LT1568
1
1568f
The LT
®
1568 is an easy-to-use, active-RC filter building
block with rail-to-rail inputs and outputs. The internal ca-
pacitors of the IC and the GBW product of the internal low
noise op amps are trimmed such that consistent and repeat-
able filter responses can be achieved. With a single resis-
tor value, the LT1568 provides a pair of matched 2-pole
Butterworth lowpass filters with unity gain suitable for I/Q
channels.
By using unequal-valued external resistors, the two 2-pole
sections can create different frequency responses or
gains. In addition, the two stages may be cascaded to
create a single 4-pole filter with a programmable re-
sponse. Capable of cutoff frequencies up to 10MHz, the
LT1568 is ideal for antialiasing or channel filtering in high
speed data communications systems. The LT1568 can
also be used as a bandpass filter.
The LT1568 features very low noise, supporting signal-to-
noise ratios of over 90dB. It also provides single-ended to
differential signal conversion for directly driving high
speed A/D converters. The LT1568 has a shutdown mode
that reduces supply current to approximately 0.5mA on a
5V supply.
The LT1568 is available in a narrow 16-lead SSOP
package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
■
Up to 10MHz Center Frequency on a Single 3V
Supply
■
Easy to Use—A Single Resistor Value Sets Lowpass
Cutoff Frequency (200kHz ≤ ƒ
C
≤ 5MHz), Unequal
Resistor Values Extend Cutoff Frequency Up to
10MHz
■
Extremely Flexible—Different Resistor Values
Allow Lowpass Transfer Functions with or Without
Gain (Butterworth, Chebyshev or Custom)
■
SNR = 92dB (ƒ
C
= 2MHz, 2V
P-P
)
■
THD = –84dB (ƒ
C
= 2MHz, 1V
P-P
)
■
Internal Capacitors Trimmed to ±0.75%
■
Single 4-Pole Lowpass Filter or Matched Pair of
2-Pole Lowpass Filters
■
Can be Connected as a Bandpass Filter
■
Single-Ended or Differential Output
■
Operates from Single 3V (2.7V Min) to ±5V Supply
■
Rail-to-Rail Input and Output Voltages
Very Low Noise,
High Frequency Active RC,
Filter Building Block
■
Replaces Discrete RC Active Filters and LC Filter
Modules
■
Antialiasing/Reconstruction Filters
■
Dual or I-and-Q Channels (Two Matched 2nd Order
Filters in One Package)
■
Single-Ended to Differential Conversion
■
Video Signal Processing
V
+
INVA
SA
OUTA
OUTA
GNDA
NC
V
–
V
+
INVB
SB
OUTB
OUTB
GNDB
EN
V
–
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
LT1568
511Ω
0.1µF
0.1µF
511Ω511Ω511Ω
511Ω
1568 TA01
V
INA
3V
V
OUTA
V
OUTA
V
INB
THE PROPRIETARY ARCHITECTURE
ALLOWS FOR A SIMPLE RESISTOR
CALCULATION:
V
OUTB
V
OUTB
511Ω
R = 128Ω •; ƒ
C
= CUTOFF FREQUENCY
10MHz
ƒ
C
Amplitude and Phase Matched Dual Butterworth 2.5MHz Lowpass
Filter with Differential Output. Single 3V Supply Operation
FREQUENCY (Hz)
100k
–27
GAIN (dB)
–21
–15
–9
–3
1M 10M
1568 TA02
3
–24
–18
–12
–6
0
Amplitude Response
DESCRIPTIO
U
FEATURES
APPLICATIO S
U
TYPICAL APPLICATIO
U
LT1568
2
1568f
Total Supply Voltage (V
+
to V
–
)........................... 11.6V
Input Voltage on INVA, INVB, GNDA and
GNDB Pins....................................................... V
+
to V
–
Input Current on INVA, INVB, GNDA and
GNDB Pins (Note 2)........................................... ±10mA
Output Short-Circuit Duration on OUTA, OUTB, OUTA
and OUTB Pins ............................................... Indefinite
Maximum Continuous Output Current (Note 3)
DC ............................................................... ±100mA
Specified Temperature Range (Note 9)
LT1568C............................................ –40°C to 85°C
LT1568I ............................................. –40°C to 85°C
Junction Temperature.......................................... 150°C
Storage Temperature Range ................ –65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
ORDER PART
NUMBER
(Note 1)
ABSOLUTE AXI U RATI GS
WWWU
PACKAGE/ORDER I FOR ATIO
UU
W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
LT1568CGN
LT1568IGN
T
JMAX
= 150°C, θ
JA
= 135°C/W
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications and typical values are at TA = 25°C. VS = single 5V, EN pin to logic “low,” RL = 400Ω,
connected to midsupply, RFIL = R11 = R21 = R31 = R12 = R22 = R32, unless otherwise noted (see Block Diagram).
