I'm brainstroming topologies for a circuit that can perform the following:

1) Maximum differential signal can be as high as 300V, either polarity. (300 peak) AC or DC

2) For AC measuring, bandwidth required is at least to 100kHz, faster would be even better. Needs to be "good" beyond 100kHz to get a clean 100kHz measurement (low distortion, say <0.1%)

3) Measurement circuit must not load input signal. I.e., high impedance, how high? not sure yet but you get the idea.. the higher the better over frequency.

4) output will get scaled to a 10V peak (FS). 300V in = 10v out.

5) all the usual nice things like good linearity, low drift etc... I'd be willing to compromise on some of this though if a tougher problem is solved like the relatively high bandwidth.

Well, I think this is a tough nut to crack. I've considered high voltage input instrumentation amps and my own discrete versions of the same. I'm not too happy with this idea mainly because of the god-awful bandwidths of such contraptions..

Now I'm considering a discrete transistor high voltage front end stage. (I think High voltage mosfets are a must and biasing them is a drag.)

Who's got ideas? While I try and work this out, I'll entertain any thought fragment, or block diagram or comments. Much appreciated!

Oh, one more thing, this is a from scratch design so I do not have restrictions on things like power supplies available etc.. (although, I dont wanna have to make a 300v+ supply - thats ugly in many regards) and I'll consider doing whatever might work..

Thanks!

Originally Posted by Styx

voltage divider to pot the signal down (good quality resistors)
use a isolation amplifier to get a differential voltage signal

Yes, I thought of trying this. If I just use resistors, I don't think I can get to the bandwidth I need (the resistors will be large & LPF with parasitics)

However, I might be able to make impedance networks that are resistive only at DC. But the prospect of matching capacitors etc.. seems unappealing.

Originally Posted by zachtheterrible

i think you can find high voltage probes for o-scopes?

Thanks for the tip Zach. However, I am trying to design a high voltage measurement circuit. I may be able to borrow the ideas of these probes though. I think they might be resistors & differential FET amplifiers? Not sure..

Originally Posted by Nigel Goodwin

Originally Posted by Optikon

Originally Posted by zachtheterrible

i think you can find high voltage probes for o-scopes?

Thanks for the tip Zach. However, I am trying to design a high voltage measurement circuit. I may be able to borrow the ideas of these probes though. I think they might be resistors & differential FET amplifiers? Not sure..

A x10 probe is just a 9Mohm resistor, with a small trimmer capacitor across it - it forms a potential divider with the 1Mohm impedance of the scope and it's input capacitance.

I am aware of this... this is the usual passive 10x probe. I have a set of matched differential "high voltage" probes that provide 10x or 100x attentuation and the enclosure they connect to looks way too large to be just some resistors and caps.. also the power supply wire to it seems like a giveaway that it has an active circuit in it... not sure if it's what I need.

I also like the idea of active probes that can provide G-Ohms of input impedance for light loading.

But, if I use a regular divider, I'll need two tuning capacitors(one for each side) and they'll need to match and stay put over time.. (asking for a tweak from time to time will be unacceptable) not sure if I'll have trouble with such a simple arrangement.. though I like the simplicity of it!

Any one else with ideas? Anyone think I can get away with a high voltage discrete diff amp for input?

Thank you all so far!

Originally Posted by Ron H

The possibility of transformer coupling on the input implies that you don't need response down to DC. Is that correct?
If you can use a transformer, you can use a step-down transformer, which will preserve your bandwidth, give you the attenuation you need, and make it easy to have a high input impedance , since Zin equals Zload times the square of the turns ratio.

Input could be AC or DC.. I only mention transformer coupled input to illustrate that input circuit might not be referenced to ground. Though, connecting input side to ground and applying 300VDC is still a required case.

So far my two leading ideas are:

1) Go with resistor dividers with a "flatening network" to keep frequency resonse flat up to requirement. follow this with a sufficiently high speed diffamp. This works down to DC.

2) Try to build a high voltage FET input diffamp that can simply handle the 300V requirement directly. Flattening network might still be needed but input impedance would be much better than other idea. Biasing, making low drift & linearity might be difficult.

Any other thoughts to throw out?

Thank you all so far for the great input!

Originally Posted by Ron H

It would really be helpful if you could define the maximum load you can tolerate. I don't see how you can proceed without defining it.

Ron, my input stage will connect to a variety of voltage sources producing various transient effects (up to 300V peak) and are desirable to measure (accurately in a 100KHz bandwidth). Some of these sources will not have a low output impedance (how high? I dont know how to characterize that and it wont be constant anyways.) So for starters I will say that an input impedance (up to 100KHz) of less than 10 MegOhms will be unacceptable.

Right now a solution for these kinds of measurements is with an oscilloscope so my input requirements are very similar but I also desire an improvement in loading effects if it can be done. I will not be digitizing waveforms like a DSO does. Accuracy needs to be better than 0.1%. If better is achievable, then that will be determined by cost constraints..

I'll admit, alot of the detail is not worked out yet, but I'd like to throw away dead-end ideas as soon as possible.. but I've only got two ideas that feel like I can get there.

Any help = much appreciated!