Know of a good high voltage, very low load probe?
Know of a good high voltage, very low load probe?
(OP)
(If this isn't the right forum for this let me know)
I'm looking for a probe that can handle +/-100V with very low capacitve loading (<1 pF). Rise and fall time of about 1mS is acceptable.
I have an Agilent 10076A (66 Megohm // 3pF) that doesn't quite do it so I'm searching for something better. I'm probing a pixel on a TFT (thin-film-transistor) panel; did you ever try probing a 0.3pF cap that gets charged through a MOSFET with a RDS-on of about 1 megohm?!?
I have a proprietary, two of a kind on the planet, probe with a leakage current of about 2 pA and lots of temperature drift. I'd rather not put that one on the production line...
I'm looking for a probe that can handle +/-100V with very low capacitve loading (<1 pF). Rise and fall time of about 1mS is acceptable.
I have an Agilent 10076A (66 Megohm // 3pF) that doesn't quite do it so I'm searching for something better. I'm probing a pixel on a TFT (thin-film-transistor) panel; did you ever try probing a 0.3pF cap that gets charged through a MOSFET with a RDS-on of about 1 megohm?!?
I have a proprietary, two of a kind on the planet, probe with a leakage current of about 2 pA and lots of temperature drift. I'd rather not put that one on the production line...





RE: Know of a good high voltage, very low load probe?
They have a 1,000:1 voltage reduction and are basically a simple voltage divider with a 1,000Meg in series with a 1meg resistor. I have no idea what the input capacitance or rise-time would be.
Apart from that, a proper FET oscilloscope probe might do the job, but the safe maximum input voltage may be rather limited.
RE: Know of a good high voltage, very low load probe?
take a look at a Tektronix P6015 freq compensated probe or there newer replacement model and see if their specs meet your needs.
-elf
RE: Know of a good high voltage, very low load probe?
The Tek 6015 is a physically big HV probe and has 1000:1 attenuation, and a slower rise time & bandwidth.
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If we learn from our mistakes,
I'm getting a great education!
RE: Know of a good high voltage, very low load probe?
Your modest rise-time needs make a multi-megohms passive resistor a viable solution. The time constant of the input is about 1 Megohms times 30 pF - or around 30 microseconds. Adding a resistor will not increase that time constant, it will decrease it (or leave it unchanged) since the added resistor is in parallel to the internal resistance. So you can add as much resistor as you want without having a longer time constant.
If you put a 100 Megohms (probably need to use ten 10 Megohms) resistor as close to the TFT pixel as possible and make the cable to the scope input very short, you will have something that might work. The sensitivity of a normal scope can usually be set to 2 or 5 mV/div and with this "probe" you will have 200 or 500 mV/div.
Why shall the connection be short? Simply because a metre of wire will add something like 100 pF capacitance to your input and turn your time constant up to more than 0.1 ms. Still acceptable for your application - but unecessary.
Using a 1 Gohms resistor will reduce loading even more and you will still have a less than 0.1 ms time constant. Remember that you need a very good screen - right down to the probe tip's last millimeter - to avoid any stray voltage pick up.
Gunnar Englund
www.gke.org
RE: Know of a good high voltage, very low load probe?
Totally agree that +/- 100V does not need an HV probe. Most of the commercial low capcitance probes are limited to about +/-20 to 30V, which makes them unsuitable.
----------------------------------
If we learn from our mistakes,
I'm getting a great education!
RE: Know of a good high voltage, very low load probe?
Gunnar Englund
www.gke.org
RE: Know of a good high voltage, very low load probe?
I've used something similar to poke sharp sticks into the guts of integrated circuits.
TTFN
RE: Know of a good high voltage, very low load probe?
BTW - Fluke 40Kv probe (80K-40) - While it is great for DC, Fluke says the frequency response (-3dB) is only about 200 Hz, so they don't recommend using with any scope.
RE: Know of a good high voltage, very low load probe?
TTFN
RE: Know of a good high voltage, very low load probe?
Gunnar Englund
www.gke.org
RE: Know of a good high voltage, very low load probe?
RE: Know of a good high voltage, very low load probe?
TTFN
RE: Know of a good high voltage, very low load probe?
RE: Know of a good high voltage, very low load probe?
There are, however, SEM's modified for detecting voltages on circuits.
TTFN
RE: Know of a good high voltage, very low load probe?
RE: Know of a good high voltage, very low load probe?
Suppose you use a very high value series resistor right at the probe tip as suggested previously, and then switch the other end electronically. When it was "off" input loading and leakage would be very low. When ot was "on" the input voltage could be sampled at a very small duty cycle. That would go towards solving the constant circuit loading problem.
The output signal might be reconstructed with a sample and hold circuit.
This is not going to be easy to implement, but it is just suggested as a line of possible further thought.