Datasheet

AD621
REV. B
–11–
–
+
AD705
5V
3k⍀
3k⍀
3k⍀
3k⍀
AD621B
ADC
REF
IN
AGND
DIGITAL
DATA
OUTPUT
20k⍀
10k⍀
20k⍀
–
+
0.6mA
MAX
0.10mA1.3mA
MAX
1.7mA
Figure 5. A Pressure Monitor Circuit which Operates on a 5 V Power Supply
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
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
OS
drift and 0.55 µ/V p-p typical 0.1 Hz
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.
Table I. Make vs. Buy Error Budget
AD621 Circuit Discrete Circuit Error, ppm of Full Scale
Error Source Calculation Calculation AD621 Discrete
ABSOLUTE ACCURACY at T
A
= +25°C
Input Offset Voltage, µV 125 µV/20 mV (150 µV × 2/20 mV 16,250 15,000
Output Offset Voltage, µV N/A ((150 µV × 2)/100)/20 mV N/A 12,150
Input Offset Current, nA 2 nA × 350 Ω/20 mV (6 nA × 350 Ω)/20 mV 12,118 121,53
CMR, dB 110 dB→3.16 ppm, × 5 V/20 mV (0.02% Match × 5 V)/20 mV 12,791 14,988
Total Absolute Error 17,558 20,191
DRIFT TO +85°C
Gain Drift, ppm/°C 5 ppm × 60°C 100 ppm/°C Track × 60°C 13,300 12,600
Input Offset Voltage Drift, µV/°C1µV/°C × 60°C/20 mV (2.5 µV/°C × 2 × 60°C)/20 mV 13,000 15,000
Output Offset Voltage Drift, µV/°C N/A (2.5 µV/°C × 2 × 60°C)/100/20 mV N/A 12,150
Total Drift Error 13,690 15,750
RESOLUTION
Gain Nonlinearity, ppm of Full Scale 40 ppm 40 ppm 12,140 12,140
Typ 0.1 Hz–10 Hz Voltage Noise, µV p-p 0.28 µV p-p/20 mV (0.38 µV p-p × √2)120 mV 121,14 12,127
Total Resolution Error 121,54 121,67
Grand Total Error 11,472 36,008
G = 100, V
S
= ±15 V.
(All errors are min/max and referred to input.)