OP-Amp disable

Of course, the polarity will change, but that's no problem.

Cheers

The 'disable' configuration inverts (or rather doesn't invert) the signal, right?

To me, the "shutdown output leakage" spec (100nA, typ) says the output is safely tri-stated in shutdown.

It's not clear for the inputs-the internal schematic isn't detailed enough. You might try it--drive a shut-down part thru 10k, check for input clamping / distorted output.

A little relay?

What's the source resistance and feed back resistance?

George H.

Yes, that's firmware-fixable.

Cheers

Harder to retrofit compared to a track cut and short piece of wire.

This uses 100R source, 1k feedback.

Cheers

Except if you still want the good noise performance at unity gain. How about a nice relay?

Cheers

Phil Hobbs

So when the opamp's disabled you've got ~1.1k of source imepdance...

That 'defeats' the nice low noise opamp... what's after the LT6200? Is it low noise too?

George H.

Noise at unity gain doesn't really matter. There are PGA stages after this which get wound right down when the input is high. Normally, the input is a low level from a very long cable, but a short cable is used for lab testing which then gives too high a level for the fixed gain front end.

The LTspice simulation works - often things like shutdown aren't modelled.

Cheers

Interesting thought.

One could moderately light up a good LED with the bias current of that op-amp.. 50uA worst-case... wow. There is a heavy price to be paid for the high GBW and

Noise at unity gain doesn't really matter. There are PGA stages after this which get wound right down when the input is high. Normally, the input is a low level from a very long cable, but a short cable is used for lab testing which then gives too high a level for the fixed gain front end.

Cheers

Good spot, I'd clocked the supply rail diodes but hadn't noticed the diode pair across the inputs. That will kill it, damn.

Now where did I put that relay catalogue?

Cheers

Schematic? ...Jim Thompson

Nah, Spehro's pointed out the flaw, namely the internal back-to-back diodes between the inputs :-(

Cheers

Yep. I just opened the datasheet and saw the same thing. Bipolar doesn't do all that well in shutdown situations... the output is fine, but the input screws you over. ...Jim Thompson

Is your configuration such that the +Input of the OpAmps go to ground, and you're driving thru a resistor to the -Input with a feedback resistor from Output to -Input? ...Jim Thompson

Yes, except single supply, so +ve input to half supply instead of ground, and signal capacitor coupled to input resistor.

Cheers

How about disconnecting the noninverting inputs - i.e. using an analog switch to connect it to ground 'as needed'?

Yes, Disconnect +Input from (virtual) ground, but also then connect