AD620: Decrease in CMRR

Document created by analog-archivist Employee on Feb 23, 2016
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I use the AD620 for ECG Measurements. For coupling in an additional Impedance
Plehtysmographie Signal (AC, 100 kHz, 2Vpp) I use two resistors 10 K Ohms in
series with a 0,022uF Capacitor from the Plethysmographie Signal Generator to
the +Pin of the AD620 and the same circuit for coupling out the Signal from the
inverting input of the AD620 to GND. This circuit decreases the CMMR for 50Hz
extremely, why?

P. S. all Resistors and Capacitors 1% tolerance

 

The tolerances of the RC input structure are causing the 50Hz common mode
signal to be converted to a differential signal and reducing the CMR. If you
calculate the max and minimum attenuation of the RC High pass filter at 50Hz
for a 1% tolerance, the minimum attenution is 0.140 and the maximum attenuation
is 0.128 . A 1V 50Hz common mode input signal applied to the input of the RC
hipass filter will produce a 12mV differential signal at the inputs to the
AD620, the AD620 will amplify it as if it were any other differential signal.

Assuming a differential gain of 1, the CMMR (common mode gain / differential
mode gain ) for the system as a whole (AD620 and the RC input) is reduced to
38dB. From a typical value of 90dB for the AD620 alone.

The solution is to increase the size of the coupling capacitors such that the
hi pass corner frequency is well below 50Hz, you can then rely on the common
mode rejection of the AD620.

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