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V
S
Total Supply Voltage ●2.7 11 V
I
S
Supply Current V
S
= 3V ●24 35 mA
V
S
= 5V ●26 36 mA
V
S
= ±5V ●28 38 mA
Shutdown Supply Current V
S
= 3V, V
EN
= 2.4V ●0.3 1.0 mA
V
S
= 5V, V
EN
= 4.4V ●0.5 1.3 mA
V
S
= ±5V, V
EN
= 4.4V ●1.0 2.5 mA
Output Voltage Swing High V
S
= 3V, R
FIL
= 1.28k, R
L
= 1k ●2.75 2.85 V
(OUTA, OUTA, OUTB, OUTB Pins) V
S
= 5V, R
FIL
= 1.28k, R
L
= 1k ●4.60 4.80 V
V
S
= 5V, R
FIL
= 128Ω, R
L
= 400Ω●4.50 4.65 V
V
S
= ±5V, R
FIL
= 1.28k, R
L
= 1k ●4.60 4.75 V
Output Voltage Swing Low V
S
= 3V, R
FIL
= 1.28k, R
L
= 1k ●0.05 0.12 V
(OUTA, OUTA, OUTB, OUTB Pins) V
S
= 5V, R
FIL
= 1.28k, R
L
= 1k ●0.07 0.15 V
V
S
= 5V, R
FIL
= 128Ω, R
L
= 400Ω●0.20 0.40 V
V
S
= ±5V, R
FIL
= 1.28k, R
L
= 1k ●–4.7 V
I
OUT
Maximum Output Current ±80 mA
Op Amp Input Offset Voltage V
S
= 3V ●–2.5 –0.5 1.5 mV
V
S
= 5V ●–2.5 0.2 2.5 mV
V
S
= ±5V ●–2.0 1.2 4.5 mV
Inverter Output Offset Voltage V
S
= 3V ●–2 2.5 7.0 mV
V
S
= 5V ●–10 0.6 4.5 mV
V
S
= ±5V ●–12 –4.0 2.0 mV
ELECTRICAL CHARACTERISTICS
GN PACKAGE
16-LEAD PLASTIC SSOP
1
2
3
4
5
6
7
8
TOP VIEW
16
15
14
13
12
11
10
9
V+
INVA
SA
OUTA
OUTA
GNDA
NC
V–
V+
INVB
SB
OUTB
OUTB
GNDB
EN
V–
GN PART
MARKING
1568
1568I
LT1568
3
1568f
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications and typical values are at TA = 25°C. VS = single 5V, EN pin to logic “low,” RL = 400Ω,
connected to midsupply, RFIL = R11 = R21 = R31 = R12 = R22 = R32, unless otherwise noted (see Block Diagram).
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
I
B
Op Amp Input Bias Current V
S
= 3V ● 0.5 2 µA
V
S
= 5V ● 0.4 2 µA
V
S
= ±5V ●–0.2 2 µA
Inverter Bandwidth (Note 4) 55 MHz
Inverter Gain (Sections A and B, Note 5) Frequency = DC ●–0.2 0.01 0.2 dB
Frequency = 2MHz 0.01 dB
Frequency = 10MHz 0.27 dB
Inverter Phase Shift (Sections A and B, Frequency = DC 180 DEG
Note 5) Frequency = 2MHz 179 DEG
Frequency = 10MHz 176 DEG
SR Slew Rate (OUTA, OUTB, OUTA, 53 V/µs
OUTB) Pins
V
CM
Common Mode Input Voltage Range V
S
= 3V 1 to 1.9 V
(GNDA and GNDB Pins, Note 6) V
S
= ±5V –3.4 to 2.7 V
Single Supply GND Reference Voltage V
S
= 5V, GNDA Tied to GNDB 2.5 V
V
IL
EN Input Logic Low Level V
S
= 3V, 5V or ±5V ●V
+
– 2.1 V
V
IH
EN Input Logic High Level V
S
= 3V, 5V or ±5V ●V
+
– 0.6 V
EN Input Pull-Up Resistor 30 40 kΩ
t
DIS
Disable (Shutdown) Time EN Pin Steps from 0V to V
+
20 µs
t
EN
Enable (Start-Up) Time EN Pin Steps from V
+
to 0V 100 µs
ELECTRICAL CHARACTERISTICS
Specifications are for the output (OUTA or OUTB) of a
single
2nd order section
(A or B) with respect to VGND = VGNDA = VGNDB,
gain = –1, RFIL = R11 = R21 = R31 = R12 = R22 = R32, (see Block Diagram). The ● denotes the specifications which apply over the
full operating temperature range, otherwise specifications and typical values are at TA = 25°C. VS = single 5V, EN pin to logic “low,”
RL = 400Ω, unless otherwise noted.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
ADC DC Gain ●–1.01 –1 –0.99 V/V
V
OS(OUT)
DC Offset Voltage V
S
= 3V, f
C
= 1MHz, R
FIL
= 1.28k ●–5 2.6 15 mV
(V
OUTA
– V
GNDA
) or (V
OUTB
– V
GNDB
)V
S
= 5V, f
C
= 1MHz, R
FIL
= 1.28k ●–10 0.6 10 mV
V
S
= ±5V, f
C
= 1MHz, R
FIL
= 1.28k ●–12 –4.0 4 mV
∆V
OS(OUT)
DC Offset Voltage Mismatch V
S
= 3V, f
C
= 1MHz, R
FIL
= 1.28k ●–8 ±48 mV
(V
OUTA
– V
GNDA
) – (V
OUTB
– V
GNDB
)V
S
= 5V, V
S
= ±5V, f
C
= 1MHz, R
FIL
= 1.28k ●–10 ±410 mV
Transfer Function Characteristics for Each Section (A or B) to Single-Ended Output (OUTA or OUTB)
f
C
Cutoff Frequency Range (Note 7) V
S
= 3V, V
S
= 5V, V
S
= ±5V ●0.2 10 MHz
TC Cutoff Frequency Temperature Coefficient ●±1 ppm/°C
FILTER ELECTRICAL CHARACTERISTICS
LT1568
4
1568f
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Filter Gain, f
C
= 1MHz, Test Frequency = 300kHz (0.3 • f
C
)●–0.05 0.05 0.25 dB
V
S
= 5V, R
FIL
= 1.28k Test Frequency = 750kHz (0.75 • f
C
)●–1.45 –1.20 –0.85 dB
(Measured with Respect to DC Gain) Test Frequency = 1MHz (1 • f
C
)●–3.60 –3.20 –2.80 dB
Test Frequency = 2MHz (2 • f
C
)●–13.7 –13.2 –12.5 dB
Test Frequency = 4MHz (4 • f
C
) –25.0 dB
Filter Gain, f
C
= 10MHz, Test Frequency = 1MHz (0.1 • f
C
)●–0.1 0.02 0.25 dB
V
S
= 5V, R
FIL
= 128ΩTest Frequency = 7.5MHz (0.75 • f
C
)●–1.5 –1.0 –0.50 dB
(Measured with Respect to DC Gain) Test Frequency = 10MHz (1 • f
C
)●–3.5 –3.0 –2.40 dB
Test Frequency = 20MHz (2 • f
C
)●–14.2 –13.2 –12.2 dB
Test Frequency = 40MHz (4 • f
C
) –27.5 dB
Filter Gain Mismatch f
C
= 1MHz, f
IN
= f
C
●–0.25 ±0.02 0.25 dB
(V
OUTA
– V
OUTB
)f
C
= 10MHz, f
IN
= f
C
●–0.30 ±0.02 0.30 dB
Wideband Output Noise f
C
= 1MHz, R
FIL
= 1.28k, BW = 2MHz 18 µV
RMS
f
C
= 10MHz, R
FIL
= 128Ω, BW = 20MHz 34 µV
RMS
THD Total Harmonic Distortion f
C
= 1MHz, R
FIL
= 1.28k, –84 dB
f
IN
= 200kHz, V
IN
= 1V
P-P
f
C
= 10MHz, R
FIL
= 128Ω,–69dB
f
IN
= 2MHz, V
IN
= 1V
P-P
Specifications are for the
OUTA or OUTB of a single 2nd order section
(A or B) with respect to VGND = VGNDA = VGNDB, gain = 1,
RFIL = R11 = R21 = R31 = R12 = R22 = R32, (see Block Diagram) The ● denotes the specifications which apply over the full
operating temperature range, otherwise specifications and typical values are at TA = 25°C. VS = single 5V, EN pin to logic “low,”
RL = 400Ω connected to midsupply, unless otherwise noted.
