency of approximately 16 MHz. This sets the
minimum operating frequency. Figure 40, Figure 41, and Figure 42
show three options for input coupling. A broadband resistive
match can be implemented by connecting a shunt resistor to
ground at RFIN (see Figure 40). This 52.3 Ω resistor (other
values can also be used to select different overall input impedances)
combines with the input impedance of the AD8315 to give a
broadband input impedance of 50 Ω. While the input resistance
and capacitance (CIN and RIN) of the AD8315 varies from device
to device by approximately ±20%, and over frequency (see
Figure 11), the dominance of the external shunt resistor means
that the variation in the overall input impedance is close to the
tolerance of the external resistor. This method of matching is
most useful in wideband applications or in multiband systems
where there is more than one operating frequency.
A reactive match can also be implemented as shown in
Figure 41. This is not recommended at low frequencies as
device tolerances dramatically vary the quality of the match
because of the large input resistance. For low frequencies,
Figure 40 or Figure 42 is recommended.
In Figure 41, the matching components are drawn as generic
reactances. Depending on the frequency, the input impedance
and the availability of standard value components, either a
capacitor or an inductor is used. As in the previous case, the
input impedance at a particular frequency is plotted on a Smith
Chart and matching components are chosen (shunt or series L,
shunt or series C) to move the impedance to the center of the chart.
AD8315
RFIN C
C
R
SHUNT
52.3V R
IN
C
IN
0
1520-040
Figure 40. Broadband Resistive Input Coupling Option
X2
X1
AD8315
RFIN C
C
R
IN
C
IN
01520-041
Figure 41. Narrow-Band Reactive Input Coupling Option
AD8315
Rev. C | Page 20 of 24
ANTENNA
STRIPLINE
PA
AD8315
RFIN CC
RIN
CIN
R
ATTN
0
1520-042
Figure 42. Series Attention Input Coupling Option
Figure 42 shows a third method for coupling the input
signal into the AD8315. A series resistor, connected to the RF
source, combines with the input impedance of the AD8315 to
resistively divide the input signal being applied to the input.
This has the advantage of very little power being tapped off in
RF power transmission applications.
USING THE CHIP SCALE PACKAGE
On the underside of the chip scale package, there is an exposed
paddle. This paddle is internally connected to the chip’s ground.
There is no thermal requirement to solder the paddle down to
the printed circuit board’s ground plane. However, soldering
down the paddle has been shown to increase the stability over
frequency of the AD8315 ACP’s response at low input power
levels (that is, at around −45 dBm) in the DCS and PCS bands.
EVALUATION BOARD
Figure 43 shows the schematic of the AD8315 MSOP evaluation
board. The layout and silkscreen of the component side are
shown in Figure 44 and Figure 45. An evaluation board is also
available for the LFCSP package (see the Ordering Guide for
exact part numbers). Apart from the slightly smaller device
footprint, the LFCSP evaluation board is identical to the MSOP
board. The board is powered by a single supply in the 2.7 V to
5.5 V range. The power supply is decoupled by a single 0.1 µF
capacitor.
Table 5 details the various configuration options of the
evaluation board.
1
2
3
45
6
7
8
C1
0.1µF TP1
V
POS
R3
0Ω
R4
(OPEN)
C2
(OPEN)
TP2
RFIN
ENBL
VSET
FLTR
VPOS
VAPC
NC
COMM
AD8315
R1
0Ω
V
POS
SW1
J1
J2
RFIN
V
SET
C4
(OPEN)
LK1 LK2
NC = NO CONNECT
V
POS
C5
0.1µF
R8
10kΩ
C3
0.1µF
R7
16.2kΩ
R6
17.8kΩ
R5
10kΩ
01520-043
AD8031
R2
52.3Ω
J2
VAPC
Figure 43. Evaluation Board Schematic (MSOP)
Table 5. Evaluation Board Configuration Options
Component Function Default Condition
TP1, TP2 Supply and Ground Vector Pins. Not Applicable
SW1 Device Enable. When in Position A, the ENBL pin is connected to VPOS and the AD8315 is
in operating mode. In Position B, the ENBL pin is grounded putting the device in power-down mode.
SW1 = A
R1, R2 Input Interface. The 52.3 Ω resistor in Position R2 combines with the AD8315’s internal input
impedance to give a broadband input impedance of around 50 Ω. A reactive match can be
implemented by replacing R2 with an inductor and R1 (0 Ω) with a capacitor. Note that the
AD8315’s RF input is internally ac-coupled.
R2 = 52.3 Ω (Size 0603)
R1 = 0 Ω (Size 0402)
R3, R4, C2 Output Interface. R4 and C2 can be used to check the response of VAPC to capacitive and resistive
loading. R3/R4 can be used to reduce the slope of VAPC.
R4 = C2 = Open (Size 0603)
R3 = 0 Ω (Size 0603)
C1 Power Supply Decoupling. The nominal supply decoupling consists of a 0.1 μF capacitor. C1 = 0.1 μF (Size 0603)
C4 Filter Capacitor. The response time of VAPC can be modified by placing a capacitor between
FLTR (Pin 4) and ground.
