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OK there's just too many opamps. Help me choose for a difficult audio job?

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(from the "Why doesn't this product already exist?" department)

I'm building a box to allow use of oscilloscope probes with my audio test equipment.  Because it's a front end for test gear I'd like to be as clean and quiet as practical.

My thought is to passively terminate the probe with 1M and use an AC coupled unity gain buffer to send the signal along.  But this buffer has gotta be clean and it's gotta be quiet.  We're feeding test equipment.

I'm hoping to use an opamp for this but I'm numb from the sheer quantity available.  I need something that is simultaneously really quiet with a 1 Meg input Z, happy used as a follower, & has solid audio performance.  It's the noise shopping that's difficult, the Internet does not readily hand over noise current info.

I'm thinking one of the 327 offerings from TI/Burr Brown but damn which one?  It's an embarrassment of riches these days.

What would you use in this application?  And thank you for your input.

I would go for a JFET input opamp because with the 1M input impedance current noise is more important than voltage noise. I think you already understand that and the OPA827, which I think you meant, looks like a good alternative with 4nV/Hz^0.5 and 2.2fA/Hz^0.5.

It's not just the selection of a magic part but having a noise free supply, maybe differential output. I'd guess for experimentation any low noise amplifier will do.
You could also start with an OPA140: Low power consumption, pretty low noise, precision (THD=0.00005% buffering 1 KHz 3.5 V RMS), high input impedance. It needs input protection.

Regards, Dieter

Hi cvanc,  :D

Both the OPA140 and also the OPA827 are nice opamps.
Both have fet inputs and the characteristics are almost the same.

If you want a little more bandwidth, choose the OPA827.
Do choose your resistors well that determine the gain.
You probably need a small capacitor from the output to the inverting input.

Let the stage amplify 10x, that compensates for the probe attenuation, you will still keep about 500KHz bandwidth flat with the OPA827.
It does depend on how well you perform the feedback resistor compensation.

Another tip, include at the output a series resistor of 49.9 Ohms driving the coax to your scope or measuring instrument.

You can protect the input with two 1N4148 diodes from the +input to the supply lines.
The best is if you connect the diodes directly on the + input and from there with 100 Ohm to your input circuit.
The 100 Ohm limits the peak currents through the diodes.

Of course decouple the supply lines well, with at least 100uF.
That seems a lot , but just think where the energy goes when you overload the input, yes that's right depending on the polarity of the signal the supply voltage is lifted.

If i have some more time this evening i want to draw a schematic, so you have something to start with.

Kind regards

I would consider the OPA1641:  relatively similar to the OPA140 with AC performance, but with some extra circuitry, so that the input capacitance does not change much with the DC level.
Many JFET input amplifier have a much stronger voltage dependence of the input capacitane.
The voltage dependence of the input capacitance can be an issue with some high impedance source (e.g. a 1:10 probe).


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