DGK Electronics

I’ve had the need  for a high-voltage differential probe for a long time. Mainly for probing mains connected switch-mode power supplies and such. I looked in to buying one but they are fairly expensive for what they are. You can get 25 MHz bandwidth Chinese ones for around 150 € but anything from Agilent/Tektronix/Fluke will quickly run up to multiple hundreds. I looked in to making one and they seemed fairly straightforward.

Design

Pretty much all the HV differential probes are made out from discrete op-amps forming a basic instrumentation amplifier:

There are integrated single-chip solutions for instrumentation amplifiers but they are usually designed for different kind of applications requiring high DC accuracy and common-mode rejection ratio (CMRR) and very little bandwidth. I tried to find a suitable integrated instrumentation amplifier but I really couldn’t find any that would have a bandwidth of more than a few MHz. Not really good for a differential probe which needs to have a bandwidth of at least 20 MHz or so for most applications.

Finding suitable op-amps was still fairly challenging. The devices need to have good CMRR, lots of bandwidth and high slew-rate. The best candidates I found were LT1818 and LT1819 (dual version of LT1818), which have a gain-bandwidth product of 400 MHz and 2500 V/us slew-rate. I was a bit worried about the fairly high input bias current of +-8 uA though. I ran a bunch of simulations with LTspice (which already had models for these op-amps) which were very encouraging, indicating a -3 dB bandwidth of around 150 MHz for the design. Simulating CMRR seemed to give identical results with different op-amp models so I didn’t really trust them, it seemed like the models are very simplified in terms of CMRR.

The front-end of the HV differential probe is fairly simple, consisting of basically two resistive voltage dividers. The front-end also has capacitive dividers in parallel with the resistive dividers to adjust the AC response of the probe. There is also a 200 ohm trimmer pot to adjust the balance (which results in DC offset in the output) between the sides. The probe is designed for 100:1 (-40 dB) gain and 500 Vrms.

HV differential probe schematic

Construction

The most critical thing in this type of a probe are the resistive dividers. Any difference between the two dividers gets directly converted to a common-mode signal which is undesired. For the dividers I used 0.1 % Panasonic ERA series resistors. There are a bunch of resistors in series to withstand the voltage and also reduce the error in resistance. I didn’t bother measuring and adjusting both dividers to match each other but that is a possibility.

The inputs are shorted together in this picture for the CMRR measurement. I had one small bug in the layout; The supply pin for the single op-amp was connected on the wrong pin. Easily fixed by just bridging the adjacent NC and supply pins with solder.

The PCB is designed to fit in to a Hammond 1455 series aluminum extrusion enclosure. Haven’t gotten around to enclosing it yet.

Measurements

I measured the bandwidth and the CMRR of the probe using my Rohde & Schwarz SME 03 signal generator and R&S FSIQ spectrum analyzer by feeding 10 dBm to the probe and sweeping over the bandwidth (5 to 250 MHz for both measurements) in max hold mode. The probe has a gain of -43 dB (200:1) due to the fact that the spectrum analyzer is 50 ohm terminated as is the probe.

Performance of the probe came out pretty decent I’d say. It is comparable to lower-range models from Agilent. There seems to be some kind of resonance near 70 MHz which drops the response around 4 dB down. It might be related to the fact that the input of the probe is not 50 ohm terminated or some other resonance like the BNC->2x banana connector adapter. If you discount that dip the -3 dB bandwidth looks to be around 100 MHz with which I’m quite happy with.

I’m quite happy with the CMRR as well. There might be some potential for more performance here, I quite quickly tuned the probe and didn’t measure or adjust the voltage dividers in any way. The adjustable capacitors were just tuned by feeding a constant HF signal in common-mode and then I tried to minimize it by adjusting the trimmer caps.

Here is a sample measurement from a Meanwell 5 V switch mode power supply, which seemed to have a flyback topology. Just a single FET with some controller IC. The measurement is over drain-source of the FET. The voltage is quite low (around 100 V peak) in this measurement because I just triggered on the signal when I plugged the PSU in to the wall and the voltage hasn’t had time to rise too high yet. Seems to work!

Here is the PCB design: differential_probe_v1