REV. B
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. No license is granted by implication or otherwise
under any patent or patent rights of Analog Devices.
a
LC
2
MOS High Speed, P Compatible
8-Bit ADC with Track/Hold Function
AD7821
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700 www.analog.com
Fax: 781/326-8703 © Analog Devices, Inc., 2002
FUNCTIONAL BLOCK DIAGRAM
FEATURES
Fast Conversion Time: 660 ns Max
100 kHz Track-and-Hold Function
1 MHz Sample Rate
Unipolar and Bipolar Input Ranges
Ratiometric Reference Inputs
No External Clock
Extended Temperature Range Operation
Skinny 20-Lead DlPs, SOIC, and 20-Terminal
Surface-Mount Packages
GENERAL DESCRIPTION
The AD7821 is a high speed, 8-bit, sampling, analog-to-digital
converter that offers improved performance over the popular
AD7820. It offers a conversion time of 660 ns (versus 1.36 µs
for the AD7820) and 100 kHz signal bandwidth (versus 6.4
kHz). The sampling instant is better defined and occurs on the
falling edge of WR or RD. The provision of a V
SS
pin (Pin 19)
allows the part to operate from ±5 V supplies and to digitize
bipolar input signals. Alternatively, for unipolar inputs, the V
SS
pin
can be grounded and the AD7821 will operate from a single +5 V
supply, like the AD7820.
The AD7821 has a built-in track-and-hold function capable of
digitizing full-scale signals up to 100 kHz max. It also uses a
half-flash conversion technique that eliminates the need to gen-
erate a CLK signal for the ADC.
The AD7821 is designed with standard microprocessor control
signals (CS, RD, WR, RDY, INT) and latched, three-state data
outputs capable of interfacing to high speed data buses. An
overflow output (OFL) is also provided for cascading devices to
achieve higher resolution.
The AD7821 is fabricated in Linear Compatible CMOS
(LC
2
MOS), an advanced, mixed technology process combining
precision bipolar circuits with low power CMOS logic. The part
features a low power dissipation of 50 mW.
PRODUCT HIGHLIGHTS
1. Fast Conversion Time
The half-flash conversion technique, coupled with fabrication
on Analog Devices’ LC
2
MOS process, enables a very fast con-
version time. The conversion time for the WR-RD mode is
660 ns, with 700 ns for the RD mode.
2. Built-In Track-and-Hold
This allows input signals with slew rates up to 1.6 V/µs to be
converted to 8 bits without an external track-and-hold. This
corresponds to a 5 V peak-to-peak, 100 kHz sine wave signal.
3. Total Unadjusted Error
The AD7821 features an excellent total unadjusted error figure
of less than ±1 LSB over the full operating temperature range.
4. Unipolar/Bipolar Input Ranges
The AD7821 is specified for single-supply (+5 V) operation
with a unipolar full-scale range of 0 to +5 V, and for dual-supply
(±5 V) operation with a bipolar input range of ±2.5 V. Typical
performance characteristics are given for other input ranges.
5. Dynamic Specifications for DSP Users
In addition to the traditional ADC specifications, the
AD7821 is specified for ac parameters, including signal-to-
noise ratio, distortion, and slew rate.
REV. B
AD7821–SPECIFICATIONS
VDD = +5 V ⴞ 5%, GND = 0 V. Unipolar Input Range: VSS = GND, VREF(+) = 5 V,
VREF(–) = GND. Bipolar Input Range: VSS = –5 V ⴞ 5%, VREF(+) = 2.5 V,
VREF(–) = –2.5 V. These test conditions apply unless otherwise stated. All specifications TMIN to TMAX unless otherwise noted. Specifications
apply for RD Mode (Pin 7 = 0 V).
Parameter K Version
1
B, T Versions Unit Comments
UNIPOLAR INPUT RANGE
Resolution
2
88Bits
Total Unadjusted Error
3
±1±1LSB max
Minimum Resolution for which
No Missing Codes are Guaranteed 8 8 Bits
BIPOLAR INPUT RANGE
Resolution
2
88Bits
Zero Code Error ±1±1LSB max
Full Scale Error ±1±1LSB max
Signal-to-Noise Ratio (SNR)
3
45 45 dB min V
IN
= 99.85 kHz Full-Scale Sine Wave with f
SAMPLING
= 500 kHz
Total Harmonic Distortion (THD)
3
–50 –50 dB max V
IN
= 99.85 kHz Full-Scale Sine Wave with f
SAMPLING
= 500 kHz
Peak Harmonic or Spurious Noise
3
