AMP04 查看數據表（PDF） - Analog Devices

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AMP04

Precision Single Supply Instrumentation Amplifier

Analog Devices 

AMP04

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 × log10 of the ratio of the user-selected

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

order for the output to go positive, amplifier A’s output (VOA)

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(+)

RGAIN

INPUT BUFFERS

100k⍀

VOUT

11k⍀

11k⍀

100k⍀

REF

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-

mode voltage of 10 volts. For this circuit to function, VOB must

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.

10.00V

10.01V

A

R

200⍀

100k⍀ 5V R

50␮A

VOA

VOB R

100k⍀ 15.01V R

B

10.01V

Figure 3. Gain = 1001, Three Op Amp Instrumentation

Amplifier

10.00V

10.01V

100k⍀

100⍀

10.01V

0.1␮A

+15V

100␮A

+15V 11k⍀

–15V

100.1␮A

VOUT

10V

Figure 1. Functional Block Diagram

0.01V

+

VIN

0V

–

VOA

A

VOB

B

100k⍀

0V

20k⍀

20k⍀

–0.094V

0.01V

4.7␮A

5.2␮A

4.7␮A

2127⍀

100k⍀

VOUT

100k⍀

–15V

11k⍀

11.111V

Figure 4. Gain = 1000, AMP04

Figure 2. Gain = 100 Instrumentation Amplifier

–6–

REV. C

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