Specifications are for the output
(OUTA or OUTB) of a single 2nd order section
(A or B) with respect to VGND = VGNDA = VGNDB,
gain = –1, RFIL = R11 = R21 = R31 = R12 = R22 = R32, (see Block Diagram). The ● denotes the specifications which apply over the
full operating temperature range, otherwise specifications and typical values are at TA = 25°C. VS = single 5V, EN pin to logic “low,”
RL = 400Ω connected to midsupply, unless otherwise noted.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
ADC DC Gain ●0.99 1 1.01 V/V
V
OS(OUT)
DC Offset Voltage V
S
= 3V, f
C
= 1MHz, R
FIL
= 1.28k ●–9 –2 5 mV
(V
OUTA
– V
GNDA
) or (V
OUTB
– V
GNDB
)V
S
= 5V, V
S
= ±5V, f
C
= 1MHz, R
FIL
= 1.28k ●–10 –1 10 mV
∆V
OS(OUT)
DC Offset Voltage Mismatch V
S
= 3V, f
C
= 1MHz, R
FIL
= 1.28k ●–8 ±28 mV
(V
OUTA
– V
GNDA
) – (V
OUTB
– V
GNDB
)V
S
= 5V, V
S
= ±5V, f
C
= 1MHz, R
FIL
= 1.28k ●–10 ±210 mV
Transfer Function Characteristics for Each Section (A or B) to Single-Ended Output (OUTA or OUTB)
f
C
Cutoff Frequency Range (Note 7) V
S
= 3V, V
S
= 5V, V
S
= ±5V ●0.2 10 MHz
TC Cutoff Frequency Temperature Coefficient ●±1 ppm/°C
Filter Gain, f
C
= 1MHz, Test Frequency = 300kHz (0.3 • f
C
)●–0.10 0.15 0.40 dB
V
S
= 5V, R
FIL
= 1.28k Test Frequency = 750kHz (0.75 • f
C
)●–1.40 –1.00 –0.65 dB
(Measured with Respect to DC Gain) Test Frequency = 1MHz (1 • f
C
)●–3.50 –3.10 –2.60 dB
Test Frequency = 2MHz (2 • f
C
)●–13.7 –13.0 –12.5 dB
Test Frequency = 4MHz (4 • f
C
) –25.0 dB
FILTER ELECTRICAL CHARACTERISTICS
LT1568
5
1568f
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Filter Gain, f
C
= 10MHz, Test Frequency = 1MHz (0.1 • f
C
)●–0.3 0.15 0.5 dB
V
S
= 5V, R
FIL
= 128ΩTest Frequency = 7.5MHz (0.75 • f
C
)●–1.2 –0.50 0.0 dB
(Measured with Respect to DC Gain) Test Frequency = 10MHz (1 • f
C
)●–3.1 –2.30 –1.5 dB
Test Frequency = 20MHz (2 • f
C
)●–12.2 –11.2 –10.2 dB
Test Frequency = 40MHz (4 • f
C
) –19.1 dB
Filter Gain Mismatch f
C
= 1MHz, f
IN
= f
C
●–0.4 ±0.02 0.4 dB
(V
OUTA
– V
OUTB
)f
C
= 10MHz, f
IN
= f
C
●–0.5 ±0.02 0.5 dB
Wideband Output Noise f
C
= 1MHz, R
FIL
= 1.28k, BW = 2MHz 22 µV
RMS
f
C
= 10MHz, R
FIL
= 128Ω, BW = 20MHz 60 µV
RMS
THD Total Harmonic Distortion f
C
= 1MHz, R
FIL
= 1.28k, –84 dB
f
IN
= 200kHz, V
IN
= 1V
P-P
f
C
= 10MHz, R
FIL
= 128Ω,–75dB
f
IN
= 2MHz, V
IN
= 1V
P-P
Specifications are for the
OUTA or OUTB of a single 2nd order section
(A or B) with respect to VGND = VGNDA = VGNDB, gain = 1,
RFIL = R11 = R21 = R31 = R12 = R22 = R32, (see Block Diagram) The ● denotes the specifications which apply over the full
operating temperature range, otherwise specifications and typcial values are at TA = 25°C. VS = single 5V, EN pin to logic “low,”
RL = 400Ω connected to midsupply, unless otherwise noted.
