LB-21| Application Note

Instrumentational Amplifiers

Instrumentational Amplifiers
Introduction
One of the most useful analog subsystems is the true instrumentation amplifier. It can faithfully amplify low level signals in the presence of high common mode noise. This aspect of its performance makes it especially useful as the input amplifier of a signal processing system. Other features of the instrumentation amplifier are high input impedance, low input current, and good linearity. It has never been easy to design a high performance instrumentation amplifier; however, the availability of high performance IC's considerably simplifies the problem. IC op amps are available today that can give very low drifts as well as low bias currents; however, most of the circuits have some drawbacks. The most commonly used instrumentation amplifier designs utilize either 2 or 3 op amps and several precision resistors. These are capable of excellent performance; however, for high performance they require very precisely matched resistors. The common mode rejection of these designs depends on resistor matching and overa
ll gain. Since op amps are now available with exceedingly high CMRR, this is no longer a problem. The CMRR of the instrumentation amplifier is approximately equal to half resistor mismatch plus the gain. For a 1% resistor mismatch the CMRR is limited to 46 dB plus the gain -- referred to the input.

National Semiconductor Linear Brief 21 June 1973

Referred to the output, the common mode error is independent of gain and fixed by the resistor mismatch. For 1% match the error is 0.5%, and for 0.1% match the error is 0.05%. These errors are not trivial in high precision systems. An instrumentation amplifier is shown here that compares favorably with multiple op amp designs, yet does not require precisely matched resistors. Further, the design allows a single resistor to adjust the gain. In comparing this instrumentation amp to multiple op amp types there are of course some drawbacks. The gain linearity and accuracy are not as good as the multiple op amp circuits. The errors appearing in multiple op amp circuits are independent of the output signal level. For example, a common mode error at the output of 0.5% of full scale is a 33% error if the desired output signal is only 1.5% of full scale. With the new circuit maximum errors at full scale output and the percentage of output error decreases at lower output levels. Figure 1 shows a general purpose instru
mentation amplifier optimized for wide bandwidth. It can provide gains from under 1 to over 1000 with a single resistor adjustment. Gain linearity is worst for unity-gain at 0.4%, and gain stability is better than 1.5% from -55 C to +125 C. Typically over a 0 C to +70 C range gain stability is 0.2%. Common mode rejection ratio is about 100 dB -- independent of gain.

00872701

Note: Since the LM114 is an obsolete part, substitution of the LM194 is recommended, along with the removal of the two LM194 diodes. This circuit has not been tested with the LM194 included. Also, the LM185-1.2 could be substituted for the LM113.

FIGURE 1. Instrumentation Amplifier

LB-21

2002 National Semiconductor Corporation

AN008727

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