AD625

REV. D

–12–

GROUND RETURNS FOR BIAS CURRENTS

Input bias currents are those currents necessary to bias the input

transistors of a dc amplifier. There must be a direct return path

for these currents, otherwise they will charge external capaci-

tances, causing the output to drift uncontrollably or saturate.

Therefore, when amplifying “floating” input sources such as

transformers, or ac-coupled sources, there must be a dc path

from each input to ground as shown in Figure 35.

AD625

LOAD

TO POWER

SUPPLY

GROUND

SENSE

REFERENCE

Figure 35a. Ground Returns for Bias Currents with

Transformer Coupled Inputs

AD625

LOAD

TO POWER

SUPPLY

GROUND

SENSE

REFERENCE

Figure 35b. Ground Returns for Bias Currents with

Thermocouple Input

AD625

LOAD

TO POWER

SUPPLY

GROUND

SENSE

REFERENCE

100k

100k

Figure 35c. Ground Returns for Bias Currents with AC

Coupled Inputs

AUTOZERO CIRCUITS

In many applications it is necessary to maintain high accuracy.

At room temperature, offset effects can be nulled by the use of

offset trimpots. Over the operating temperature range, however,

offset nulling becomes a problem. For these applications the

autozero circuit of Figure 36 provides a hardware solution.

OTHER CONSIDERATIONS

One of the more overlooked problems in designing ultralow-

drift dc amplifiers is thermocouple induced offset. In a circuit

comprised of two dissimilar conductors (i.e., copper, kovar), a

current flows when the two junctions are at different tempera-

tures. When this circuit is broken, a voltage known as the

“Seebeck” or thermocouple emf can be measured. Standard IC

lead material (kovar) and copper form a thermocouple with a

high thermoelectric potential (about 35

µV°C). This means that

care must be taken to insure that all connections (especially

those in the input circuit of the AD625) remain isothermal. This

includes the input leads (1, 16) and the gain sense lines (2, 15).

These pins were chosen for symmetry, helping to desensitize the

input circuit to thermal gradients. In addition, the user should

also avoid air currents over the circuitry since slowly fluctuating

AD625

AD7502

15

16

13

14

GND

AD7510DIKD

A1

A2

A3

A4

200 s

ZERO PULSE

AD711

+

–

0.1 F LOW

LEAKAGE

1k

12

11

9

Figure 36. Auto-Zero Circuit

thermocouple voltages will appear as “flicker” noise. In SPGA

applications relay contacts and CMOS mux leads are both

potential sources of additional thermocouple errors.

The base emitter junction of an input transistor can rectify out

of band signals (i.e., RF interference). When amplifying small

signals, these rectified voltages act as small dc offset errors. The

AD625 allows direct access to the input transistors’ bases and

emitters enabling the user to apply some first order filtering to

these unwanted signals. In Figure 37, the RC time constant

should be chosen for desired attenuation of the interfering signals.

In the case of a resistive transducer, the capacitance alone work-

ing against the internal resistance of the transducer may suffice.

+GAIN SENSE

+IN

–IN

RTI NULL

RTI NULL

RTO

NULL

RTO

NULL

+V

+GAIN DRIVE

–GAIN DRIVE

NC

REF

A1

A2

AD625

10k

10k

10k

10k

A3

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

FILTER

CAP

R

R

FILTER

CAP

C

C

–GAIN SENSE

+IN

SENSE

–IN

Figure 37. Circuit to Attenuate RF Interference