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Re: Contd

Posted by RJM (A) on December 6, 2002 at 00:46:39

In Reply to: Contd posted by Jon Risch on December 5, 2002 at 20:05:07:

Thanks for clearing that up.

At the risk of seeming stubborn, though, this all seems backwards to me. Am I totally off or am I incorrectly applying voltage mode rules to current mode operation?

Normally, the noise is equal to the voltage noise plus the current noise multiplied by the source impedance. (non-mathmatically speaking) so, normally, for high source impedances you choose a JFET input op-amp with low current noise, and for low impedance sources you choose a bipolar, high current input stage with high current noise and low voltage noise, since the voltage noise component dominates.

As you said, the input impedance of this circuit is essentially just the impedance of the cartridge, so, I would have said, this is just an extrapolation of the low impedance case to near zero, and thus a low voltage noise is critical.

I think I see your argument: since we are now measuring the current and not the voltage, voltage noise is no longer relevent. I can't quite convince myself of that, yet. The formula for the noise at the output of the opamp remains unchanged regardless of the conceptual mode of operation, no? well ... just gimme a couple of days to mull this over, ok?

I'm in a similar position on the DC offset point, too. The input offset voltage is the output offset, referenced to the input, given the inputs are shorted together and to ground. (I thought) In the present case, this is essentially the working circuit, so I would have expected the DC offset to still be present.

Again though, I think I understand your counter-argument. The op-amp is now measuring current, so the offset is determined by the current offset at the inputs, not the voltage offset ... just gimme a couple of days to mull this over too, ok?

rjm

PS: in my example, yes I wrote uV instead of mV. I was going to write 400uV and then switched the numbers without converting the units.

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