–50 –50 dB max V
IN
= 99.85 kHz Full-Scale Sine Wave with f
SAMPLING
= 500 kHz
Intermodulation Distortion (IMD)
3
fa (84.72 kHz) and fb (94.97 kHz) Full-Scale Sine Waves
with f
SAMPLING
= 500 kHz
–50 –50 dB max Second Order Terms
–50 –50 dB max Third Order Terms
Slew Rate, Tracking
3
1.6 1.6 V/µs max
2.36 2.36 V/µs typ
REFERENCE INPUT
Input Resistance 1.0/4.0 1.0/4.0 kΩ min/kΩ max
V
REF
(+) Input Voltage Range V
REF
(–)/V
DD
V
REF
(–)/V
DD
V min/V max
V
REF
(–) Input Voltage Range V
SS
/V
REF
(+) V
SS
/V
REF
(+) V min/V max
ANALOG INPUT
Input Voltage Range V
REF
(–)/V
REF
(+) V
REF
(–)/V
REF
(+) V min/ max
Input Leakage Current ±3±3µA max –5 V ≤ V
IN
≤ +5 V
Input Capacitance 55 55 pF typ
LOGIC INPUTS
CS, WR, RD
V
INH
2.4 2.4 V min
V
INL
0.8 0.8 V max
I
INH
(CS, RD)11µA max
I
INH
(WR)33µA max
I
INL
–1 –1 µA max
Input Capacitance
4
88pF max Typically 5 pF
MODE
V
INH
3.5 3.5 V min
V
INL
1.5 1.5 V max
I
INH
200 200 µA max 50 µA typ
I
INL
–1 –1 µA max
Input Capacitance
4
88pF max Typically 5 pF
LOGIC OUTPUTS
DB0–DB7, OFL, INT
V
OH
4.0 4.0 V min I
SOURCE
= 360 µA
V
OL
0.4 0.4 V max I
SINK
= 1.6 mA
I
OUT
(DB0–DB7) ±3±3µA max Floating State Leakage
Output Capacitance
4
(DB0–DB7) 8 8 pF max Typically 5 pF
RDY
V
OL
0.4 0.4 V max I
SINK
= 2.6 mA
I
OUT
±3±3µA max Floating State Leakage
Output Capacitance
4
88pF max Typically 5 pF
POWER SUPPLY
I
DD5
20 20 mA max CS = RD = 0 V
I
SS
100 100 µA max CS = RD = 0 V
Power Dissipation 50 50 mW typ
Power Supply Sensitivity ±1/4 ±1/4 LSB max ±1/16 LSB typ, V
DD
= 4.75 V to 5.25 V,
(V
REF
(+) = 4.75 V max for Unipolar Mode)
NOTES
1
Temperature Ranges are as follows: K Version = –40°C to +85°C; B Version = –40°C to +85°C; T Version = –55°C to +125°C.
2
1 LSB = 19.53 mV for both the unipolar (0 V to +5 V) and bipolar (–2.5 V to +2.5 V) input ranges.
3
See Terminology.
4
Sample tested at +25°C to ensure compliance.
5
See Typical Performance Characteristics.
Specifications subject to change without notice.
–2–
AD7821
REV. B –3–
TIMING CHARACTERISTICS
1
(VDD = +5 V ⴞ 5%, VSS = 0 V or –5 V ⴞ 5%; Unipolar or Bipolar Input Range)
Limit at Limit at
Limit at +25ⴗCT
MIN
, T
MAX
T
MIN
, T
MAX
Parameter (All Versions) (K, B Versions) (T Version) Unit Conditions/Comments
t
CSS
000ns min CS to RD/WR Setup Time
t
CSH
000ns min CS to RD/WR Hold Time
t
RDY2
70 85 100 ns max CS to RDY Delay. Pull-Up
Resistor 5 kΩ
t
CRD
700 875 975 ns max Conversion Time (RD Mode)
t
ACC03
Data Access Time (RD Mode)
t
CRD
+ 25 t
CRD
+ 30 t
CRD
+ 35 ns max C
L
= 20 pF
t
CRD
+ 50 t
CRD
+ 65 t
CRD
+ 75 ns max C
L
= 100 pF
t
INTH2
50 – – ns typ RD to INT Delay (RD Mode)
80 85 90 ns max
t
DH4
15 15 15 ns min Data Hold Time
60 70 80 ns max
t
P
350 425 500 ns min Delay Time Between Conversions
t
WR
250 325 400 ns min Write Pulsewidth
10 10 10 µs max
t
RD
250 350 450 ns min Delay Time between WR and RD Pulses
t
READ1
160 205 240 ns min RD Pulsewidth (WR-RD Mode, see Figure 12b)
Determined by t
ACC1
t
ACC13
Data Access Time (WR-RD Mode, see Figure 12b)
160 205 240 ns max C
L
= 20 pF
185 235 275 ns max C
L
= 100 pF
t
RI
150 185 220 ns max RD to INT Delay
t
INTL2
380 – – ns typ WR to INT Delay
500 610 700 ns max
t
READ2
65 75 85 ns min RD Pulsewidth (WR-RD Mode, see Figure 12a)
Determined by t
ACC2
Data Access Time (WR-RD Mode, see Figure 12a)
t
ACC23
65 75 85 ns max C
L
= 20 pF
90 110 130 ns max C
L
= 100 pF
t
IHWR2
80 100 120 ns max WR to INT Delay (Stand-Alone Operation)
t
ID3
Data Access Time after INT
(Stand-Alone Operation)
30 35 40 ns max C
L
= 20 pF
45 60 70 ns max C
L
= 100 pF
NOTES
1
Sample tested at +25°C to ensure compliance. All input control signals are specified with t
RISE
= t
FALL
= 5 ns (10% to 90% of +5 V) and timed from a voltage level of 1.6 V.
2
C
L
= 50 pF.
3
Measured with load circuits of Figure 1 and defined as the time required for an output to cross 0.8 V or 2.4 V.
4
Defined as the time required for the data lines to change 0.5 V when loaded with the circuits of Figure 2.
Specifications subject to change without notice.
