RE: Differential voltage probes 1

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>

Hi,

>Could you suggest "good" op-amps to use?  I have not worked with op-amps 
>recently and I am not sure what the modern good ones are these days.  I 
>could go then look them up and work on it.  The supply rails and all could 
>just be voltage regulated.  Given the prices of "real" diff probes, we 
>have a lot of room to work with!!

I note the THS4012 goes to 300MHz and is a nice match to the THS4150 diff 
amp at 150MHz.

http://hot-streamer-dot-com/temp/ths4012.pdf

http://hot-streamer-dot-com/temp/ths4150.pdf

They pop right into the usual instrument amp circuit:

http://hot-streamer-dot-com/temp/DiffProbe11.gif

It appears they could still run off two 9V batteries with regulated supply 
rails and lots of NPO bypass caps...

The TLC4502 if far slower but does have that 10uV offset...

http://hot-streamer-dot-com/temp/tlc4502.pdf

It is pretty easy to pick out these chips since I just choose the most 
expensive ones they have :o)))

But messy supply voltage for our use...  Probably best to go with very high 
frequency parts and just adjust out the offset with a pot.  Of course, 
getting to 100MHz requires more layout "art" than just getting fast 
parts...  Probably looking at a $75 probe here with vastly improved 
performance over the simple one.  Everything is surface mount now...  I am 
thinking +-1000 volts peak on the inputs with about 10kV safety 
margin.  Biggest problem is keeping the HV resistors looking like 
"resistors" at high frequency.  I bet we would have to put in low pass 
filters to stop it when things go weird at some high frequency point.

So I am wondering what a good max voltage number and top frequency should 
be?  We could push the voltage up higher I think, but the frequency will 
probably just turn out at around 5 to 10MHz.  The high resistance dividers 
are the problem.  I note Tek drops Z wildly at high frequencies to get 
higher frequency response...  Note sure that is a great idea...

This type of probe would drop CMRR and all that to "don't care"...

Cheers,

         Terry