LM7322: LM358 replacement in a differential attenuation circuit

Hello Andrew,

Input bias current should not be a factor because the inputs see the same the same impedance. The offset current would create a DC shift, not an AC attenuation. Have you tried more than one sample of LM7322?  

Thanks, I haven't yet but the chip is part of ten bought from RS, so are genuine and I'm not expecting a dud.

A DC shift would be factored in, I measure it as part of start-up and remove it in software from the AC reads. I'm not seeing any DC shift at the output between either chips, or very small, which suggests it's something to do with the attenuation.

Could be time to shift to an integrated differential like INA142!

Hi Andrew,

where is the input signal coming from? Is it fully floating? Or is it referenced to the signal ground of circuit? Can you expand your schematic also showing the input voltage source?

Also, J6 and J8 are confusing me. What are they for? And what is connected to R18?

What AC frequency range are we talking about?

Kai

It's fully floating, ac1/ac2x goes straight to a dynamo.

J6/J8 are just pads for tuning the resistors to be equal, otherwise disconnected.

Frequency 10-200Hz, though for normal use it's 40Hz. Slow, in otherwords.

It is possible that the LM7322 was damaged by electrostatic discharge. Please try another unit.