Hi All,

I am going to use programmable gain amplifier AD8251 in a data acquisition system.

I want to know how to measure input impedance and capacitance of AD8251 when it present in circuit.

Please anybody suggest me as soon as possible.

Thanks,

Abhratanu

Hi Abhratanu,

Measuring the input impedance and input capacitance of an instrumentation amplifier can be accomplished by adding a large resistor, approx 1 MEG, in series with one of the instrumentation amplifier inputs.

With this circuit setup, apply an AC signal approximately 100 mVp-p and measure the gain of the AD8251. Increase the frequency until the gain become -3dB of the expected value. This -3 dB frequency is the equal to 1/(2*pi*R*C) where R is the 1 MEG and C is the input capacitance.

To measure input impedance, use the same 1 MEG resistor, but apply a DC value of 10 V. Measure the drop across the resistor. Then apply -10 V to the input and remeasure the voltage across the resistor.

The input impedance is then a calculation from these two measurements. If you think of the input impedance as the change in input voltage divided by the change in input current then we get R = 20V / (meas1/1MEG - meas2/1MEG).

Does this answer your question?

Regards,

Paul