AMP04

–6–

APPLICATIONS

Common-Mode Rejection

The purpose of the instrumentation amplifier is to amplify the

difference between the two input signals while ignoring offset

and noise voltages common to both inputs. One way of judging

the device’s ability to reject this offset is the common-mode

gain, which is the ratio between a change in the common-mode

voltage and the resulting output voltage change. Instrumenta-

tion amplifiers are often judged by the common-mode rejection

ratio, which is equal to 20

× log

differential signal gain to the common-mode gain, commonly

called the CMRR. The AMP04 offers excellent CMRR, guaran-

teed to be greater than 90 dB at gains of 100 or greater. Input

offsets attain very low temperature drift by proprietary laser-

trimmed thin-film resistors and high gain amplifiers.

Input Common-Mode Range Includes Ground

The AMP04 employs a topology (Figure 1) that uniquely allows

the common-mode input voltage to truly extend to zero volts

where other instrumentation amplifiers fail. To illustrate, take

for example the single supply, gain of 100 instrumentation

amplifier as in Figure 2. As the inputs approach zero volts, in

must be allowed to go below ground, to –0.094 volts. Clearly

this is not possible in a single supply environment. Consequently

this instrumentation amplifier configuration’s input common-mode

voltage cannot go below about 0.4 volts. In comparison, the

AMP04 has no such restriction. Its inputs will function with a

zero-volt common-mode voltage.

IN(–)

IN(+)

INPUT BUFFERS

REF

100k

11k

11k

100k

Figure 1. Functional Block Diagram

0.01V

+

20k

100k

–

4.7 A

4.7 A

20k

0.01V

5.2 A

2127

100k

0V

B

A

–0.094V

0V

Figure 2. Gain = 100 Instrumentation Amplifier

Input Common-Mode Voltage Below Ground

Although not tested and guaranteed, the AMP04 inputs are

biased in a way that they can amplify signals linearly with common-

mode voltage as low as –0.25 volts below ground. This holds

true over the industrial temperature range from –40

°C to +85°C.

Extended Positive Common-Mode Range

On the high side, other instrumentation amplifier configurations,

such as the three op amp instrumentation amplifier, can have

severe positive common-mode range limitations. Figure 3 shows

an example of a gain of 1001 amplifier, with an input common-

swing to 15.01 volts in order for the output to go to 10.01 volts.

Clearly no op amp can handle this swing range (given a 15 V

supply) as the output will saturate long before it reaches the

supply rails. Again the AMP04’s topology does not have this

limitation. Figure 4 illustrates the AMP04 operating at the same

common-mode conditions as in Figure 3. None of the internal

nodes has a signal high enough to cause amplifier saturation. As

a result, the AMP04 can accommodate much wider common-

mode range than most instrumentation amplifiers.

100k

R

10.00V

A

10.01V

15.01V

5V

R

R

R

50 A

100k

10.01V

200

B

Figure 3. Gain = 1001, Three Op Amp Instrumentation

Amplifier

+15V

–15V

100k

11k

100k

+15V

–15V

11k

100.1 A

11.111V

10.01V

10.00V

100

10.01V

0.1 A

10V

100 A

Figure 4. Gain = 1000, AMP04

REV. C