AD624 查看數據表（PDF） - Analog Devices

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AD624

Precision Instrumentation Amplifier

Analog Devices 

AD624

GROUNDING

Many data-acquisition components have two or more ground

pins which are not connected together within the device. These

grounds must be tied together at one point, usually at the sys-

tem power supply ground. Ideally, a single solid ground would

be desirable. However, since current flows through the ground

wires and etch stripes of the circuit cards, and since these paths

have resistance and inductance, hundreds of millivolts can be

generated between the system ground point and the data acqui-

sition components. Separate ground returns should be provided

to minimize the current flow in the path from the most sensitive

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 (see Figure 34).

ANALOG P.S.

+15V C –15V

DIGITAL P.S.

C +5V

0.1 0.1

␮F ␮F

0.1 0.1

␮F ␮F

1␮F 1␮F 1␮F

DIG

COM

+

AD624

OUTPUT

REFERENCE

ANALOG

GROUND*

AD583

SAMPLE

AND HOLD

AD574A

SIGNAL

GROUND

DIGITAL

DATA

OUTPUT

*IF INDEPENDENT, OTHERWISE RETURN AMPLIFIER REFERENCE

TO MECCA AT ANALOG P.S. COMMON

Figure 34. Basic Grounding Practice

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

tial on the reference terminal an instrumentation amplifier can

solve many grounding problems.

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.

VIN+

V+

AD624

(SENSE)

OUTPUT

CURRENT

BOOSTER

X1

VIN–

RL

(REF)

V–

Figure 35. AD624 Instrumentation Amplifier with Output

Current Booster

Typically, IC instrumentation amplifiers are rated for a full

± 10 volt output swing into 2 kΩ. In some applications, how-

ever, the need exists to drive more current into heavier loads.

Figure 35 shows how a current booster may be connected

“inside the loop” of an instrumentation amplifier to provide the

required current without significantly degrading overall perfor-

mance. The effects of nonlinearities, offset and gain inaccuracies

of the buffer are reduced by the loop gain of the IA output

amplifier. Offset 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, from ground, to

be shared between signal and reference offset.

VIN+

+VS

SENSE

AD624

VIN–

REF

LOAD

–VS

AD711

VOFFSET

Figure 36. Use of Reference Terminal to Provide Output

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, including those caused by

PC layouts or other connection techniques, which appears

between the reference pin and ground will increase the gain of

the noninverting signal path, thereby upsetting the common-

mode rejection of the IA. Inadvertent thermocouple connections

created in the sense and reference lines should also be avoided

as they will directly affect the output offset voltage and output

offset voltage drift.

In the AD624 a reference source resistance will unbalance the

CMR trim by the ratio of 10 kΩ/RREF. For example, if the refer-

ence source impedance is 1 Ω, CMR will be reduced to 80 dB

(10 kΩ/1 Ω = 80 dB). An operational amplifier may be used to

provide that low impedance reference point as shown in Figure

36. The input offset voltage characteristics of that amplifier will

add directly to the output offset voltage performance of the

instrumentation amplifier.

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 37.

+INPUT

SENSE

AD624

R1

+VX–

IL

–INPUT

AD711

REF

A2

IL =

VX

R1

=

VIN

R1

1 + 40.000

RG

LOAD

Figure 37. Voltage-to-Current Converter

–10–

REV. C

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