Oscillioscope pre amp.

[ashaw]

I do not know where to post this. However as the problems I will face are similar to those faced with MC preamps, I thought I would post here.

I want to design a oscillioscope pre amp this will need to have the following specifications:
A selectable gain of 10,100,1000.
A bandwidth of 1MHz with a gain of 100.
Low THD.
Tolerant of 20V across the input.
An input impedance of 1MΩ||20pf.
Noise as good as any phono preamp.

Differential inputs would be useful but not crucial.
I was thinking of using a push pull Jfet amplifier. Powered by
Batteries however I would like some advice.

Regards
Alexis.

[thaumaturge]

Hey Alexis, why not learn from the best? Download a couple of Tek scope plug in schizmos from the boat anchor archive and see how they did it. However one common feature I recall from direct experience was nearly all of those I worked on used top quality mylar or polypropylene feed through caps for AC coupling. A quick glance of a 3A74 plug in sitting by my feet shows a 0.1uF on each of the four channels. I recall 465's had similar. Another high end plug in would be a Tek 7A26.
For op amps you might look at TI OPA1632 full differential and analog devices AD605 variable gain op amps.

Doc

[ashaw]

The problem is that the sensitivity of those amps is too low for my purposes. I want to use this preamp to reach 10nV per division sensitivity.

[ashaw]

Sorry that last line was a typo. I meant to say 10μV per division.

[thaumaturge]

To pull that off you'll likely have to go with a chopper stabilized auto-zero op amp. See Analog Devices: Analog Dialogue: Demystifying Auto-Zero Amplifiers-Part 1 and http://www.analog.com/static/importe...als/MT-055.pdf for more info. These unfortunately typically suffer from limited bandwidth (~200khz) but there may now be much faster models/variations/subs available. You might take a look at a Tek 7a22 diff plug in as a ref.
Doc

[blakkshepeaudio]

Originally Posted by ashaw

The problem is that the sensitivity of those amps is too low for my purposes. I want to use this preamp to reach 10nV per division sensitivity.

Sorry that last line was a typo. I meant to say 10μV per division.

[phonomac]

Hello there,
Take a look at the Tektronix 7A22 - differential input, 10μV per division, 12V pk-pk measurement, DC-offsettable to 55V, safe to 200V.

Hope this helps.

Regards

Angus

[ashaw]

can this be used with a modern scope?

[luvdunhill]

http://tangentsoft.net/elec/lnmp/

[jackinnj]

Originally Posted by ashaw

can this be used with a modern scope?

The 7xxx line of scopes from Tek are still regarded as the "ne plus ultra" of analog scopes, and the manuals for the plug-ins are pretty generally available. There is a really active Yahoo group as well.

It might be less expensive for you to purchase a 7A22N or 5A22N and have it shipped to NZ.

One thing you want with a scope amplifier of this sensitivity is the ability to select bandwidth, and as mentioned previously, the ability to null the DC component.

Tektronix also makes a low noise differential amplifier plug-in, the ADA400A. It's still in production -- 5uV sensitivity to 1MHz with selectable bandwidth. Some of the Tekscopes will power this preamp directly.