AD629

Rev. B | Page 12 of 16

OUTPUT CURRENT AND BUFFERING

The AD629 is designed to drive loads of 2 kΩ to within 2 V of

the rails but can deliver higher output currents at lower output

voltages (see Figure 15). If higher output current is required, the

output of the AD629 should be buffered with a precision op amp,

such as the OP113, as shown in Figure 36. This op amp can swing

to within 1 V of either rail while driving a load as small as 600 Ω.

REF (–)

REF (+)

NC

–IN

+IN

0.1µF

0.1µF

0.1µF

0.1µF

NC = NO CONNECT

21.1kΩ

380kΩ

380kΩ

20kΩ

380kΩ

AD629

1

2

3

4

8

7

6

5

OP113

Figure 36. Output Buffering Application

A GAIN OF 19 DIFFERENTIAL AMPLIFIER

While low level signals can be connected directly to the –IN and

+IN inputs of the AD629, differential input signals can also be

connected, as shown in Figure 37, to give a precise gain of 19.

However, large common-mode voltages are no longer permissible.

Cold junction compensation can be implemented using a

temperature sensor, such as the AD590.

REF (–)

REF (+)

NC

–IN

+IN

0.1µF

NC = NO CONNECT

21.1kΩ

380kΩ

380kΩ

20kΩ

380kΩ

AD629

1

2

3

4

8

7

6

5

THERMOCOUPLE

Figure 37. A Gain of 19 Thermocouple Amplifier

ERROR BUDGET ANALYSIS EXAMPLE 1

In the dc application that follows, the 10 A output current from

a device with a high common-mode voltage (such as a power

supply or current-mode amplifier) is sensed across a 1 Ω shunt

resistor (see Figure 38). The common-mode voltage is 200 V,

and the resistor terminals are connected through a long pair of

lead wires located in a high noise environment, for example,

50 Hz/60 Hz, 440 V ac power lines. The calculations in Table 5

assume an induced noise level of 1 V at 60 Hz on the leads, in

addition to a full-scale dc differential voltage of 10 V. The error

budget table quantifies the contribution of each error source.

Note that the dominant error source in this example is due to

the dc common-mode voltage.

REF (–)

OUTPUT

CURRENT

60Hz

POWER LINE

1Ω

SHUNT

REF (+)

NC

–IN

+IN

0.1µF

0.1µF

NC = NO CONNECT

21.1kΩ

380kΩ

380kΩ

20kΩ

380kΩ

AD629

1

2

3

4

8

7

6

5

10 AMPS

TO GROUND

Error, ppm of FS

Error Source

AD629

INA117

AD629

INA117

Initial Gain Error

500

500

Offset Voltage

100

200

DC CMR (Over Temperature)

4480

10,000

Total Accuracy Error

5080

10,700

TEMPERATURE DRIFT (85°C)

Gain

10 ppm/°C × 60°C

10 ppm/°C × 60°C

600

600

Offset Voltage

120

240

Total Drift Error

720

840

RESOLUTION

Noise, Typical, 0.01 Hz to 10 Hz, μV p-p

2

3

CMR, 60 Hz

14

50

Nonlinearity

10

10

Total Resolution Error

26

63

Total Error

5826

11,603