Common_mode_rejection

Published:2009/7/24 20:40:00 Author:Jessie | From:SeekIC

There are many definitions for common-mode rejection (CMR), also known as common-mode rejection ratio (CMRR). One definition is the ratio of differential gain (usually large) to common-mode gain (usually a fraction). That is, the amplifier might have a large gain of differential signals (different signals at each input terminal or with one input terminal grounded and the opposite input terminal with a signal), but little gain (or possibly a loss) of common-mode signals (same signal at both terminals). Another definition for CMR is the relationship of change in output voltage to change in input common-mode voltage producing the change, divided by the open-loop gain (amplifier gain without feedback). No matter what definition is used, the first step to measure CMR is to find the open-loop gain of the IC at the desired operating frequency (chapter 1). Then, connect the IC in the common-mode circuit of Fig. 10-D. Increase common-mode voltage (at the same frequency used for the open-loop gain test) until a measurable output is obtained. Be careful not to exceed the maximum specified input common-mode voltage swing. If no such value is shown, do not exceed the normal input voltage of the IC. Then, find CMR using the equation. To simplify the calculation, increase the input voltage until the output is 1 mV. With an open-loop gain of 100, this provides an equivalent differential input signal of 0.00001 V. Then, measure the input voltage. Move the input-voltage decimal point over five places to find CMR.



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