Re: Further thoughts on the opamp

Posted by Jon Risch (B) on December 5, 2002 at 18:00:47

In Reply to: Further thoughts on the opamp posted by RJM on December 5, 2002 at 01:40:48:

One more time.

It is not the input offset voltage that is critical for a current sensing virtual ground input, it is the current offset.

For the noise, it is not the voltage noise that is critical, but rather the current noise.

The AD797 has a HUGE amount of input bias current compared to any FET input op-amp.

Further more, if you want to get all the gain of an MC head amp (typ. 36-40 dB), AND a typical phono preamp (40 dB at 1 kHz), and a preamp (typ. gain 20 dB), all rolled into one box, then I strongly recommend that you not try to get it from just two op-amp stages. I certainly would not try to get 60 dB, or even 45 dB of gain from just one op-amp.

This kind of MC level to preamp out level really should have the gain spread out over three op-amps if the typical 106 dB of total gain is desired.

Even 90 dB is too much for just two op-amps.

Just to make sure:

THE AD 797 IS NOT OPTIMAL FOR CURRENT SENSING VIRTUAL GROUND INPUT CIRCUITS DUE TO VERY HIGN BIAS CURRENTS AND RELATIVELY HIGH INPUT OFFSET CURRENTS.

Jon Risch

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Follow Ups Full Thread

Topic - A current input MC phono preamplifier project - RJM 06:37:01 12/4/02 (16)

Re: A current input MC phono preamplifier project - jcarr 09:18:14 12/11/02 (0)

Induction & voltage/current debate rebuttal - Claude Abraham 12:26:56 12/10/02 (0)

Further thoughts on the opamp - RJM 01:40:48 12/5/02 (1)

Re: Further thoughts on the opamp - Jon Risch 12/5/0218:00:47 12/5/02 (0)

My experience with current sensing MC phono pre's - Jon Risch 19:46:39 12/4/02 (11)

Re: My experience with current sensing MC phono pre's - RJM 00:59:21 12/5/02 (7)

Contd - Jon Risch 20:05:07 12/5/02 (5)

Re: Contd - RJM 00:46:39 12/6/02 (4)

MC phono cartridge is a Voltage Generator: e=B*l*u - jcox 11:24:52 12/7/02 (0)

Re: Contd - Jon Risch 08:23:52 12/7/02 (2)

Re: Contd - RJM 07:21:01 12/9/02 (1)

I actually went with LT1028 over AD797 in strain gage amplifiers - jcox 08:54:16 12/9/02 (0)

Re: My experience with current sensing MC phono pre's - Jon Risch 10:26:45 12/5/02 (0)

Re: My experience with current sensing MC phono pre's - jcarr 23:55:23 12/4/02 (2)

Re: My experience with current sensing MC phono pre's - Jon Risch 10:36:30 12/5/02 (1)

Re: My experience with current sensing MC phono pre's - jcarr 12:16:48 12/5/02 (0)

Follow Ups

Re: Further thoughts on the opamp - Jon Risch 12/5/0218:00:47 12/5/02 (0)

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