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What is the AC input impedance of the ADA4817

Question asked by lsevero on Sep 28, 2017
Latest reply on Oct 11, 2017 by JinoL

Hi,

I want to use the ADA4817 opamp as a buffer to measure a high output impedance filter (my DUT) and to obtain the transfer function using a spectrum analyzer.

The main idea is to design a device that has high input impedance and is able to drive a 50ohms load.

I'm trying to use the ADA4817 in non-inverting configuration by using two 510 ohms resistors.

However, I've checked in my measurements that the non-inverting input is not a high impedance input. It has about 10kohms of input impedance at 1MHz. As the amplifier has a JFET input I thought that it has an impedance over 1Mohms, but it is not!

As the amplifier has a JFET input I thought that it has an impedance over 1Mohms, but it is not!

 

In the datasheet, there is only the information about input resistance of 500Gohms and input capacitance of 1.2pF.

 

How can I obtain the Analog Devices measurements data of the input impedance vs frequency for non-inverting configuration?

 

Attached is my circuit schematic. It is a fully differential buffer using an ADA4817-2 and a balun to perform the differential to single-ended conversion.

 

Can someone help me?

 

Thank you.

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