Specifications are for the
differential output (OUTA – OUTA or OUTB
– OUTB) of a single 2nd order section
(A or B), gain = –2,
RFIL = R11 = R21 = R31 = R12 = R22 = R32. All voltages are with respect to VGND = VGNDA = VGNDB. The ● denotes the specifications
which apply over the full operating temperature range, otherwise specifications and typical values are at TA = 25°C.
VS = single 5V, EN pin to logic “low,” RLDIFF = 800Ω connected at midsupply, unless otherwise noted.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
ADC DC Gain ●–2 V/V
V
OS(OUT)
DC Offset Voltage V
S
= 3V, f
C
= 1MHz, R
FIL
= 1.28k ●–4 6 16 mV
(OUTA – OUTA) or (OUTB – OUTB) V
S
= 5V, f
C
= 1MHz, R
FIL
= 1.28k ●–12 2 15 mV
V
S
= ±5V, f
C
= 1MHz, R
FIL
= 1.28k ●–20 –5 10 mV
∆V
OS(OUT)
DC Offset Voltage Mismatch V
S
= 3V, f
C
= 1MHz, R
FIL
= 1.28k ●–8 2 8 mV
(OUTA – OUTA) – (OUTB – OUTB) V
S
= 5V, f
C
= 1MHz, R
FIL
= 1.28k ●–12 –2 12 mV
V
S
= ±5V, f
C
= 1MHz, R
FIL
= 1.28k ●–15 2 15 mV
Transfer Function Characteristics for Each Section (A or B) to Differential Output (OUTA – OUTA or OUTB – OUTB)
f
C
Cutoff Frequency Range (Note 7) V
S
= 3V, V
S
= 5V, V
S
= ±5V ●0.2 10 MHz
TC Cutoff Frequency Temperature Coefficient ●±1 ppm/°C
Filter Gain, f
C
= 1MHz, Test Frequency = 300kHz (0.3 • f
C
)●–0.05 0.10 0.25 dB
V
S
= 5V, R
FIL
= 1.28k (Note 8) Test Frequency = 750kHz (0.75 • f
C
)●–1.40 –1.10 –0.80 dB
(Measured with Respect to DC Gain) Test Frequency = 1MHz (1 • f
C
)●–3.60 –3.20 –2.70 dB
Test Frequency = 2MHz (2 • f
C
)●–13.7 –13.1 –12.5 dB
Test Frequency = 4MHz (4 • f
C
) –25.0 dB
Filter Gain, f
C
= 10MHz, Test Frequency = 1MHz (0.1 • f
C
)●–0.20 0.1 0.30 dB
V
S
= 5V, R
FL
= 128Ω (Note 8) Test Frequency = 7.5MHz (0.75 • f
C
)●–1.30 –0.8 –0.20 dB
(Measured with Respect to DC Gain) Test Frequency = 10MHz (1 • f
C
)●–3.30 –2.6 –1.90 dB
Test Frequency = 20MHz (2 • f
C
)●–13.1 –12.1 –11.1 dB
Test Frequency = 40MHz (4 • f
C
) –24.3 dB
FILTER ELECTRICAL CHARACTERISTICS
LT1568
6
1568f
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Filter Gain Mismatch f
C
= 1MHz, f
IN
= f
C
●–0.3 ±0.10 0.3 dB
(V
OUTA
– V
OUTA
) – (V
OUTB
– V
OUTB
)f
C
= 10MHz, f
IN
= f
C
●–0.4 ±0.15 0.4 dB
Wideband Output Noise f
C
= 1MHz, R
FIL
= 1.28k, BW = 2MHz 36 µV
RMS
f
C
= 10MHz, R
FIL
= 128Ω, BW = 20MHz 88 µV
RMS
THD Total Harmonic Distortion f
C
= 1MHz, R
FIL
= 1.28k, –84 dB
f
IN
= 200kHz, V
IN
= 1V
P-P
f
C
= 10MHz, R
FIL
= 128Ω,–69dB
f
IN
= 2MHz, V
IN
= 1V
P-P
Note 1: Absolute Maximum Ratings are those values beyond which the life
of the device may be impaired.
Note 2: The inputs of each op amp are protected by back-to-back diodes.
If either differential input voltage exceeds 1.4V, the input current should be
limited to less than 10mA.
Note 3: A heat sink may be required to keep the junction temperature
below the absolute maximum rating when the output is shorted
indefinitely.
Note 4: The inverter bandwidth is measured with the SA or SB output
floating, and is defined as the frequency at which the phase shift from
OUTA (OUTB) to OUTA (OUTB) drops from 180° to 135°.
Note 5: Measured with the SA or SB output connected in the filter
application circuit as shown in the Block Diagram.
Note 6: The common mode input voltage range is measured by shorting
the filter input to the common mode reference (GNDA or GNDB) and
applying a DC input voltage to search for the common mode voltage range
that creates a ±2mV (V
S
= 3V) or ±5mV (V
S
= ±5V) change in the (OUTA
or OUTB) voltage (measured with respect to GNDA or GNDB).
Note 7: The minimum cutoff frequency of the LT1568 is arbitrarily listed
as 200kHz. The limit is arrived at by setting the maximum resistor value
limit at 6.4k. Due to input bias current, the output DC offset through a
single section can be as high as 25mV with resistors this large. The
LT1568 can be used with even larger resistors if the large offset voltages
can be tolerated. For cutoff frequencies below 200kHz, refer to the
LTC1563-2, LTC1563-3.
Note 8: With equal-sized resistors, the differential DC gain through either a
single section or cascaded sections is 6dB.
Note 9: The LT1568C is guaranteed to meet specified performance from
0°C to 70°C. The LT1568C is designed, characterized and expected to
meet specified performance from –40°C to 85°C but is not tested or QA
sampled at these temperatures. The LT1568I is guaranteed to meet
specified performance from –40°C to 85°C.
