AD524

REV. E

–11–

acquisition components. Separate ground returns should be

provided to minimize the current flow in the path from the sensi-

tive points to the system ground point. In this way supply currents

and logic-gate return currents are not summed into the same

return path as analog signals where they would cause measure-

ment errors.

Since the output voltage is developed with respect to the poten-

tial on the reference terminal, an instrumentation amplifier can

solve many grounding problems.

0.1

F

0.1

F

DIGITAL P.S.

+5V

C

–15V

ANALOG P.S.

1 F

DIG

COM

AD574A

C

+15V

6

OUTPUT

REFERENCE

GROUND

AD524

DIGITAL

DATA

OUTPUT

SIGNAL

GROUND

*IF INDEPENDENT; OTHERWISE RETURN AMPLIFIER REFERENCE

TO MECCA AT ANALOG P.S. COMMON

1 F1 F

0.1

F

0.1

F

AD583

SAMPLE

AND HOLD

Figure 37. Basic Grounding Practice

SENSE TERMINAL

The sense terminal is the feedback point for the instrument

amplifier’s output amplifier. Normally it is connected to the

instrument amplifier output. If heavy load currents are to be

drawn through long leads, voltage drops due to current flowing

through lead resistance can cause errors. The sense terminal can

be wired to the instrument amplifier at the load, thus putting

the IxR drops “inside the loop” and virtually eliminating this

error source.

V–

V+

X1

AD524

OUTPUT

CURRENT

BOOSTER

(REF)

(SENSE)

Figure 38. AD524 Instrumentation Amplifier with Output

Current Booster

Typically, IC instrumentation amplifiers are rated for a full

±10

volt output swing into 2 k

Ω. In some applications, however, the

need exists to drive more current into heavier loads. Figure 38

shows how a high-current booster may be connected “inside the

loop” of an instrumentation amplifier to provide the required

current boost without significantly degrading overall perfor-

mance. Nonlinearities, offset and gain inaccuracies of the buffer

are minimized by the loop gain of the IA output amplifier. Off-

set drift of the buffer is similarly reduced.

REFERENCE TERMINAL

The reference terminal may be used to offset the output by up

to

±10 V. This is useful when the load is “floating” or does not

share a ground with the rest of the system. It also provides a

direct means of injecting a precise offset. It must be remem-

bered that the total output swing is

±10 volts to be shared be-

tween signal and reference offset.

When the IA is of the three-amplifier configuration it is neces-

sary that nearly zero impedance be presented to the reference

terminal.

Any significant resistance from the reference terminal to ground

increases the gain of the noninverting signal path, thereby upset-

ting the common-mode rejection of the IA.

In the AD524 a reference source resistance will unbalance the

CMR trim by the ratio of 20 k

Ω/R

ence source impedance is 1

Ω, CMR will be reduced to 86 dB

(20 k

Ω/1 Ω = 86 dB). An operational amplifier may be used to

provide that low impedance reference point as shown in Figure

39. The input offset voltage characteristics of that amplifier will

add directly to the output offset voltage performance of the

instrumentation amplifier.

AD524

REF

SENSE

LOAD

AD711

Figure 39. Use of Reference Terminal to Provide Output

Offset

An instrumentation amplifier can be turned into a voltage-to-

current converter by taking advantage of the sense and reference

terminals as shown in Figure 40.

AD524

REF

SENSE

LOAD

AD711

+INPUT

–INPUT

R1

R1

=

=

R1

)

40,000

A2

Figure 40. Voltage-to-Current Converter

By establishing a reference at the “low” side of a current setting

resistor, an output current may be defined as a function of input

voltage, gain and the value of that resistor. Since only a small

drift specifications of the IA.