AD621

REV. B

–11–

–

+

AD705

5V

3k

3k

3k

3k

AD621B

ADC

REF

IN

AGND

DIGITAL

DATA

OUTPUT

20k

10k

20k

–

+

0.6mA

MAX

0.10mA

1.3mA

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 k

Ω 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

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

°C

Input Offset Voltage,

µV

125

µV/20 mV

(150

µV × 2/20 mV

15,000

Output Offset Voltage,

µV

N/A

((150

µV × 2)/100)/20 mV

N/A

Input Offset Current, nA

2 nA

× 350 Ω/20 mV

(6 nA

× 350 Ω)/20 mV

CMR, dB

110 dB

→3.16 ppm, × 5 V/20 mV (0.02% Match × 5 V)/20 mV

Total Absolute Error

20,191

DRIFT TO +85

°C

Gain Drift, ppm/

°C

5 ppm

× 60°C

100 ppm/

°C Track × 60°C

Input Offset Voltage Drift,

µV/°C1 µV/°C × 60°C/20 mV

(2.5

µV/°C × 2 × 60°C)/20 mV

15,000

Output Offset Voltage Drift,

µV/°C

N/A

(2.5

µV/°C × 2 × 60°C)/100/20 mV

N/A

Total Drift Error

15,750

RESOLUTION

Gain Nonlinearity, ppm of Full Scale

40 ppm

40 ppm

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

Total Resolution Error

Grand Total Error

11,472

36,008

± 15 V.

(All errors are min/max and referred to input.)