Specifications are for the
differential output (OUTA – OUTA or OUTB
– OUTB) of a single 2nd order section
(A or B), gain = –2,
RFIL = R11 = R21 = R31 = R12 = R22 = R32. All voltages are with respect to VGND = VGNDA = VGNDB. The ● denotes the specifications
which apply over the full operating temperature range, otherwise specifications and typical values are at TA = 25°C.
VS = single 5V, EN pin to logic “low,” RLDIFF = 800Ω connected to midsupply, unless otherwise noted.
FILTER ELECTRICAL CHARACTERISTICS
LT1568
7
1568f
TYPICAL PERFOR A CE CHARACTERISTICS
UW
Supply Current vs Temperature Crosstalk vs Frequency
fCUTOFF = 1MHz Crosstalk vs Frequency
fCUTOFF = 10MHz
TEMPERATURE (°C)
–40
15
ICC (mA)
20
25
30
35
40
–25 025
1568 G01
50 75 85
VS = ±5V
VS = 5V
VS = 3V
FREQUENCY (Hz)
–110
CROSSTALK (dB)
–100
–95
–85
–80
1k 100k 1M 10M
1568 G02
–120 10k
–90
–105
–115
OUTA, OUTB
OUTA, OUTB
V
IN
= 2V
P-P
V
S
= 5V
FREQUENCY (Hz)
–90
CROSSTALK (dB)
–70
–50
–100
–80
–60
10k 1M 10M 100M
1568 G03
–110 100k
OUTA, OUTB
OUTA, OUTB
VIN = 2VP-P
VS = 5V
Distortion vs Frequency
VS = ±5V, fCUTOFF = 5MHz
FREQUENCY (Hz)
200k
–90
DISTORTION (dB)
–85
–80
–75
–70
2ND
–60
1M 5M
1568 G04
–65
R
L
= 400Ω
V
IN
= 2V
P-P
3RD
Distortion vs Frequency
VS = ±5V, fCUTOFF = 10MHz
FREQUENCY (Hz)
500k 1M
–90
DISTORTION (dB)
–65
–60
–55
–50
–85
–80
–75
–70 2ND
3RD
10M
1568 G05
–45 R
L
= 400Ω
V
IN
= 1V
P-P
Distortion vs Output Voltage Swing
VS = ±5V, fCUTOFF = 5MHz
OUTA (V
P-P
)
0
DISTORTION (dB)
–60
–50
–40
8
1568 G06
–70
–80
–65
–55
–45
–75
–85
–90 246
19
35710 11
2ND
3RD
R
L
= 400Ω
f
IN
= 2.5MHz
Distortion vs Output Voltage Swing
VS = 5V, fCUTOFF = 5MHz
OUTA (VP-P)
0
–90
DISTORTION (dB)
–80
–70
–60
1234
1568 G07
5
–50
–40
–85
–75
–65
2ND
3RD
–55
–45
6
RL = 400Ω
fIN = 2.5MHz
Distortion vs Output Voltage Swing
VS = 3V, fCUTOFF = 5MHz
OUTA (V
P-P
)
0
–90
DISTORTION (dB)
–80
–70
–60
2ND
3RD
–50
–40
–30
1234
1568 G08
R
L
= 400Ω
f
IN
= 2.5MHz
Power Supply Rejection
vs Frequency
FREQUENCY (Hz)
20
POWER SUPPLY REJECTION (dB)
40
60
70
10k 1M 10M 100M
1568 G09
0100k
50
30
10
OUTA, OUTB
OUTA, OUTB
LT1568
8
1568f
UU
U
PI FU CTIO S
V
+
(Pins 1, 16): The V
+
positive supply voltage pins should
be tied together and bypassed with a 0.1µF capacitor to an
adequate analog ground plane using the shortest possible
wiring.
INVA, INVB (Pins 2, 15): Inverting Input. Each of the INV
pins is an inverting input of an op amp. Note that the INV
pins are high impedance, and are susceptible to coupling
of unintended signals. External parasitic capacitance on
the INV nodes will also affect the frequency response of
the filter sections. For these reasons, printed circuit con-
nections to the INV pins must be kept as short as possible.
SA, SB (Pins 3, 14): Summing Pins. These pins are a
summing junction for input signals. Stray capacitance on
the SA or SB pins may cause “small” frequency errors of
the frequency response near the cutoff frequency (or
center frequency). The three external resistors for each
section should be located as close as possible to the SA or
SB pin to minimize stray capacitance (one picofarad of
stray capacitance may add up to 0.1% frequency error).
OUTA, OUTB (Pins 4, 13): Lowpass Output. These pins
are the rail-to-rail outputs of op amps. Each output is
designed to drive a nominal net load of 400Ω and 30pF.
OUTA, OUTB (Pins 5, 12): These pins are the inverted
versions of the OUTA and OUTB outputs respectively. Each
output is designed to drive a nominal load of 400Ω and
30pF.
GNDA (Pin 6): GNDA serves as the common mode refer-
ence voltage for section A. It should be tied to the analog
ground plane in a dual supply system. In a single-supply
system, an internal resistor divider can be used to estab-
lish a half-supply reference point. In that case, GNDA must
be bypassed to V
–
(Pins 8, 9) by a 0.1µF capacitor.
NC (Pin 7): This pin is not connected internally and can be
connected to ground.
V
–
(Pins 8, 9): The V
–
negative supply voltage pins should
be tied together and bypassed to GND by a 0.1µF capacitor
in a dual-supply system. In a single-supply system, tie
these pins to the ground plane.
EN (Pin 10): ENABLE. When the EN input goes high or is
open circuited, the LT1568 enters a shutdown state which
reduces the supply current to approximately 0.5mA
(VS = 5V). The OUTA, OUTB, OUTA and OUTB pins
assume high impedance states. GNDA will continue to be
biased at half-supply. If an input signal is applied to a
complete filter circuit while the LT1568 is in shutdown,
some signal will normally flow to the output through
passive components around the inactive IC.
