AMP04

REV. A

–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 patented topology (Figure 1) that

uniquely allows the common-mode input voltage to truly extend

to zero volts where other instrumentation amplifiers fail. To il-

lustrate, take for example the single supply, gain of 100 instru-

mentation amplifier as in Figure 2. As the inputs approach zero

volts, in order for the output to go positive, amplifier A’s output

Clearly this is not possible in a single supply environment. Con-

sequently 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.

2

3

8

1

6

5

IN(–)

IN(+)

INPUT BUFFERS

R

GAIN

100k

REF

100k

V

OUT

11k

11k

Figure 1. Functional Block Diagram

B

A

V

OA

100k

20k

20k

100k

2127

Ω

0V

V

OUT

0V

0.01V

0.01V

+

–

4.7

µA

5.2

µA

4.7

µA

V

OB

–.094V

V

IN

Figure 2. Gain = 100 Instrumentation Amplifier

Input Common-Mode Voltage Below Ground

Although not tested and guaranteed, the AMP04 inputs are bi-

ased 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 configura-

tions, 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-mode voltage of 10 volts. For this circuit to function,

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 satu-

ration. As a result, the AMP04 can accommodate much wider

common-mode range than most instrumentation amplifiers.

B

V

OA

100k

100k

A

V

OB

R

R

R

+5V

+15.01V

R

10.01

+10.00V

+10.01V

200

Ω

50

µA

Figure 3. Gain = 1001, Three Op Amp Instrumentation

Amplifier

+10.00V

+10.01V

100k

100k

11k

–15V

+10.01V

100

Ω

100

µA

+15V

11k

+11.111V

+

–

0.1

µA

100.1

µA

–15V

+15V

+10V

Figure 4. Gain = 1000, AMP04