High-performance adder uses instrumentation amplifiers

Moshe Gerstenhaber and Michael O'Sullivan, Analog Devices, Wilmington, MA; Edited by Martin Rowe and Fran Granville -September 03, 2009

As instrumentation amplifiers become less costly, they can provide improved performance in applications that operational amplifiers traditionally served. The op-amp adder in Figure 1 has a few shortcomings. First, the inputs have low to medium input impedance, which the input resistor of each signal determines. This arrangement causes gain errors when the source impedance of the driving signal is large or requires the design of low-impedance driving sources. This circuit also has no common-mode-rejection capability, so inputs must be single-ended. The channel with the largest gain limits the performance of the entire system. Higher gain on one channel results in lower bandwidth, higher distortion, and increased system noise on all channels. To limit these effects, even low-performance adders require high-performance, high-bandwidth op amps.

Figure 1 A typical adder configuration uses a single op amp.

The noise gain of this op-amp adder is 1+10,000/(10||10,000). The input signal with the highest gain and 10Ω input dominates the noise gain, but all inputs suffer increased offset voltage, gain error, noise, and distortion. You can increase input impedance and improve common-mode rejection by using instrumentation amplifiers. The output voltage of an instrumentation amplifier is proportional to the voltage difference between the positive and the negative inputs. You can amplify this signal by connecting a resistor, RGAIN, to the RG pins (Figure 2). The output voltage is generated between the reference pin and the output pin. This arrangement allows you to use the reference pin to cascade multiple signals together in an adder configuration. You can set each instrumentation amplifier to a different gain.

Figure 2 Two instrumentation amplifiers provide increased input impedance in this adder circuit.

This system has several advantages over the simple op-amp adder. For example, each input has extremely high input impedance and has independent common-mode rejection, which the instrumentation amp connected to that channel determines. The higher the channel gain, the higher the common-mode rejection, and the smaller the resulting error. You can also easily add or subtract signals by using the inverting or noninverting terminals of the instrumentation amplifier, and the amplifier enables the use of differential input signals if you wish. Further, the distortion, noise gain, and bandwidth of each signal are independent of the other signals, leading to lower offset voltage, gain error, noise, and distortion. Figure 3’s THD+N (total-harmonic-distortion-plus-noise) plot demonstrates five times less distortion for the instrumentation-amplifier adder than that of the op-amp adder, even though the instrumentation amplifier has 1-MHz bandwidth and operates at 1 mA, whereas the op amp has 8-MHz bandwidth and operates at 4.5 mA.

Figure 3 The instrumentation-amplifier configuration shows improved THD+N at frequencies greater than 300 Hz.

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