C4 = Open (Size 0603)
LK1, LK2 Measurement Mode. A quasimeasurement mode can be implemented by installing LK1 and LK2
(connecting an inverted VAPC to VSET) to yield the nominal relationship between RFIN and VSET.
In this mode, a large capacitor (0.01 μF or greater) must be installed in C4.
LK1, LK2 = Installed
AD8315
Rev. C | Page 21 of 24
01520-044
Figure 44. Layout of Component Side (MSOP)
EV ALUA TION BOARD REV A
PWUP GND
TP2
AD8315
VAPC
J2
VPOS
TP1 R3
R4
C2
C1
R2
R1
C4
Z1
C5
R7
R8
LK2
R5
PWDN
A
B
SW1
RFIN
J1
J3
C3
R6
A1
LK1
VSET 08 - 006794 REV A
COMPONENT SIDE
01520-045
Figure 45. Silkscreen of Component Side (MSOP)
For operation in controller mode, both jumpers, LK1 and LK2,
should be removed. The setpoint voltage is applied to VSET,
RFIN is connected to the RF source (PA output or directional
coupler), and VAPC is connected to the gain control pin of the
PA. When used in controller mode, a capacitor must be installed in
C4 for loop stability. For GSM/DCS handset power amplifiers,
this capacitor should typically range from 150 pF to 300 pF.
A quasimeasurement mode (where the AD8315 delivers an
output voltage that is proportional to the log of the input signal)
can be implemented, to establish the relationship between VSET
and RFIN, by installing the two jumpers, LK1 and LK2. This
mimics an AGC loop. To establish the transfer function of the
log amp, the RF input should be swept while the voltage on
VSET is measured, that is, the SMA connector labeled VSET
now acts as an output. This is the simplest method to validate
operation of the evaluation board. When operated in this mode,
a large capacitor (0.01 µF or greater) must be installed in C4
(filter capacitor) to ensure loop stability.
AD8315
Rev. C | Page 22 of 24
OUTLINE DIMENSIONS
COMPLIANT TO JEDEC STANDARDS MO-187-AA
0.80
0.60
0.40
8°
0°
4
8
1
5
PIN 1
0.65 BSC
SEATING
PLANE
0.38
0.22
1.10 MAX
3.20
3.00
2.80
COPLANARITY
0.10
0.23
0.08
3.20
3.00
2.80
5.15
4.90
4.65
0.15
0.00
0.95
0.85
0.75
Figure 46. 8-Lead Mini Small Outline Package [MSOP]
(RM-8)
Dimensions shown in millimeters
0.30
0.23
0.18
SEATING
PLANE 0.20 REF
0.80 MAX
0.65 TYP
1.00
0.85
0.80
1.89
1.74
1.59
0.50 BSC
0.60
0.45
0.30
0.55
0.40
0.30
0.15
0.10
0.05
0.25
0.20
0.15
BOTTOM VIEW
4 1
58
3.25
3.00
2.75
1.95
1.75
1.55
2.95
2.75
2.55
PIN 1
INDICATOR
2.25
2.00
1.75
TOP VIEW
0.05 MAX
0.02 NOM
12° MAX
EXPOSEDPAD
Figure 47. 8-Lead Lead Frame Chip Scale Package [LFCSP_VD]
2 mm × 3 mm Body, Very Thin, Dual Lead
(CP-8-1)
Dimensions shown in millimeters
AD8315
Rev. C | Page 23 of 24
ORDERING GUIDE
Model Temperature Range Package Description Package Option Ordering Quantity Branding
AD8315ARM −30°C to +85°C 8-Lead MSOP, Tube RM-8 50 J7A
AD8315ARM-REEL −30°C to +85°C 8-Lead MSOP, 13" Tape and Reel RM-8 3,000 J7A
AD8315ARM-REEL7 −30°C to +85°C 8-Lead MSOP, 7" Tape and Reel RM-8 1,000 J7A
AD8315ARMZ1−30°C to +85°C 8-Lead MSOP, Tube RM-8 50 Q0S
AD8315ARMZ-RL1−30°C to +85°C 8-Lead MSOP, 13" Tape and Reel RM-8 3,000 Q0S
AD8315-EVAL MSOP Evaluation Board
AD8315ACP-REEL −30°C to +85°C 8-Lead LFCSP_VD, 13" Tape and Reel CP-8-1 10,000 J7
AD8315ACP-REEL7 −30°C to +85°C 8-Lead LFCSP_VD, 7" Tape and Reel CP-8-1 3,000 J7
AD8315ACPZ-REEL1−30°C to +85°C 8-Lead LFCSP_VD, 13" Tape and Reel CP-8-1 10,000 0J
AD8315ACPZ-REEL71−30°C to +85°C 8-Lead LFCSP_VD, 7" Tape and Reel CP-8-1 3,000 0J
AD8315ACP-EVAL LFCSP_VD Evaluation Board
AD8315CSURF Die, Surf Tape DIE 5,000
AD8315ACHIPS Die, Waffle Pack DIE 325
1 Z = Pb-free part.
AD8315
Rev. C | Page 24 of 24
NOTES
©2006 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
C01520-0-6/06(C)