EN is connected to V
+
through an internal pull-up resistor
of approximately 40k. This defaults the LT1568 to the
shutdown state if the EN pin is left floating. Therefore, the
user must connect the EN pin to a voltage equal to or less
than (V
+
– 2.1)V to enable the part for normal operation.
(For example, if V
+
is 5V, then to enable the part the EN pin
voltage should be 2.9V or less.)
GNDB (Pin 11): GNDB serves as the common mode
reference voltage for section B. It should be tied to the
analog ground plane in a dual supply system. In a single-
supply system, GNDB can be tied to GNDA to set the
common mode voltage at half-supply. If it is tied to
another reference voltage, GNDB should be bypassed to
V– (Pins 8, 9) by a 0.1µF capacitor.
LT1568
9
1568f
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
V
+
INVA
SA
OUTA
OUTA
GNDA
NC
V
–
V
+
INVB
SB
OUTB
OUTB
GNDB
EN
V
–
LT1568
V
+
V
–
0.1µF
0.1µF
SINGLE POINT
SYSTEM GROUND
1568 PF01
ANALOG
GROUND
PLANE
DIGITAL GROUND PLANE
(IF ANY)
Dual Supply Power and Ground Connections Single Supply Power and Ground Connections
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
V
+
INVA
SA
OUTA
OUTA
GNDA
NC
V
–
V
+
INVB
SB
OUTB
OUTB
GNDB
EN
V
–
LT1568
V
+
0.1µF
0.1µF
SINGLE POINT
SYSTEM GROUND
1568 PF02
ANALOG
GROUND
PLANE
DIGITAL GROUND PLANE
(IF ANY)
UU
U
PI FU CTIO S
–
+
–
+
C1B
INA INB
OUTA
OUTA
V
–
+
–
C1A
C2A C2B
A1A
–1
INVB
OUTB
OUTB
B-SIDE
DIFFERENTIAL
OUTPUTS
TYPICAL CAPACITOR VALUES:
C1 = 105.7pF ±0.75%
C2 = 141.3pF ±0.75%
A-SIDE
DIFFERENTIAL
OUTPUTS
OUTB
+
–
R22
1.27k
R32
1.27k R12
1.27k
V
+
SB
GNDB
EN
1568BD
V
–
INVA
C
BP1
0.1µF
V
+
SA
GNDA
NC
V
–
A1B
–1
V
+
V
–
5k
5k
0.1µF
R31
1.27k
V
+
R11
1.27k
R21
1.27k
16
15
14
13
12
11
10
98
7
6
5
4
3
2
1
OUTB
OUTA
OUTA
BLOCK DIAGRA A D TEST CIRCUIT
WU
LT1568
10
1568f
APPLICATIO S I FOR ATIO
WUUU
The LT1568 has been designed to make the implementa-
tion of high frequency filtering functions very easy. Inter-
nal low noise amplifiers and capacitors are configured in
a topology that requires only three external resistors to
implement a 2nd order filter stage. The two 2nd order
stages can be used independently or cascaded for simple
4th order filter functions. With two stages integrated on
the same die, the matching of the independent sections is
better than what can be achieved with separate amplifier
components.
OPERATING WITH SINGLE OR DUAL SUPPLIES
Figure 1 shows the recommended connection of an analog
ground plane with the LT1568 biased from either symmetri-
cal dual (±V) power supplies or a single supply. Connec-
tion of the two GND pins is important to properly DC bias
the internal amplifiers. The use of a ground plane helps to
minimize noise and stray components to preserve signal
integrity and maintain frequency response accuracy.
When biasing from a dual supply, it is recommended that
a Schottky diode clamp (BAT54S) be added as shown.
These diodes ensure that improper supply voltages,
through either reverse polarity or power-up sequencing,
do not damage the LT1568.
SIMPLE FILTER IMPLEMENTATIONS
The basic 2nd order filter block of the LT1568, with three
external resistors connected as shown in the Block Dia-
gram, has the following lowpass transfer function:
e
e
DC f
sf
Qsf
OUT
IN
GAIN O
OO
=π
()
+π+π
()
–•2
22
2
22
where e
OUT
is either OUTA or OUTB,
DC R
RfRRCC
GAIN O
==
π
2
1
1
22312
,•••
and
QCCRRR f
CRRR RR CRR
O
=π+
()
+
[]
212123
1123 23 212
••••••
•• •–••
The typical values of the internal capacitors are:
C1= 105.7pF
C2 = 141.3pF
These filter functions assume ideal amplifiers.
Dual Supply Power and Ground Connections Single Supply Power and Ground Connections
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
V
+
INVA
SA
OUTA
OUTA
GNDA
NC
V
–
V
+
INVB
SB
OUTB
OUTB
GNDB
EN
V
–
LT1568
BAT54S
V
+
V
–
0.1µF
0.1µF
SINGLE POINT
SYSTEM GROUND
1568 F01a
ANALOG
GROUND
PLANE
DIGITAL GROUND PLANE
(IF ANY)
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
V
+
INVA
SA
OUTA
OUTA
GNDA
NC
V
–
V
+
INVB
SB
OUTB
OUTB
GNDB
EN
V
–
LT1568
V
+
0.1µF
0.1µF
SINGLE POINT
SYSTEM GROUND
1568 F01b
ANALOG
GROUND
PLANE
DIGITAL GROUND PLANE
(IF ANY)
Figure 1. Dual and Single Supply and Ground Plane Connections
LT1568
11
1568f
The following filter examples are provided to make it easy
to design a variety of filter stages. Both 2nd and 4th order
filters are shown. For each filer, a table of external resistor
values (standard 1% tolerance) is provided. These resistor
values have been adjusted to compensate for the finite
gain bandwidth product of the LT1568 amplifiers.
To implement a filter, simply connect the resistor values
shown in the table for the cutoff frequency desired. If the
desired cutoff frequency is not shown in the table of
values, use interpolation as recommended in the next
section.
