Mutant op amp becomes instrumentation amp

Op amps are the fundamental building blocks of analog circuits; you can build all analog functions from op amps. Add digital output circuitry to an op amp to get a comparator that can also function as a 1-bit ADC. Add switched resistors or current sources to the op-amp front end to get a DAC. Add some diodes in the feedback loop, and the op amp functions as a log or antilog circuit; thus, you obtain multipliers and dividers by adding or subtracting logs. However, op amps offer no free lunch: Op amps' versatility stems from their open-loop configuration, but external resistors that degrade CMRR must close the loop.

Instrumentation amplifiers target a specific task that requires a difference amplifier with a very high CMRR. You can configure an op amp as a difference amplifier, but the external resistors dramatically reduce the CMR. The primary job of an instrumentation amplifier is to interface sensors into the electronics package. Sensors are normally remote from the electronics package, so connecting cabling picks up noise and is susceptible to ground-voltage differences. Noise and ground-voltage differences become common-mode voltages by carefully interconnecting the sensor or transducer. Common-mode voltages often exceed the signal voltages. Instrumentation amplifiers must have an outstanding CMRR because this parameter reduces common-mode noise to acceptable levels.

The following transfer equation for the difference amplifier in Figure 1 op amp or instrumentation amplifier) assumes that the amplifier internal gain is large (a→∞).

When R₁=R₃ and R₂=R₄ the transfer equation reduces to

Consider when V₁ and V₂ contain a common voltage, V_CM. When the resistor ratios don't match, the signal gains for V₁ and V₂ are different, and the difference amplifier amplifies some of the common-mode signal, V_CM. If one resistor ratio has a 0.01% mismatch, the best achievable CMR is –86 dB. This performance is almost impossible to achieve on a pc board with discrete components, but you can readily achieve it in a laser-trimmed IC. The fundamental difference between the op amp and instrumentation amp is that the op amp requires external feedback resistors, and the instrumentation amp has internal feedback resistors. Both op- and instrumentation-amp designers must minimize input errors, such as offset voltage and bias current, and match drift parameters to preserve the CMR. However, the error specifications for instrumentation amps are much tighter than those for op amps.

Although instrumentation-amp difference amplifiers perform much better than do op amps, the difference amplifiers still have problems. All sensors have output impedance, and the transfer equation for some sensors is a function of the load resistance. The termination resistance for V₁ is R₁, and the termination resistance for V₂ is (R₃+R₄). You can't use instrumentation-amp difference amplifiers in load-sensitive applications because you can't match the termination resistors. Consider an INA106, in which R₁=R₂=10,000 and the gain is 10. The bridge resistance enters into the INA106 gain equation because the equivalent sensor is two voltage sources, each in series with a resistance. If the bridge legs mismatch by 10Ω out of 1 kΩ, the CMR for the INA106 reduces from 86 to 60.8 dB. Never put external resistors in series with a difference amplifier, even if you are trying to make a filter, because the resistor tolerances degrade the instrumentation amp's CMR. Some instrumentation amps use two and three op-amp-circuit configurations to circumvent this problem.

Author Information

Ron Mancini is staff scientist at Texas Instruments. You can reach him at 1-352-569-9401, rmancini@ti.com.