Test Circuits
a. High Z to V
OH
b. High Z to V
OL
Figure 1. Load Circuits for Data Access Time Test
a. V
OH
to High Z b. V
OL
to High Z
Figure 2. Load Circuits for Data Hold Time Test
ORDERING GUIDE
Total
Temperature Unadjusted Package
Model
1
Range Error (LSB) Option
2
AD7821KN –40°C to +85°C±1 max N-20
AD7821KP –40°C to +85°C±1 max P-20A
AD7821KR –40°C to +85°C±1 max RW-20
AD7821BQ –40°C to +85°C±1 max Q-20
AD7821TQ –55°C to +125°C±1 max Q-20
AD7821TE –55°C to +125°C±1 max E-20A
NOTES
1
To order MIL-STD-883, Class B processed parts, add /883B to part
number. Contact local sales office for military data sheet.
2
E = Leadless Ceramic Chip Carrier; N = Plastic DIP; P = Plastic Leaded
Chip Carrier; Q = Cerdip; R = SOIC.
AD7821
REV. B
–4–
ABSOLUTE MAXIMUM RATINGS*
V
DD
to GND . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V, + 7 V
V
SS
to GND . . . . . . . . . . . . . . . . . . . . . . . . . . . . +0.3 V, + 7 V
Digital Input Voltage to GND
(Pins 6–8, 13) . . . . . . . . . . . . . . . . . . . –0.3 V, V
DD
+ 0.3 V
Digital Output Voltage to GND
(Pins 2–5, 9, 14–18) . . . . . . . . . . . . . . . –0.3 V, V
DD
+ 0.3 V
V
REF
(+) to GND . . . . . . . . . . . . . . . V
SS
– 0.3 V, V
DD
+ 0.3 V
V
REF
(–) to GND . . . . . . . . . . . . . . . V
SS
– 0.3 V, V
DD
+ 0.3 V
V
IN
to GND . . . . . . . . . . . . . . . . . . . V
SS
– 0.3 V, V
DD
+ 0.3 V
Operating Temperature Range
Commercial (K Version) . . . . . . . . . . . . . . –40°C to +85°C
Industrial (B Version) . . . . . . . . . . . . . . . . –40°C to +85°C
Extended (T Version) . . . . . . . . . . . . . . . –55°C to +125°C
Storage Temperature Range . . . . . . . . . . . . –65°C to +150°C
Lead Temperature (Soldering, 10 sec) . . . . . . . . . . . . +300°C
Power Dissipation (Any Package) to +75°C . . . . . . . 450 mW
Derates above +75°C by . . . . . . . . . . . . . . . . . . . . . 6 mW/°C
*Stresses above those listed under “Absolute Maximum Ratings” may cause
permanent damage to the device. This is a stress rating only and functional
operation of the device at these or any other conditions above those indicated in the
operational sections of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect device reliability.
PIN CONFIGURATIONS
DIP AND SOIC LCCC PLCC
WARNING!
ESD SENSITIVE DEVICE
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 the AD7821 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.
PIN FUNCTION DESCRIPTIONS
Pin Mnemonic Description
1V
IN
Analog Input: Range V
REF
(–) ≤ V
IN
≤ V
REF
(+)
2DB0 Three-State Data Output (LSB)
3–5 DB1–DB3 Three-State Data Outputs
6WR/RDY WRITE control input/READY status output. See Digital Interface section.
7MODE Mode Selection Input. It determines whether the device operates in the WR-RD or RD mode. This input is internally
pulled low through a 50 µA current source. See Digital Interface section.
8RD READ Input. RD must be low to access data from the part. See Digital Interface section.
9INT INTERRUPT Output. INT going low indicates that the conversion is complete. INT returns high on the rising
edge of CS or RD. See Digital Interface section.
10 GND Ground
11 V
REF
(–) Lower limit of reference span.
Range: V
SS
≤ V
REF
(–) ≤ V
REF
(+).
12 V
REF
(+) Upper limit of reference span.
Range: V
REF
(–) < V
REF
(+) ≤ V
DD
.
13 CS Chip Select Input. The device is selected when this input is low.
14–16 DB4–DB6 Three-State Data Outputs
17 DB7 Three-State Data Output (MSB)
18 OFL Overflow Output. If the analog input is higher than (V
REF
(+) – 1/2 LSB), OFL will be low at the end of conversion. It
is a non-three-state output which can be used to cascade two or more devices to increase resolution.
19 V
SS
Negative Supply Voltage
V
SS
= 0 V; Unipolar Operation
V
SS
= –5 V; Bipolar Operation
20 V
DD
Positive Supply Voltage, +5 V
AD7821
REV. B –5–
TERMINOLOGY
LEAST SIGNIFICANT BIT (LSB)
An ADC with 8-bit resolution can resolve one part in 2
8
(1/256
of full scale). For the AD7821 operating in either the unipolar
or bipolar input range with 5 V full scale, one LSB is 19.53 mV.
TOTAL UNADJUSTED ERROR
This is a comprehensive specification which includes relative
accuracy, offset error, and full-scale error.
SLEW RATE
Slew rate is the maximum allowable rate of change of input
signal such that the digital sample values are not in error.
TOTAL HARMONIC DISTORTION (THD)
Total harmonic distortion is the ratio of the square root of the
sum of the squares of the rms value of the harmonics to the rms
value of the fundamental. For the AD7821, total harmonic dis-
tortion is defined as
20
2
2
3
2
4
2
5
2
6
2
1
log
VVVVV
VdB
++++
()
where V
1
is the rms amplitude of the fundamental and V
2
, V
3
, V
4
,
V
5
, and V
6
are the rms amplitudes of the individual harmonics.
