AD621SQ_883B Просмотр технического описания (PDF) - Analog Devices
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AD621SQ_883B Datasheet PDF : 16 Pages
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5V
3k⍀
3k⍀
3k⍀
3k⍀
1.7mA
AD621
20k⍀
+
AD621B
–
1.3mA
MAX
10k⍀
20k⍀
0.10mA
+
AD705
–
0.6mA
MAX
REF
IN
DIGITAL
ADC DATA
OUTPUT
AGND
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. VOS 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
Error Source
AD621 Circuit
Calculation
Discrete Circuit
Calculation
ABSOLUTE ACCURACY at TA = +25°C
Input Offset Voltage, µV
Output Offset Voltage, µV
Input Offset Current, nA
CMR, dB
125 µV/20 mV
N/A
2 nA × 350 Ω/20 mV
110 dB→3.16 ppm, × 5 V/20 mV
(150 µV × 2/20 mV
((150 µV × 2)/100)/20 mV
(6 nA × 350 Ω)/20 mV
(0.02% Match × 5 V)/20 mV
DRIFT TO +85°C
Gain Drift, ppm/°C
Input Offset Voltage Drift, µV/°C
Output Offset Voltage Drift, µV/°C
5 ppm × 60°C
1 µV/°C × 60°C/20 mV
N/A
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
RESOLUTION
Gain Nonlinearity, ppm of Full Scale
40 ppm
Typ 0.1 Hz–10 Hz Voltage Noise, µV p-p 0.28 µV p-p/20 mV
Total Drift Error
40 ppm
(0.38 µV p-p × √2)120 mV
Total Resolution Error
Grand Total Error
G = 100, VS = ± 15 V.
(All errors are min/max and referred to input.)
Error, ppm of Full Scale
AD621
Discrete
16,250
N/A
12,118
12,791
17,558
13,300
13,000
N/A
13,690
12,140
121,14
121,54
11,472
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
REV. B
–11–
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