How Do I measure Op Amp Input Voltages?

[CJ]

Is there a way I can measure the voltages on pin 2 and 3 without disturbing the circuit? Even my best meter does not have enough internal resistance to keep from loading, especially on fet input op amps.
Thanks!
cj

 

[bcarso]

Not easy without a very specialized buffer probe. I have been known to roll my own in some cases. Essentially, a naked input offset-corrected FET source follower. Even that won't work for super low input bias current amps like the National CMOS series, but they aren't very good for audio to begin with.

Typically, one simply infers the input voltages from observing the other low-Z ones (input , feedback network input, output).

 

[jhaible]

What exactly do you want to measure?

Artefacts that come from the opamp's own bias currents?

Or dynamic behaviour, glitches etc, that come from the opamp's limited open loop gain at high frequencies?

If you want to measure the latter, IMO your probe doesn't have to be high Z compared to the naked opamp inputs - only high Z compared to the feedback resistors.

JH.

 

[Mark Burnley]

CJ,

There is a NatSem app note pdf somewhere on t'internet that is "measuring op-amp specifications" or something similar. It shows you how to measure offset voltages and currents, may be worth a google.

Mark

 

[bcarso]

That material is good for characterizing the amplifiers although doesn't address measuring them in-circuit iirc. It sounds like CJ wants to look at static and dynamic voltages in-circuit, which requires something like the probe described unless the surrounding impedances are quite low and the frequencies not too high.

Tek and HP have made some active probes that could approach the job, but I would counsel a homebrew with a FET follower and maybe a bootstrapped drain for lowest input C. The gate could simply float until connected to the circuit point of interest.