INTERMODULATION DISTORTION
With inputs consisting of sine waves at two frequencies, fa and
fb, any active device with nonlinearities will create distortion
products, of order (m+n), at sum and difference frequencies of
mfa+nfb, where m, n = 0, 1, 2, 3…. Intermodulation terms are
those for which m or n is not equal to zero. For example, the
second order terms include (fa + fb) and (fa – fb), and the third
order terms include (2fa + fb), (2fa – fb), (fa + 2fb) and
(fa – 2fb). For the AD7821 intermodulation distortion is calcu-
lated separately for both the second and third order terms.
SIGNAL-TO-NOISE RATIO (SNR)
Signal-to-noise ratio is measured signal-to-noise at the output of
the ADC. The signal is the rms magnitude of the fundamental.
Noise is the rms sum of all nonfundamental signals (excluding
dc) up to half the sampling frequency. SNR is dependent on the
number of quantization levels used in the digitization process.
The theoretical SNR for a sine wave input is given by:
SNR N dB=+
()
602 176..
where N is the number of bits in the ADC. Thus, for an ideal
8-bit ADC, SNR = 50 dB.
PEAK HARMONIC OR SPURIOUS NOISE
Peak harmonic or spurious noise is the rms value of the largest
nonfundamental frequency (excluding dc) up to half the sam-
pling frequency to the rms value of the fundamental.
AD7821
REV. B
–6–
—Typical Performance Characteristics
TPC 2. Power Supply Current vs.
Temperature (Not Including
Reference Ladder)
TPC 8. t
INTL
, Internal Time Delay vs.
Temperature
TPC 3. Accuracy vs. t
WR
TPC 1. Conversion Time
(RD Mode) vs.Temperature
TPC 4. Accuracy vs. t
RD
TPC 7. Effective Number of Bits vs.
Input Signal (
±
2.5 V) Frequency
TPC 9. Output Current vs.
Temperature
TPC 5. Accuracy vs. t
P
TPC 6. Accuracy vs. V
REF
[V
REF
= V
REF
(+) – V
REF
(–)]
AD7821
REV. B –7–
CIRCUIT INFORMATION
BASIC DESCRIPTION
The AD7821 uses a half flash conversion technique (see Func-
tional Block Diagram), whereby two 4-bit flash ADCs are used to
achieve an 8-bit result. Each 4-bit flash ADC contains 15
comparators, which compare an unknown input voltage to the
reference ladder, to achieve a 4-bit result. The MS (most signifi-
cant) flash ADC converts an unknown analog input voltage (V
IN
)
to provide the 4 MS data bits. An internal DAC, driven by the 4 MS
data bits, then recreates an analog approximation of the input
voltage. The DAC output voltage is subtracted from the analog
input, and the difference is converted by the LS (least significant)
ADC to provide the 4 LS data bits. The MS flash ADC also has one
additional comparator to detect over-range on the analog input.
OPERATING SEQUENCE
The AD7821 has two operating modes. The RD mode allows a con-
version to be started and data to be read with a single, extended,
READ operation (i.e., CS and RD are taken low). The conversion
process is timed out by internal one-shots. The WR-RD mode uses
WR to start a conversion and RD to read the data and allows the
conversion timing to be externally controlled. The operating sequence
for the WR-RD mode is shown in Figure 3.
Figure 3. Operating Sequence (WR-RD Mode)
A conversion is initiated and the analog input signal (V
IN
) sampled
on the falling edge of WR (falling edge of RD, RD mode). A setup
time (t
P
, delay time between conversions) of 350 ns is required
prior to this falling edge. See the Digital Interface section for more
details. When WR is low, the internal MS (most significant) ADC
compares the sampled analog input with the reference ladder to
provide the 4 MS data bits. A minimum of 250 ns is required for
this comparison. On the rising edge of WR, the MS data result is
latched internally and the LS (least significant) conversion begins,
to yield the 4 LS data bits. INT goes low typically 380 ns after the
rising edge of WR. This indicates the LS conversion is complete
and that both the LS and MS data results are latched into the
output buffer. RD going low then enables the output data. If a
faster conversion time is required, the RD line can be brought low
250 ns after WR goes high. This latches both the LS and MS
data bits and outputs the conversion result on DB0–DB7.
REFERENCE AND INPUT
The V
REF
(–) and V
REF
(+) reference inputs on the AD7821 are
fully differential and define the zero and full-scale input range of
the ADC. The transfer characteristic of the part is defined by
the integer value of the following expression:
Data (LSBs)=256 V
IN
−V
REF
(−)
V
REF
(+)−V
REF
(−)
+0.5
As a result, the analog input (V
IN
) of the device can easily be set
up to provide both unipolar and bipolar operation. The data
output code for unipolar and bipolar operation is Natural Binary
and Offset Binary, respectively.
The span of the analog input voltage can easily be varied. By
reducing the reference span, V
REF
(+) – V
REF
(–), to less than 5 V,
the sensitivity of the converter can be increased (i.e., if V
REF
= 2 V
then 1 LSB = 7.8 mV). The reference flexibility also allows the
input span for unipolar operation to be offset from zero (V
REF
(–) >
GND). Additionally, the input/reference arrangement facilitates
ratiometric operation.
Figures 4 and 5 show some configurations that are possible. For
minimum noise, a 47 µF capacitor in parallel with a 0.1 µF ca-
pacitor should be connected between the reference inputs and
GND.
