REV. E

AMP02

–9–

25k

R

25k

25k

R

25k

+IN

–IN

25k

25k

SENSE

(SOIC-16 ONLY)

OUT

REFERENCE

3

8

1

2

5

6

Figure 4. Triple Op Amp Topology

Grounding

The majority of instruments and data acquisition systems have

separate grounds for analog and digital signals. Analog ground may

also be divided into two or more grounds that will be tied together

at one point, usually at the analog power supply ground. In

addition, the digital and analog grounds may be joined—normally

at the analog ground pin on the A/D converter. Following 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 resistance

and inductance, hundreds of millivolts can be developed between

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. Consequently, noisy

ground currents from logic gates interact with the analog signals.

Inevitably, two or more circuits will be joined together with

their grounds at differential potentials. In these situations, the

differential 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 instrumen-

tation amplifier output stage and is internally connected directly

to the output. For SOIC devices, connect the sense terminal to

the output. The output signal is specified with respect to the refer-

ence terminal, which is normally connected to analog ground.

The reference may also be used for offset correction level shift-

ing. A reference source resistance will reduce the common-mode

rejection by the ratio of 25 k

Ω/R

tance is 1

Ω, the CMR will be reduced 88 dB (25 kΩ/1 Ω = 88 dB).

Figure 4 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.

V

R

R

V

V

G

V

V

V

R

R

V

V

G

V

V

G

D

CM

D

CM

G

D

CM

D

CM

1

2

1

2

2

2

1

2

2

2

=+









+

=+

=+









+

=+

–

–

where:

= Differential input voltage

= (+IN) – (–IN)

G

= Gain of instrumentation amplifier

±12 V maximum, the common-mode

input voltage range is:

CMVR

V

GV

D

=±

−









12

2