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Measurement of the input current noise of an OPAMP

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frikau00
Posts: 2
Joined: March 15th, 2017, 5:40 am
Country: Austria

Measurement of the input current noise of an OPAMP

Post by frikau00 » March 15th, 2017, 5:50 am

Hello,

I´m searching for a measurement device for measuring the input current noise of the OPA376 from Texas Instruments.
The input current noise is specified by 2fA/sqrt(Hz) typical at 1kHz.
So I need a current measurement device, which is able to measure AC signals at 1kHz at very low current levels.
If you can give me a hint, I would appreciate.


Best regards,

Kaufmann Friedrich

Norbert W
Keithley Applications
Keithley Applications
Posts: 227
Joined: October 21st, 2010, 7:18 am
Country: Germany

Re: Measurement of the input current noise of an OPAMP

Post by Norbert W » April 18th, 2017, 5:34 am

Hello,
my assumption is that the op-amp with a feedback resistor (or network) in inverting mode is used to
inherently convert the current noise to be available as voltage noise on the amplifier output. You may
want to contact the op-amp vendor for details of his recommended method.

frikau00
Posts: 2
Joined: March 15th, 2017, 5:40 am
Country: Austria

Re: Measurement of the input current noise of an OPAMP

Post by frikau00 » April 21st, 2017, 2:14 am

Hello,
I know the recommended circuits from the op-amp vendor. It is a non inverting amplifier (output connected to inverting input(minus)) and a high value resistor at the non-inverting input (plus) to ground. If I use a 5GOhm resistor at the non-inverting input, the current noise of the resistor is 1.8fA/sqrt(Hz) and this current has to be considered too. But the problem is the input capacitance of the op-amp, which is 13pF and this will create a lowpass filter with a corner frequency of 2.4Hz. But I would like to measure at 1kHz and this lowpass will attenuate the noise signal.
So I'm searing for a measurement device, which is able to measure the input current noise of the op-amp having a low input impedance.

I thought, that Keithley has any measurement device for measuring the input current noise at that low level at a signal frequency of 1kHz. But it seams, that there is no such device available.

Actually I know from Linear Technology, that they use for the LTC6240 for the input current noise in the data sheet a theoretical value sqrt(2 * Ib * e), where e is the electrical charge of an electron (1.6E-19 As). But I don't believe that theoretical results, I would like to measure the current noise of the op-amp. I know, that there is clearly the shot noise due to the quantization of the electrical charge, but I suppose there are further effects, which produce noise but which are not considered by the theoretical results. For example there are clamping diodes to prevent ESD failures at each input, which will add noise.

best regards,
Kaufmann Friedrich

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