Figure 4. Power Supply as Reference;
Unipolar Operation (0 to + 5 V)
Figure 5. External Reference;
Bipolar Operation (–2.5 V to +2.5 V)
INPUT CURRENT
The analog input of the AD7821 behaves somewhat differently
than conventional ADCs. This is due to the ADC’s sampled
data comparators, which take varying amounts of input current
depending on the cycle of the converter.
The equivalent input circuit of the AD7821 is shown in Figure 6.
When a conversion ends (e.g., falling edge of INT, WR-RD
mode, t
RD
> t
INTL
) all the input switches are closed and V
IN
is
connected to the comparators of the internal LS and MS ADCs.
Therefore, V
IN
is simultaneously connected to 31 input capacitors
of 1 pF each.
AD7821
REV. B
–8–
Figure 6. AD7821 Equivalent Input Circuit
The input capacitors must charge to the input voltage through the
on resistance of the analog switches (about 2 kΩ to 5 kΩ). In
addition, about 12 pF of input stray capacitance must be charged.
The analog input can be modeled as an equivalent RC network
as shown in Figure 7. As R
S
(source impedance) increases, the
input capacitance takes longer to charge.
The comparators track the analog input between conversions.
A minimum delay time (t
P
) of 350 ns is required between
conversions to allow for voltage source settling and comparator
tracking time. This allows input time constants of 50 ns without
settling time problems. Typical total input capacitance values of
55 pF allow R
S
to be 0.9 kΩ without lengthening t
P
to give V
IN
more time to settle.
Figure 7. RC Network Model
INPUT TRANSIENTS
Transients on the analog input signal caused by charging current
flowing into V
IN
will not normally degrade the ADC’s perfor-
mance. In effect, the AD7821 does not “look” at the input when
these transients occur. The comparators’ inputs track V
IN
and are
not sampled until the falling edge of WR (WR-RD Mode) or
RD (RD Mode), so at least 350 ns (t
P
) is provided to charge the
ADC’s input capacitance. It is, therefore, not necessary to filter
out these transients with an external capacitor at the V
IN
terminal.
INHERENT TRACK-AND-HOLD
A major benefit of the AD7821’s input structure is its ability to
measure a variety of high speed signals without the help of an
external track-and-hold. Any ADC which does not have a built-in
track-and-hold, regardless of its speed, requires the analog input to
remain stable to at least 1/2 LSB for the duration of the conver-
sion to maintain full accuracy. This requires the use of a
track-and-hold whenever the input is a high-speed signal. The
AD7821’s sampled-data comparators, by nature of their input
switching, inherently accomplish this track-and-hold function.
Although the conversion time for the AD7821 is 660 ns (WR-RD
mode, t
WR
+ t
RD
+ t
ACC1
), the time for which V
IN
must be stable
to 1/2 LSB is much smaller. The AD7821 tracks V
IN
between
conversions only, and its value on the falling edge of WR or RD in
the WR-RD or RD modes, respectively, is the measured value.
SINUSOIDAL INPUTS
The bandwidth of the built-in track-and-hold is 100 kHz max
(150 kHz typ, 5 V p-p). This is limited by the analog bandwidth
of the comparators and timing skew between the comparator
switches. This means that the analog input frequency can be up
to 100 kHz without the aid of an external track-and-hold. The
Nyquist criterion requires that the sampling rate be at least
twice the input frequency (i.e., ≥2 ⫻ 100 kHz). This requires an
ideal antialiasing filter with an infinite roll-off. To ease the prob-
lem of antialiasing filter design, the sampling rate is usually set
much greater than the Nyquist criterion. The maximum sampling
rate (f
MAX
) for the AD7821 in the WR-RD mode, (t
RD
< t
INTL
)
can be calculated as follows:
ftttt
f
MAX
WR RD RI P
MAX
=+++
=×
()
+×
()
+×
()
+×
()
−−−−
1
1
025 10 025 10 0 15 10 0 35 10
6666
....
tWR = Write Pulsewidth
tRD = Delay Time between WR and RD Pulses
tRI = RD to INT Delay
tP = Delay Time between Conversions
This permits a maximum sampling rate for the AD7821 of
1 MHz, which is much greater than the Nyquist criterion for
sampling a 100 kHz analog input signal.
DIGITAL SIGNAL PROCESSING APPLICATIONS
In Digital Signal Processing (DSP) application areas such as voice
recognition, echo cancellation, and adaptive filtering, the dynamic
characteristics (Signal-to-Noise Ratio, Harmonic Distortion,
Intermodulation Distortion) of an ADC are critical. Since the
AD7821 is a very fast ADC with a built-in track-and-hold function,
it is specified dynamically as well as with standard dc specifications
(Total Unadjusted Error, and so on).
AD7821
REV. B –9–
SIGNAL-TO-NOISE RATIO AND DISTORTION
The dynamic performance of the AD7821 is evaluated by apply-
ing a very low distortion sine wave signal to the analog input
(V
IN
) which is then sampled at a 512 kHz sampling rate. A Fast
Fourier Transform (FFT) plot is then generated from which
Signal-to-Noise Ratio (SNR) and harmonic distortion data are
obtained.
