Question 11.11: Determination of the Minimum CMRR Specification Find the min...
Determination of the Minimum CMRR Specification
Find the minimum CMRR for an electrocardiograph amplifier if the differential gain is 1000, the desired differential input signal has a peak amplitude of 1 mV, the common-mode signal is a 100-V-peak 60-Hz sinewave, and it is desired that the output contain a peak common-mode contribution that is 1 percent or less of the peak output caused by the differential signal.
Since the peak differential input is 1 mV and the differential gain is 1000, the peak output of the desired signal is 1 V. To meet the specification required, the common-mode output signal must have a peak value of 0.01 V or less. Thus, the common-mode gain is
A_{\mathrm{cm}}=\frac{0.01\mathrm{~V}}{100\mathrm{~V}}=10^{4}
(Therefore, the common-mode gain actually amounts to attenuation.) Now, we can find the CMRR by application of Equation 11.25:
\mathrm{CMRR}=20\log{\frac{\left|A_d\right| }{\left|A_{\mathrm{cm}}\right| }}=20\log{\frac{1000}{10^{-4}}}=140\mathrm{~dB}
Hence, an electrocardiograph requires a very good CMRR specification.