DESIGNING FOR ANY CUTOFF FREQUENCY
To implement a lowpass filter with a cutoff frequency not
included in the design table, resistor values can be inter-
polated in the following manner:
For a Cutoff Frequency, f
C
, Less Than 1MHz
Start with the resistor values for f
C
= 1MHz and then scale
them up by the ratio of (1MHz/f
C
).
APPLICATIO S I FOR ATIO
WUUU
Example: Implement a 2nd order lowpass Chebyshev filter
with an f
C
of 256kHz. From Table 2 the values for f
C
of
1MHz are R11 = R21 = 976Ω and R31 825Ω.
Scaling for f
C
= 256kHz:
R11 = R21 = 976Ω • (1MHz/256kHz) ≈ 3.83k
R31 = 825Ω • (1MHz/256kHz) ≈ 3.24k
For a Cutoff Frequency, f
C
, Between Values Given in a
Design Table
Start with the resistor values for the cutoff frequency
closest to the desired one and scale the values up or down
accordingly.
Example: Implement a 2nd order lowpass Chebyshev filter
with an f
C
of 3.2MHz. From Table 2 the closest values are
for f
C
of 3MHz and are R11 = R21 = 316Ω and R31 = 274Ω.
Scaling for f
C
= 3.2MHz:
R11 = R21 = 316Ω • (3MHz/3.2MHz) ≈ 294Ω
R31 = 274Ω • (3MHz/3.2MHz) ≈ 255Ω
LT1568
12
1568f
Table 1. Resistor Values in Ohms, Dual 2nd Order
Butterworth, Gain = 1, R12 = R11, R22 = R21, R32 = R31
f
CUTOFF
(MHz) R11 = R21 = R31
0.2 6340Ω
0.5 2550Ω
1 1270Ω
2 634Ω
3 422Ω
4 324Ω
5 255Ω
6 210Ω
7 182Ω
8 162Ω
9 143Ω
10 127Ω
FREQUENCY (MHz)
0.1
–50
GAIN (dB)
–40
–30
–20
–10
11020
1568 TA07
–60
–70
–80
–90
0
10
Amplitude Response
2nd Order Butterworth, fCUTOFF = 1MHz Transient Response
2nd Order Butterworth, fCUTOFF = 1MHz
INPUT
500mV/DIV
OUTPUT
200mV/DIV
1µs/DIV 1568 TA08
Dual 2nd Order Lowpass Filter,
Dual Supply Operation
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
V
+
INVA
SA
OUTA
OUTA
GNDA
NC
V
–
V
+
INVB
SB
OUTB
OUTB
GNDB
EN
V
–
0.1µF
BAT54S
V
IN1
V
OUTA
V
OUTA
V
IN2
V
OUTB
V
OUTB
R11 R31
R22
R32 R12
R21
0.1µF
–5V
1568 TA03
5V
LT1568
Dual 2nd Order Lowpass Filter,
Single Supply Operation
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
V
+
INVA
SA
OUTA
OUTA
GNDA
NC
V
–
V
+
INVB
SB
OUTB
OUTB
GNDB
EN
V
–
0.1µF
V
IN1
V
OUTA
V
OUTA
V
IN2
V
OUTB
V
OUTB
R11 R31
R22
R32 R12
R21
0.1µF
1568 TA04
2.7V ≤ V
+
≤ 10V
LT1568
R
ff
C CUTOFF
11= R21= R31=R = 128 • 10MHz
fC
Ω
=
DUAL 2nd ORDER LOWPASS FILTER DESIG S
U
LT1568
13
1568f
Table 2. Resistor Values in Ohms, Dual 2nd Order Lowpass
Chebyshev, ±0.25dB Passband Ripple, Gain = 1, R11 = R12,
R21 = R22, R31 = R32
f
CUTOFF
(MHz) R11, R21 R31
1 976Ω825Ω
2 475Ω412Ω
3 316Ω274Ω
4 226Ω205Ω
5 178Ω165Ω
6 143Ω137Ω
7 121Ω118Ω
FREQUENCY (MHz)
0.1
–50
GAIN (dB)
–40
–30
–20
–10
11020
1568 TA09
–60
–70
–80
–90
0
10
Amplitude Response 2nd Order Lowpass Chebyshev,
±0.25dB Passband Ripple, fCUTOFF = 1MHz Transient Response 2nd Order Lowpass Chebyshev,
±0.25dB Passband Ripple, fCUTOFF = 1MHz
INPUT
500mV/DIV
OUTPUT
200mV/DIV
1µs/DIV 1568 TA10
DUAL 2nd ORDER LOWPASS FILTER DESIG S
U
Table 3. Resistor Values in Ohms, Dual 2nd Order Lowpass
Bessel, Gain = 1
f
CUTOFF
(MHz) R11, R21 R31
1 866Ω1180Ω
2 422Ω590Ω
3 280Ω383Ω
4 210Ω287Ω
5 165Ω232Ω
6 137Ω191Ω
7 115Ω162Ω
Amplitude Response
2nd Order Lowpass Bessel, fCUTOFF = 1MHz
FREQUENCY (MHz)
0.1
–50
GAIN (dB)
–40
–30
–20
–10
11020
1568 TA11
–60
–70
–80
–90
0
10
Transient Response
2nd Order Lowpass Bessel, fCUTOFF = 1MHz
INPUT
500mV/DIV
OUTPUT
200mV/DIV
1µs/DIV 1568 TA12
LT1568
14
1568f
Table 4. Resistor Values in Ohms, 4th Order Lowpass
Butterworth, Gain = 1
f
CUTOFF
(MHz) R11, R21 R31 R12, R22 R32
1 1.05k 1.58k 1.82k 887Ω
2 523Ω787Ω909Ω432Ω
3 348Ω523Ω590Ω294Ω
4 255Ω383Ω432Ω215Ω
5 205Ω309Ω348Ω174Ω
6 169Ω255Ω280Ω143Ω
