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Instrumentation amplifier combination extends CMRR frequency range

The main function of an instrumentation amplifier is the amplification of differential signals while rejecting unwanted common-mode interference. This function proves particularly useful for extracting low-level transducer outputs from noisy environments.

A common application for this function is rejection of common-mode noise associated with bridge circuits that use ac excitation voltages. Most in-amps reject low-frequency common-mode interference well, but their performance typically degrades quickly as the frequency of the common-mode interference increases. For these ac applications, an ability to extend the common-mode rejection ratio (CMRR) over frequency is a plus.

The composite instrumentation amplifier, Figure 1 , amplifies a small differential signal while rejecting high-frequency common-mode interference.



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Figure 1: Composite instrumentation amplifier circuit

The circuit can be used as either differential-in/differential-out or differential-in/single-ended out. The low-level input signal will be “gained up” by a factor of 4, while the common-mode signals are greatly attenuated, even at high frequencies. The circuit has a differential output, so it can be used to drive standard differential-input A/D converters, providing a precise output with low distortion and excellent CMRR over frequency.

The circuit contains two AD8222 dual instrumentation amplifiers. These low-cost ICs contain two matching amplifiers per package. The amplifiers are connected such that one output is in phase with the input and the other output is 180° out of phase. The outputs of the first stage go through a second stage, with the output taken differentially at the output of the second stage. With no external gain resistors, the signal at the network output is gained up by four. Other gains can be set by two external resistors. Resistor matching is critical to balance positive and negative outputs.

The ADC's reference voltage is connected to the reference input of the second AD8222. Due to the ratiometric connection, system performance will remain unaffected when the reference drifts.

Figure 2 shows the outputs given a 100 kHz differential input and a gain of 4. The input signal is on top, positive and negative outputs are in the middle, and the differential output is on the bottom.



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Figure 2: Photo of actual performance. Top is input signal, single-ended, 200 mV peak-to-peak at 100 kHz; middle is the positive and negative output; bottom is the differential output, 800 mV peak-to-peak.


Figure 3 shows the measured CMRR over frequency for this circuit, and compares it to the performance of a standard instrumentation amplifier.



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Figure 3: Performance photo. Top is CMRR of the system at gain of 4; bottom is uncorrected CMRR at gain of 1.

The CMRR error for the matched channels in the AD8222 is the same. It will thus appear as a common-mode signal to the second stage, and will be further rejected. The output CMRR will be very small, reaching levels of nearly 80 dB at 70 kHz, compared to 80 dB at 5 kHz for standard amplifiers.

About the authors
Chau Tran and Paul Blanchard are with Analog Devices, Inc. and can be reached at chau.tran@analog.com and paul.blanchard@analog.com, respectively.

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