Oscillioscope pre amp.

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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.
 
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
 
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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.
 
Hello there,
As Jackinnj says, the 7Axx amplifiers are plug-ins for the Tektronix 7xxx series scope mainframes. The 7A22 has individually adjustable LF and HF filters to tune the measurement bandwidth to maximize sensitivity/minimize noise.

It would be difficult to use one without a 7-series mainframe, but the circuitry might be instructive. Look at Johnson's Home Page, select Tektronix, and you will find a synopsis of most of the range, plus links to manuals.

I don't know the later ADA series.

Regards

Angus
 
Jack, thanks for the ADA recommendation. What sort of uses does the OP have in mind for such a setup?

Would it be possible for instance with the ADA to remove the DC component of say a PSU rail and look at the resulting noise and quantify said noise to differentiate various regulation circuits? 5uV is great, but it looks like the noise figure is potentially 5x this.
 
can this be used with a modern scope?

The 7A22 or 5A22, no. The ADA400A maybe. To quote off a dealer site "The ADA400A is powered directly from TDS400, TDS500, TDS600, TDS700, TDS5000, TDS7000 (TEKPROBE), TDS7000 (TekConnect) w/ TCA-1MEG Series oscilloscopes. It may also be powered by a 1103 Probe Power Supply."

They also want $1700 for the ADA400A (not including a 1103 power supply if needed.) Depending on your budget, that might not be an option. But it might be an option to pick up any of the three (7A22, 5A22 & ADA400A) on Ebay or some such and maybe just improvise them as a front end amp.

The 7A22 for instance takes +/-50V & +/-15V supplies off any 7000 series mainframe and outputs a lovely differential signal. Not too tough to interface.
If nothing else you might download the op manuals for them and see if they wuld really apply to your application.


Doc
 

The AD797 input impedance is too low for his listed criteria (1 Meg input impedance). You would want a JFET front end, and a low capacitance one at that. (Dennis Colin's design is on the AudioXpress website, but you'd probably want to use a different JFET than the 2SK170 if you truly have to measure in the MHz). You could, as well, use a JFET operational amplifier like the OPA637.

With regard to the "antiquity" of the 7A22N circuit -- looks pretty good for a front end -- http://bama.edebris.com/manuals/tek/7a22(2)/

EDIT: There's a seller of the female mating connectors for the Tek plugins on EBay from time to time. Later today I'll take some pix of the ADA400 which I own -- I bought it in the wake of the internet bubble several years ago with 2 power supplies!
 
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Do you think that a diy version could be made with a jfet buffer based on the BF862 and a few stages of op-amps? Input protection is obviously importantas well. Is there a service manyAl dot that device. I am thinking about getting a pair of 7A22 and a pair of 7A13 and trying to figure out how to interface them with my scope. It looks like there actually was a device that let you do this made by a 3rd party so perhaps it is possible.
Though it would be nice to design a modern version that is not so big... But that may have to wait.
 
Cool!

Do you happen to know what the insides of the high voltage differential probes look like? The attached is the maximum voltage that my Tek CFG280 can output into a Tek P5210, and I'm wondering if this unit might be adjusted for better looking waveform, or if perhaps the issue is the very low input voltage. I haven't quite got around to trying it on the plate of a tube or something, but I have plans to.
 

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