Using wideband RF transformer to isolate shunt resistor - Page 1
Forum section: electronics / projects

author: daqq
author_rank: Super Contributor
timestamp: March 18, 2016, 11:54:22 am
content: Hi guys,In a project I have a shunt resistor that I would like to measure. The trick is that the source and load need to be left floating relative to the measurement device by around +/-200V of a low frequency. I don't care for the DC value of the current going through the resistor (since there will be effectively no DC current going through it), only very short pulses (1-5 us) matter - I would like to measure them as precisely as possible, both in terms of waveform shape and value.The most elegant solution seems to me to use two of these:http://www.farnell.com/datasheets/1870435.pdf,say PWB1010L, connect one side to the shunt, the other one to coax or differential pair onto the measurement board, which would have a counterpart to this transformer, termination, gain and misc. and feed an ADC (or scope with the proper termination). There will be around 1V on the shunt during the pulse.My questions are:Is this a good idea? If not, what would you recommend?What kind of termination do I need and where do I need it? The source has pretty much zero impedance (being a low value shunt), so assuming that I connect it as follows:Shunt->Transformer->Shielded Differential Pair Cable (100 Ohm)->Transformer->Termination and measurementThe Termination and measurement should have a 100 Ohm impedance as well?Thanks,David

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 19, 2016, 12:59:37 am
content: I would suggest source termination resistors, perhaps 22 ohms per lead. This will help the frequency response (bandwidth is best into matched source and load), and also prevent questionable currents from flowing (a DC offset on the shunt resistor will become a DC bias in the transformer, limited by winding resistance..).Tim

author: daqq
author_rank: Super Contributor
timestamp: March 21, 2016, 09:38:45 am
content: Thanks!How did you come by the 22 Ohms number though?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 21, 2016, 10:41:42 pm
content: 50 ohms total, split in two, 25 not being a standard value.Common mode noise will still see imbalance, because you're using it as a balun (well, I assume you'll be reading a single-ended output, anyway..). You could follow it with another wired sideways as a CMC, to help reduce that effect.I'm not sure, and I think I'd want to do some scratching on paper to figure out, if an imbalanced source impedance would complement an imbalanced load impedance (i.e., 0 and 50 ohms on the source side, GND and signal on the output side).Tim

author: nctnico
author_rank: Super Contributor
timestamp: March 21, 2016, 10:47:56 pm
content: I'd use current sense transformers and skip the shunt resistor! Wurth has several and with a 10 Ohm loading resistor they are flat until several MHz.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 22, 2016, 12:05:01 am
content: Quote from: nctnico on March 21, 2016, 10:47:56 pmAs long as the shunt resistor is wired properly, the OP's method will be flat to 100MHz+.Tim

author: daqq
author_rank: Super Contributor
timestamp: March 30, 2016, 10:52:31 am
content: So, I've wired it up, and I'm getting some nasty ringing. I'm pretty sure it's not the setup since it is just going into a resistor. I'll try to play around with the termination at the source and scope.