AMP02

–9–

REV. D

Figure 27. Triple Op Amp Topology of the AMP02

GROUNDING

The majority of instruments and data acquisition systems the

separate grounds for analog and digital signals. Analog ground

may also be divided into two or more grounds which will be tied

together at one point, usually the analog power-supply ground.

In addition, the digital and analog grounds may be joined, nor-

mally at the analog ground pin on the A to D converter. Follow

this basic practice is essential for good circuit performance.

Mixing grounds causes interactions between digital circuits and

the analog signals. Since the ground returns have finite resis-

tance and inductance, hundreds of millivolts can be develop be-

tween the system ground and the data acquisition components.

Using separate ground returns minimizes the current flow in the

sensitive analog return path to the system ground point. Conse-

quently, noisy ground currents from logic gates do interact with

the analog signals.

Inevitably, two or more circuits will be joined together with their

grounds at differential potentials. In these situations, the differ-

ential input of an instrumentation amplifier, with its high CMR,

can accurately transfer analog information from one circuit to

another.

SENSE AND REFERENCE TERMINALS

The sense terminal completes the feedback path for the instru-

mentation amplifier output stage and is internally connected

directly to the output. For SOL devices, connect the sense

terminal to the output. The output signal is specified with re-

spect to the reference terminal, which is normally connected to

analog ground. The reference may also be used for offset correc-

tion level shifting. A reference source resistance will reduce the

common-mode rejection by the ratio of 25 k

Ω/R

ence source resistance is 1

Ω, then the CMR will be reduced

88 dB (25 k

Ω/1 Ω = 88 dB).

Figure 27 shows the triple op amp configuration of the AMP02.

With all instrumentation amplifiers of this type, it is critical not

to exceed the dynamic range of the input amplifiers. The ampli-

fied differential input signal and the input common-mode volt-

age must not force the amplifier’s output voltage beyond

±12 V

±15 V) or nonlinear operation will result.

2R









2

+V

CM

2

+V

CM

2R









2

+V

CM

2

+V

CM

where

= Differential input voltage

= (+IN) – (–IN)

G

= Gain of instrumentation amplifier

±12 V maximum, then the

common-mode input voltage range is:

CMVR =

2