Figure 8 shows a 2048 point FFT plot of the AD7821 with an
input signal of 100.25 kHz. The SNR is 49.1 dB. It should be
noted that the harmonics are taken into account when calculat-
ing the SNR. The theoretical relationship between SNR and
resolution (N) is expressed by the following equation:
SNR N dB=+
()
602 176.. (1)
Figure 8. FFT Plot
EFFECTIVE NUMBER OF BITS
By working backwards from Equation (1) it is possible to get a
measure of ADC performance expressed in effective number of
bits (N). A plot of the effective number of bits versus input
frequency is given in the Typical Performance Characteristics
section. The effective number of bits typically falls between 7.7
and 7.9, corresponding to SNR figures of 48.1 dB and 49.7 dB.
INTERMODULATION DISTORTION
For intermodulation distortion (IMD), an FFT plot consisting
of very low distortion sine waves at two frequencies is generated
by sampling an analog input applied to the ADC. Figure 9
shows a 2048 point plot for IMD.
Figure 9. FFT Plot for IMD
HISTOGRAM PLOT
When a sine wave of specified frequency is applied to the V
IN
input
of the AD7821 and several thousand samples are taken, it is
possible to plot a histogram showing the frequency of occurrence
of each of the 256 ADC codes. A perfect ADC produces a
probability density function described by the equation:
P(V)=1
π(A
2
−V
2
)
1/ 2
where A is the peak amplitude of the sine wave and P(V) is the
probability of occurrence at a voltage V.
If a particular step is wider than the ideal 1 LSB width, then the
code associated with that step will accumulate more counts than
for the code for an ideal step. Likewise, a step narrower than the
ideal width will have fewer counts. Missing codes are easily seen
because a missing code means zero counts for a particular code.
The absence of large spikes in the plot indicates small differ-
ential nonlinearity.
Figure 10 shows a histogram plot for the AD7821, which corre-
sponds very well with the ideal shape. The plot indicates very
small differential nonlinearity and no missing codes for an input
frequency of 100.25 kHz.
Figure 10. Histogram Plot
In digital signal processing applications, where the AD7821 is
used to sample ac signals, it is essential that the signal sampling
occurs at exactly equal intervals. This minimizes errors due to
sampling uncertainty or jitter. A precise timer or clock source,
to start the ADC conversion process, is the best method of gen-
erating equidistant sampling intervals.
The two modes of operation given in the data sheet are suitable
for DSP applications because the sampling instant of the
AD7821 is well defined. V
IN
is sampled on the falling edge of
WR or RD in the WR-RD or RD modes, respectively.
DIGITAL INTERFACE
The AD7821 has two basic interface modes which are determined
by the status of the MODE pin. When this pin is low, the
converter is in the RD mode, with this pin high, the AD7821 is
set up for the WR-RD mode.
The RD mode is designed for microprocessors that can be driven
into a WAIT state. A READ operation (i.e., CS and RD are taken
low) starts a conversion and data is read when the conversion is
complete. The WR-RD mode does not require microprocessor
WAIT states. A WRITE operation (i.e., CS and WR are taken
low) initiates a conversion, and a READ operation reads the
result when the conversion is complete.
AD7821
REV. B
–10–
RD Mode (MODE = 0)
The timing diagram for the RD mode is shown in Figure 11.
This mode is intended for use with microprocessors that have a
WAIT state facility, whereby a READ instruction cycle can be
extended to accommodate slow memory devices. A conversion
is started by taking CS and RD low (READ operation). Both
CS and RD are then kept low until output data appears.
Figure 11. RD Mode
In this mode, Pin 6 of the AD7821 is configured as a status out-
put, RDY. This RDY output can be used to drive the processor
READY or WAIT input. It is an open-drain output (no inter-
nal-pull-up device) which goes low after the falling edge of CS
and goes high impedance at the end of conversion. An INT line is
also provided which goes low when a conversion is complete.
INT returns high on the rising edge of CS or RD.
WR-RD Mode (MODE = 1)
In the WR-RD mode, Pin 6 is configured as a WRITE (WR)
input for the AD7821. With CS low, conversion is initiated on
the falling edge of WR. Two options exist for reading data from
the converter.
In the first of these options the processor waits for the INT status
line to go low before reading the data (see Figure 12a).
INT typically goes low within 380 ns after the rising edge of WR.
It indicates that conversion is complete and that the data result is
in the output latch. With CS low, the data outputs (DB0–DB7)
are activated when RD goes low. INT is reset by the rising edge
of RD or CS.
Figure 12a. WR-RD Mode (t
RD
> t
INTL
)
The alternative option can be used to shorten the conversion time.
This is a method for bypassing the internal time-out circuit. The
INT line is ignored and RD can be brought low 250 ns after the
rising edge of WR. In this case RD going low transfers the data
result into the output latch and activates the data output
(DB0–DB7). INT is driven low on the falling edge of RD and is
reset on the rising edge of RD or CS. The timing for this interface
is shown in Figure 12b.
Figure 12b. WR-RD Mode (t
RD
< t
INTL
)
The AD7821 can also be used in standalone operation in the
WR-RD mode. CS and RD are tied low, and a conversion is initi-
ated by bringing WR low. Output data is valid 530 ns (t
INTL
+ t
ID
)
after the rising edge of WR. The timing diagram for this mode is
shown in Figure 13.
Figure 13. WR-RD Mode Stand-Alone Operation,
CS
=
RD
= 0
AD7821
REV. B –11–
MICROPROCESSOR INTERFACING
The AD7821 is designed for easy interfacing to microprocessors
as a memory mapped peripheral or an I/O device. This reduces
to a minimum the amount of external logic required for
interfacing.