7 143Ω221Ω232Ω124Ω
8 124Ω196Ω196Ω107Ω
9 107Ω174Ω169Ω97.6Ω
10 97.6Ω158Ω143Ω88.7Ω
4th Order Lowpass Filter,
Dual Supply Operation
4th Order Lowpass Filter,
Single Supply Operation
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
V
+
INVA
SA
OUTA
OUTA
GNDA
NC
V
–
V
+
INVB
SB
OUTB
OUTB
GNDB
EN
V
–
0.1µF
V
IN
V
OUT
V
OUT
R11 R31
R22
R32 R12
R21
0.1µF
–5V
1568 TA05
5V
LT1568
BAT54S
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
V+
INVA
SA
OUTA
OUTA
GNDA
NC
V–
V+
INVB
SB
OUTB
OUTB
GNDB
EN
V–
0.1µF
VIN
VOUT
VOUT
R11 R31
R22
R32 R12
R21
0.1µF
1568 TA06
V+
LT1568
Amplitude Response
4th Order Lowpass Butterworth Lowpass, fCUTOFF = 1MHz Transient Response
4th Order Lowpass Butterworth Lowpass, fCUTOFF = 1MHz
INPUT
500mV/DIV
OUTPUT
200mV/DIV
1µs/DIV 1568 TA14
FREQUENCY (MHz)
0.1
–60
GAIN (dB)
–48
–36
–24
–12
11020
1568 TA13
–72
–84
–96
–108
0
12
4th ORDER LOWPASS FILTER DESIG S
U
LT1568
15
1568f
Table 5. Resistor Values in Ohms, 4th Order Lowpass
Chebyshev, ±0.25dB Passband Ripple, Gain = 1
f
CUTOFF
(MHz) R11, R21 R31 R12, R22 R32
1 1.87k 2.05k 2.21k 634Ω
2 931Ω1.05k 1.10k 324Ω
3 604Ω681Ω698Ω205Ω
4 453Ω511Ω499Ω154Ω
5 357Ω402Ω383Ω121Ω
6 287Ω332Ω309Ω100Ω
7 243Ω287Ω255Ω86.6Ω
8 205Ω249Ω215Ω76.8Ω
9 178Ω221Ω182Ω66.5Ω
10 154Ω196Ω158Ω61.9Ω
Amplitude Response 4th Order Lowpass Chebyshev,
±0.25dB Passband Ripple, fCUTOFF = 1MHz
FREQUENCY (MHz)
0.1
–60
GAIN (dB)
–48
–36
–24
–12
11020
1568 TA13
–72
–84
–96
–108
0
12
4th ORDER LOWPASS FILTER DESIG S
U
Transient Response 4th Order Lowpass Chebyshev,
±0.25dB Passband Ripple, fCUTOFF = 1MHz
INPUT
500mV/DIV
OUTPUT
200mV/DIV
1µs/DIV 1568 TA16
Table 6. Resistor Values in Ohms, 4th Order Lowpass Bessel,
Gain = 1
f
CUTOFF
(MHz) R11, R21 R31 R12, R22 R32
1 715Ω1.15k 1.91k 324Ω
2 357Ω562Ω432Ω365Ω
3 237Ω374Ω280Ω243Ω
4 174Ω280Ω205Ω187Ω
5 137Ω221Ω162Ω147Ω
6 115Ω187Ω130Ω124Ω
Amplitude Response
4th Order Lowpass Bessel, fCUTOFF = 1MHz
FREQUENCY (MHz)
0.1
–60
GAIN (dB)
–48
–36
–24
–12
11020
1568 TA17
–72
–84
–96
–108
0
12
Transient Response
4th Order Lowpass Bessel, fCUTOFF = 1MHz
INPUT
500mV/DIV
OUTPUT
200mV/DIV
1µs/DIV 1568 TA18
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
LT1568
16
1568f
LINEAR TECHNOLOGY CO RPORATION 2003
LT/TP 0403 2K • PRINTED IN USA
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U
PACKAGE DESCRIPTIO
GN Package
16-Lead Plastic SSOP (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1641)
GN16 (SSOP) 0502
.016 – .050
(0.406 – 1.270)
.015 ± .004
(0.38 ± 0.10) × 45°
0° – 8° TYP
.007 – .0098
(0.178 – 0.249)
.053 – .068
(1.351 – 1.727)
.008 – .012
(0.203 – 0.305)
.004 – .0098
(0.102 – 0.249)
.0250
(0.635)
BSC
12
345678
.229 – .244
(5.817 – 6.198)
.150 – .157**
(3.810 – 3.988)
16 15 14 13
.189 – .196*
(4.801 – 4.978)
12 11 10 9
.009
(0.229)
REF
.254 MIN
RECOMMENDED SOLDER PAD LAYOUT
.150 – .165
.0250 TYP.0165 ±.0015
.045 ±.005
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
INCHES
(MILLIMETERS)
NOTE:
1. CONTROLLING DIMENSION: INCHES
2. DIMENSIONS ARE IN
3. DRAWING NOT TO SCALE
TYPICAL APPLICATIO S
U
V
+
INVA
SA
OUTA
OUTA
GNDA
NC
V
–
V
+
INVB
SB
OUTB
OUTB
GNDB
EN
V
–
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
LT1568
R11 93.1Ω
C
IN1
39pF
5%
C
IN2
39pF
5%
0.1µF
0.1µF
R12 93.1Ω
R22 113Ω
1568 TA19
V
IN
5V
V
OUT
V
OUT
R21 113Ω
4th Order Bandpass Filter
fCENTER = 10MHz, –3dB Passband = fCENTER/5.4 Amplitude Response
4th Order Bandpass Filter fCENTER = 10MHz
FREQUENCY (MHz)
1
–30
GAIN (dB)
–24
–18
–12
–6
10 40
1568 TA20
–36
–42
–48
–54
0
6PIN 13 OUTPUT
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507
●
www.linear.com
