# Experimentally determining the input impedence of an op amp

#### abhaymv

Joined Aug 6, 2011
105
Hello everyone,

What should I do to determine the input impedence of an opamp? I want to find the input impedence of a noninverting amplifier and it is the same as that of the opamp. I know to measure various parameters like slew rate and offset voltage, but I can't find anything on Input impedence. I know it's in the datasheet, about a few megaohms, but what circuit can I use to determine it?

#### abhaymv

Joined Aug 6, 2011
105

The impedence w.r.t GND I think. I found a circuit that I think works based on voltage divider after one hour of searching:
http://my.safaribooksonline.com/boo...asics/measurement_of_operational_amplifier_pa

The circuit is at the end of the page. However, the statement
"
With the switch open, Rs = 0, some amount of input is given to obtain a convenient value of V0. Rs is adjusted till V0 falls to
. Then Rs = Rin."
confused me. How is Rs= 0 when switch is open? Is it Rf=0?

#### Audioguru

Joined Dec 20, 2007
11,249
Don't you think the resistors that bias the opamp input affect the input impedance?

If you want a very high input impedance then use an opamp with Jfet inputs like a TL081 or TL071. The bias resistors can be bootstrapped or be a very high value.

#### MrChips

Joined Oct 2, 2009
22,099
Get a signal generator and set it to a desired frequency (say 1kHz) and amplitude (say 100mV).

Connect a resistor in series between the signal generator and the input of the amplifier.
Measure the amplitude at the input of the amplifier.
Adjust the value of the resistor until you get half the amplitude (50mV) at the input.
That resistance value would be equal to the input impedance of the amplifier.