AD7821 – 68008 INTERFACE
Figure 14 shows an AD7821 interface to the 68008 micropro-
cessor. The ADC is configured for the RD interface mode. This
means that one read instruction starts a conversion and reads
the result when the conversion is completed. The read cycle is
stretched out over the entire conversion period by taking the
INT line back to the DTACK input of the 68008. Starting a
conversion and reading the relevant data consists of a <MOVE B
Dn, addr> instruction, where addr is the decoded ADC address and
Dn is the data register into which the result is placed.
Figure 14. AD7821 to 68008 Interface
AD7821 – 8088 INTERFACE
A typical interface to the 8088 is shown in Figure 15. The AD7821
is configured for the RD interface mode. One read instruction
starts a conversion and reads the result. The read cycle is stretched
out over the entire conversion period by taking the RDY line back
to the READY input of the 8088. Starting a conversion and
reading the result consists of a <MOV AX, (addr)> instruction,
where addr is the decoded ADC address and AX is the 8088 data
register into which the conversion result is placed.
Figure 15. AD7821 to 8088 Interface
AD7821 – TMS32010 INTERFACE
A typical interface to the TMS32010 is shown in Figure 16. The
AD7821 is mapped at a port address and the interface is designed
for the maximum TMS32010 clock frequency of 20 MHz. In this
case, the AD7821 is configured in the WR-RD interface mode.
This means that a write instruction starts a conversion and a read
instruction reads the result when the conversion is completed. A
precise timer or clock source is used to start a conversion in
applications requiring equidistant sampling intervals. The
scheme used, whereby the AD7821 generates an interrupt to
the TMS32010, is limited in that it does not allow the AD7821
to be sampled at its maximum rate. This is because the time
between samples has to be long enough to allow the TMS32010
to service its interrupt and read data from the AD7821.
Constant interruption of the TMS32010 by the AD7821, every time
the ADC completes a conversion, is not a very efficient use of
the processor time. To overcome these problems, some buffer
memory or FIFO could be placed between the AD7821 and the
TMS32010. The INT line of the AD7821 could be used to
trigger a pulse which drives its CS and RD lines and places the
AD7821 data into a FIFO or buffer memory. The microproces-
sor can then read a batch of data from the FIFO or buffer memory
at some convenient time. Reading data from the AD7821, after an
INT has been received, consists of a <IN A, PA> instruction
(PA is the decoded ADC address).
Figure 16. AD7821 to TMS32010 Interface
AD7821 – 8051 INTERFACE
Figure 17 shows the AD7821 interface to the 8051 microcom-
puter. The AD7821 is configured in the WR-RD interface mode
and is connected to the 8051 ports. The processor starts conver-
sion and then polls INT, until it goes low, before reading the
conversion result. Data is read from the AD7821 by using the
<MOV A, 90H> instruction (90H is the address for Port 1).
Figure 17. AD7821 to 8051 Interface
AD7821
REV. B
–12–
APPLYING THE AD7821
The AD7821 is specified for a unipolar input range of 0 V to +5 V
and a bipolar input range of –2.5 V to +2.5 V. The V
REF
(–) and
V
REF
(+) voltages required for these input ranges are outlined
below. See the Typical Performance Characteristics section for
operation with unspecified input voltage ranges.
UNIPOLAR OPERATION
Figure 18 gives the configuration and reference voltages required
for 0 V to +5 V operation. The nominal transfer characteristic
for this input range is shown in Figure 19. The output code is
Natural Binary with 1 LSB = (5/256) V = 19.5 mV.
Figure 18. Unipolar/Bipolar Operation
Figure 19. Nominal Transfer Characteristic for Unipolar
(0 V to +5 V) Operation
BIPOLAR OPERATION
Figure 18 gives the configuration and reference voltages required
for –2.5 V to +2.5 V operation. The nominal transfer characteris-
tic for this input range is shown in Figure 20. The output code is
Offset Binary with 1 LSB = ([+2.5 – (–2.5)]/256) V = 19.5 mV.
Figure 20. Nominal Transfer Characteristic for Bipolar
(–2.5 V to +2.5 V) Operation
16-CHANNEL TELECOM A/D CONVERTER
The fast sampling rate (1 MHz) and bipolar operation of the
AD7821 makes it useful in telecom applications for sampling a
number of input channels using a multiplexer. Figure 21 shows
a circuit for such an application.
The maximum signal frequency required for acceptable quality
in telecom applications is 3 kHz. The circuit given in Figure 21
permits each of the 16-input channels to be sampled at a rate of
16 kHz maximum. The sampling rate takes into account such
multiplexer parameters as t
ON
, settling time, and so on. The
circuit also eases the problem of the antialiasing filter design by
sampling at a rate much greater than that required by the
Nyquist criterion.
AD7821
REV. B –13–
Figure 21. 16-Channel Telecom ADC System
SIMULTANEOUS SAMPLING ADC
S
The AD7821’s inherent track-and-hold and well defined sampling
instant makes it useful in such applications as sonar, where a num-
ber of input channels are required to be sampled simultaneously.
Figure 22 shows a circuit for such an application.
Figure 22. Simultaneous Sampling ADCs
The actual sampling instant at which V
IN
is measured occurs
approximately 50 ns after the falling edge of WR or RD in the
WR-RD or RD modes, respectively, due to internal logic delays.
However, the internal logic delay and, therefore, the sampling
instant can vary from device to device, but is typically within ±5 ns.
