AD524CDZ 查看數據表（PDF） - Analog Devices

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AD524CDZ

Precision Instrumentation Amplifier

Analog Devices 

AD524

GROUNDING

Many data acquisition components have two or more ground

pins that are not connected together within the device. These

grounds must be tied together at one point, usually at the system

power-supply ground. Ideally, a single solid ground would be

desirable. However, because current flows through the ground

wires and etch stripes of the circuit cards, and because these

paths have resistance and inductance, hundreds of millivolts can

be generated between the system ground point and the data

acquisition components. Separate ground returns should be

provided to minimize the current flow in the path from the

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

measurement errors.

Because the output voltage is developed with respect to the

potential on the reference terminal, an instrumentation

amplifier can solve many grounding problems.

ANALOG P.S.

+15V C –15V

DIGITAL P.S.

C

+5V

0.1 0.1

µF µF

0.1 0.1

µF µF

1µF 1µF 1µF

8

2

7

10

AD524 9

1

6

DIG

COM

AD583

SAMPLE

AND HOLD

7

9 11 15 1

AD574A

DIGITAL

DATA

OUTPUT

OUTPUT

REFERENCE

ANALOG

GROUND*

SIGNAL

GROUND

*IF INDEPENDENT; OTHERWISE, RETURN AMPLIFIER REFERENCE

TO MECCA AT ANALOG P.S. COMMON.

Figure 43. 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.

VIN+

VIN–

V+

(SENSE)

2

8

OUTPUT

CURRENT

BOOSTER

3

10

AD524

9

X1

12

6

1

7

(REF)

RL

V–

Figure 44. AD524 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, however,

the need exists to drive more current into heavier loads.

Figure 44 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

performance. Nonlinearities and offset and gain inaccuracies of

the buffer are minimized by the loop gain of the AD524 output

amplifier. Offset drift of the buffer is similarly reduced.

REFERENCE TERMINAL

The reference terminal can 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 remembered that

the total output swing is ±10 V to be shared between signal and

reference offset.

When the AD524 is of the 3-amplifier configuration it

is necessary 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 upsetting the common-mode rejection of the AD524.

In the AD524, a reference source resistance unbalances the CMR

trim by the ratio of 20 kΩ/RREF. For example, if the reference

source impedance is 1 Ω, CMR is reduced to 86 dB (20 kΩ/1 Ω

= 86 dB). An operational amplifier can be used to provide that

low impedance reference point, as shown in Figure 45. The

input offset voltage characteristics of that amplifier adds directly

to the output offset voltage performance of the instrumentation

amplifier.

Rev. F | Page 18 of 28

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