AD621 Analog Devices, Inc., AD621 Datasheet - Page 11

• Manufacturer Part Number: AD621
• Description: Low Drift, Low Power Instrumentation Amplifier
• Manufacturer: Analog Devices, Inc.

Pressure Measurement
Although useful in many bridge applications such as weigh-scales,
the AD621 is especially suited for higher resistance pressure
sensors powered at lower voltages where small size and low
power become more even significant.
Figure 5 shows a 3 kΩ pressure transducer bridge powered from
5 V. In such a circuit, the bridge consumes only 1.7 mA. Adding
the AD621 and a buffered voltage divider allows the signal to be
conditioned for only 3.8 mA of total supply current.
Small size and low cost make the AD621 especially attractive for
voltage output pressure transducers. Since it delivers low noise
and drift, it will also serve applications such as diagnostic non-
invasion blood pressure measurement.
Wide Dynamic Range Gain Block Suppresses Large Common-
Mode and Offset Signals
The AD621 is especially useful in wide dynamic range applica-
tions such as those requiring the amplification of signals in the
Error Source
ABSOLUTE ACCURACY at T
DRIFT TO +85°C
RESOLUTION
G = 100, V
(All errors are min/max and referred to input.)
Input Offset Voltage, µV
Output Offset Voltage, µV
Input Offset Current, nA
CMR, dB
Gain Drift, ppm/°C
Input Offset Voltage Drift, µV/°C
Output Offset Voltage Drift, µV/°C
Gain Nonlinearity, ppm of Full Scale
Typ 0.1 Hz–10 Hz Voltage Noise, µV p-p 0.28 µV p-p/20 mV
S
= ± 15 V.
3k
3k
1.7mA
5V
A
= +25°C
3k
3k
AD621 Circuit
Calculation
125 µV/20 mV
N/A
2 nA × 350 Ω/20 mV
110 dB→3.16 ppm, × 5 V/20 mV (0.02% Match × 5 V)/20 mV
5 ppm × 60°C
1 µV/°C × 60°C/20 mV
N/A
40 ppm
Table I. Make vs. Buy Error Budget
1.3mA
MAX
+
–
AD621B
0.10mA
20k
10k
20k
presence of large, unwanted common-mode signals or offsets.
Many monolithic in amps achieve low total input drift and noise
errors only at relatively high gains (~100). In contrast the AD621’s
low output errors allow such performance at a gain of 10, thus
allowing larger input signals and therefore greater dynamic
range. The circuit of Figure 6 (± 15 V supply, G = 10) has
only 2.5 µV/°C max. V
to 10 Hz noise, yet will amplify a ± 0.5 V differential signal while
suppressing a ± 10 V common-mode signal, or it will amplify a
± 1.25 V differential signal while suppressing a 1 V offset by use
of the DAC driving the reference pin of the AD621. An added
benefit, the offsetting DAC connected to the reference pin allows
removal of a dc signal without the associated time-constant
of ac coupling. Note the representations of a differential and
common-mode signal shown in Figure 6 such that a single-ended
(or normal mode) signal of 1 V would be composed of a 0.5 V
common-mode component and a 1 V differential component.
40 ppm
Discrete Circuit
Calculation
(150 µV × 2/20 mV
((150 µV × 2)/100)/20 mV
(6 nA × 350 Ω)/20 mV
Total Absolute Error
100 ppm/°C Track × 60°C
(2.5 µV/°C × 2 × 60°C)/20 mV
(2.5 µV/°C × 2 × 60°C)/100/20 mV
Total Drift Error
(0.38 µV p-p × √2)120 mV
Total Resolution Error
Grand Total Error
+
AD705
–
0.6mA
MAX
OS
REF
IN
AGND
drift and 0.55 µ/V p-p typical 0.1 Hz
ADC
DIGITAL
DATA
OUTPUT
Error, ppm of Full Scale
AD621
16,250
12,118
12,791
17,558
13,300
13,000
13,690
12,140
121,14
121,54
11,472
N/A
N/A
AD621
Discrete
15,000
12,150
121,53
14,988
20,191
12,600
15,000
12,150
15,750
12,140
12,127
121,67
36,008