Datasheet AMP04 (Analog Devices) - 6

AMP04 APPLICATIONS Input Common-Mode Voltage Below Ground Common-Mode Rejection
Although not tested and guaranteed, the AMP04 inputs are The purpose of the instrumentation amplifier is to amplify the biased in a way that they can amplify signals linearly with common- difference between the two input signals while ignoring offset mode voltage as low as –0.25 volts below ground. This holds and noise voltages common to both inputs. One way of judging true over the industrial temperature range from –40°C to +85°C. the device’s ability to reject this offset is the common-mode
Extended Positive Common-Mode Range
gain, which is the ratio between a change in the common-mode On the high side, other instrumentation amplifier configurations, voltage and the resulting output voltage change. Instrumenta- such as the three op amp instrumentation amplifier, can have tion amplifiers are often judged by the common-mode rejection severe positive common-mode range limitations. Figure 3 shows ratio, which is equal to 20 × log10 of the ratio of the user-selected an example of a gain of 1001 amplifier, with an input common- differential signal gain to the common-mode gain, commonly mode voltage of 10 volts. For this circuit to function, VOB must called the CMRR. The AMP04 offers excellent CMRR, guaran- swing to 15.01 volts in order for the output to go to 10.01 volts. teed to be greater than 90 dB at gains of 100 or greater. Input Clearly no op amp can handle this swing range (given a 15 V offsets attain very low temperature drift by proprietary laser- supply) as the output will saturate long before it reaches the trimmed thin-film resistors and high gain amplifiers. supply rails. Again the AMP04’s topology does not have this
Input Common-Mode Range Includes Ground
limitation. Figure 4 illustrates the AMP04 operating at the same The AMP04 employs a topology (Figure 1) that uniquely allows common-mode conditions as in Figure 3. None of the internal the common-mode input voltage to truly extend to zero volts nodes has a signal high enough to cause amplifier saturation. As where other instrumentation amplifiers fail. To illustrate, take a result, the AMP04 can accommodate much wider common- for example the single supply, gain of 100 instrumentation mode range than most instrumentation amplifiers. amplifier as in Figure 2. As the inputs approach zero volts, in order for the output to go positive, amplifier A’s output (VOA)
10.00V
must be allowed to go below ground, to –0.094 volts. Clearly
A R
this is not possible in a single supply environment. Consequently
100k 5V R
this instrumentation amplifier configuration’s input common-mode
VOA 200 50 A 10.01V
voltage cannot go below about 0.4 volts. In comparison, the
VOB R
AMP04 has no such restriction. Its inputs will function with a
100k 15.01V R
zero-volt common-mode voltage.
B 10.01V 100k RGAIN
Figure 3. Gain = 1001, Three Op Amp Instrumentation
IN(–)
Amplifier
INPUT BUFFERS VOUT IN(+) 100k 11k 0.1 A +15V 11k 10.00V 100 10.01V VOUT 100 A 10V 100k 10.01V –15V +15V 11k 100.1 A REF
Figure 1. Functional Block Diagram
–15V 11.111V 11k 100k 0.01V + VOB V V IN B OUT 0V
Figure 4. Gain = 1000, AMP04
– VOA A 100k 20k 20k 100k –0.094V 0.01V 0V 4.7 A 5.2 A 4.7 A 2127
Figure 2. Gain = 100 Instrumentation Amplifier –6– REV. C