This means that a maximum common input sine wave of ±2.5 V
at 32 kHz, applied to any number of AD7821s in the circuit of
Figure 22, will yield a maximum difference between the converter
outputs of typically ±1/4 LSB.
AD7821
REV. B
–14–
OUTLINE DIMENSIONS
20-Lead Plastic Dual-in-Line Package [PDIP]
(N-20)
Dimensions shown in inches and (millimeters)
20
110
11
0.985 (25.02)
0.965 (24.51)
0.945 (24.00) 0.295 (7.49)
0.285 (7.24)
0.275 (6.99)
0.150 (3.81)
0.135 (3.43)
0.120 (3.05)
0.015 (0.38)
0.010 (0.25)
0.008 (0.20)
0.325 (8.26)
0.310 (7.87)
0.300 (7.62)
SEATING
PLANE
0.015 (0.38) MIN
0.180 (4.57)
MAX
0.022 (0.56)
0.018 (0.46)
0.014 (0.36)
0.150 (3.81)
0.130 (3.30)
0.110 (2.79) 0.100
(2.54)
BSC
0.060 (1.52)
0.050 (1.27)
0.045 (1.14)
CONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETER DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN
COMPLIANT TO JEDEC STANDARDS MO-095-AE
20-Lead Ceramic DIP - Glass Hermetic Seal [CERDIP]
(Q-20)
Dimensions shown in inches and (millimeters)
20
110
11
0.310 (7.87)
0.220 (5.59)
PIN 1
0.005
(0.13)
MIN
0.098 (2.49)
MAX
15
0
0.320 (8.13)
0.290 (7.37)
0.015 (0.38)
0.008 (0.20)
SEATING
PLANE
0.200 (5.08)
MAX 1.060 (26.92) MAX
0.150 (3.81)
MIN
0.200 (5.08)
0.125 (3.18)
0.023 (0.58)
0.014 (0.36)
0.100
(2.54)
BSC
0.070 (1.78)
0.030 (0.76)
0.060 (1.52)
0.015 (0.38)
CONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETERS DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN
20-Terminal Ceramic Leaded Chip Carrier [LCC]
(E-20A)
Dimensions shown in millimeters and (inches)
1
20 4
9
8
13
19
14
3
18
BOTTOM
VIEW
0.71 (0.0278)
0.56 (0.0220)
45 TYP
0.38 (0.0150)
MIN
1.40 (0.0551)
1.14 (0.0449)
1.27 (0.0500)
BSC
1.91 (0.0752)
REF
0.28 (0.0110)
0.18 (0.0071)
R TYP
2.41 (0.0949)
1.90 (0.0748)
2.54 (0.1000) BSC
5.08 (0.2000)
BSC
3.81 (0.1500)
BSC
1.91
(0.0752)
REF
9.09 (0.3579)
8.69 (0.3421)
SQ
9.09
(0.3579)
MAX
SQ
2.54 (0.1000)
1.63 (0.0642)
2.24 (0.0882)
1.37 (0.0539)
20-Lead Standard Small Outline Package [SOIC]
Wide Body
(RW-20)
Dimensions shown in millimeters and (inches)
CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN
COMPLIANT TO JEDEC STANDARDS MS-013AC
0.75 (0.0295)
0.25 (0.0098)
20 11
10
1
0.32 (0.0126)
0.23 (0.0091)
8ⴗ
0ⴗ
ⴛ 45ⴗ
1.27 (0.0500)
0.40 (0.0157)
SEATING
PLANE
0.30 (0.0118)
0.10 (0.0039)
0.51 (0.0201)
0.33 (0.0130)
2.65 (0.1043)
2.35 (0.0925)
1.27
(0.0500)
BSC
10.65 (0.4193)
10.00 (0.3937)
7.60 (0.2992)
7.40 (0.2913)
13.00 (0.5118)
12.60 (0.4961)
COPLANARITY
0.10
AD7821
REV. B –15–
Revision History
Location Page
10/02—Data Sheet changed from REV. A to REV. B.
Update Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Universal
Changes to FUNCTIONAL BLOCK DIAGRAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Edit to SPECIFICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Change to TOTAL HARMONIC DISTORTION formula . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Changes to INPUT CURRENT section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Change to Figure 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Change to formula in SINUSOIDAL INPUTS section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
OUTLINE DIMENSIONS updated . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
OUTLINE DIMENSIONS
20-Lead Plastic Leaded Chip Carrier [PLCC]
(P-20A)
Dimensions shown in inches and (millimeters)
0.020 (0.50)
R
BOTTOM
VIEW
(PINS UP)
0.040 (1.01)
0.025 (0.64)
0.021 (0.53)
0.013 (0.33)
0.330 (8.38)
0.290 (7.37)
0.032 (0.81)
0.026 (0.66)
0.056 (1.42)
0.042 (1.07) 0.20 (0.51)
MIN
0.120 (3.04)
0.090 (2.29)
3
4
19
18
8
9
14
13
TOP VIEW
(PINS DOWN)
0.395 (10.02)
0.385 (9.78) SQ
0.356 (9.04)
0.350 (8.89) SQ
0.048 (1.21)
0.042 (1.07)
0.048 (1.21)
0.042 (1.07)
0.020
(0.50)
R
0.050
(1.27)
BSC
0.180 (4.57)
0.165 (4.19)
COMPLIANT TO JEDEC STANDARDS MO-047AA
CONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETER DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN
–16–
C01320–0–11/02(B)
PRINTED IN U.S.A.
