An Analog Isolator for General Measurement Use, DC to >200kHz, tested to 2.5kV - Page 1
Forum section: electronics / projects

author: Insatman
author_rank: Frequent Contributor
timestamp: February 02, 2018, 06:48:42 am
content: Hello,I built this little box after some experimenting with analog opto-isolators. I wanted something to allow for use of standard scope probes where the ground lead wasn't at ground. While I have a differential probe, this little box is still useful for a 2nd low bandwidth channel and it is fully DC isolated whereas the differential probe does have a high-impedance to ground DC. Hi-pot testing at 1kV was open-circuit on my megger. 2.5kV testing yielded an measurement of 20Gohms rising to over 40Gohms when I ended the test at 60 seconds. Bandwidth for the circuit is DC to >200KHz (3dB measured at 240kHz) without the filter circuit activated. Noise floor in this mode is 10's of mV. With the filter engaged, the noise floor drops by about a factor of 5 but the bandwidth drops to approx 12kHz (-3dB). CMMR isn't spectacular, being quite noticeable at higher frequencies of common-mode noise. At low common-mode frequencies it is difficult to measure above the noise floor. I'm sure the performance can be improved with better op-amps but it's useful to me now and the cost of the parts is reasonable.

author: PartialDischarge
author_rank: Super Contributor
timestamp: February 02, 2018, 08:30:11 am
content: Nice one. Two questions, what would you say is the biggest source of noise? and the limiting factor in bandwidth?

author: Insatman
author_rank: Frequent Contributor
timestamp: February 02, 2018, 09:21:08 am
content: Quote from: MasterTech on February 02, 2018, 08:30:11 amI don't know about either for sure...but...the noise is at a specific frequency around 170kHz..ish from memory. This could be the DC-DC converter switching frequency used for isolated power...it could also be some other unknown source in my lab.As for bandwidth, there are several places to look. Using a 1K output resistor and several feet of cable isn't helping. While looking at these issues is on my list it may be awhile before i get to looking at it again. I suppose I'll try the easy thing first and just substitute different op-amps as they are sockets and see what changes.On thing I will do if I lay out the board again is to design a better filter with a sharper falloff and a higher cutoff frequency. This should give better response with the filter on.

author: capt bullshot
author_rank: Super Contributor
timestamp: February 02, 2018, 09:33:56 am
content: Quote from: Insatman on February 02, 2018, 09:21:08 amFrom my experience, the source is these small DC/DC converters. These are really really noisy. You can filter their outputs using a RLC combination to make the supply rails quit, but they will still emit a good amount of common mode noise. I've used such converters for analog supplies in the past, and always had to filter their outputs with a "double" PI filter (kind of C - L - C - R - C), omitting the "middle" C (so C - L - R - C) should do also the job. Or just use two small mains powered transformer / linear reg. supplies.Otherwise: nice job, makes me think "want one too".

author: mikerj
author_rank: Super Contributor
timestamp: February 02, 2018, 11:26:29 am
content: Quote from: capt bullshot on February 02, 2018, 09:33:56 am100% this, I've had exactly the same experience.

author: danadak
author_rank: Super Contributor
timestamp: February 02, 2018, 12:28:01 pm
content: These might be of interest -http://www.analog.com/en/products/amplifiers/isolation-amplifiers.htmlRegards, Dana.

author: Kalvin
author_rank: Super Contributor
timestamp: February 02, 2018, 12:54:54 pm
content: From digikey selection:https://www.digikey.com/products/en/integrated-circuits-ics/linear-amplifiers-instrumentation-op-amps-buffer-amps/687?k=isolation+amplifier&k=&pkeyword=isolation+amplifier&pv2153=u100kHz&pv2153=u120kHz&pv2153=u200kHz&pv2153=u230kHzEdit: Added filtering for 100 kHz - 230 kHz -3dB point.

author: Insatman
author_rank: Frequent Contributor
timestamp: February 03, 2018, 12:24:17 am
content: Quote from: Kalvin on February 02, 2018, 12:54:54 pmI was aware of these units and in fact have a couple to experiment with in future. I chose the bare analog opto-coupler because I wanted to learn more about the principle and because of the potential of increased bandwidth. According to the datasheet 1Mhz should be achievable which is better than any of the isolation amplifiers and at much lower cost. Thus far my prototype can only match the performance of the iso-amps. And regardless I'd still have the problem of the power supply noise.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 03, 2018, 12:53:37 am
content: Be interesting to see it with balanced operation, so the isolator capacitance cancels out, for useful HF CMRR.Second (3rd? 4th?) the DC-DC noise.Tim

author: Insatman
author_rank: Frequent Contributor
timestamp: February 03, 2018, 01:09:24 am
content: Quote from: T3sl4co1l on February 03, 2018, 12:53:37 amNot sure what you mean by balanced operation. I originally built the circuit in the application notes that uses two opto-couplers, one for positive and one for negative going signals. The cross-over distortion due to the turn-on voltage of the LEDs was horrible however, so I went with a single biased opto-coupler.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 03, 2018, 01:36:27 am
content: Ah, balanced as in, you have positive and negative polarity channels, and the output is the difference of them.Tim

author: Insatman
author_rank: Frequent Contributor
timestamp: February 03, 2018, 02:13:12 am
content: It is probably possible to implement balanced operation without the prior observed crossover distortion though some kind of biasing network. I may experiment with that in future.

author: John Heath
author_rank: Frequent Contributor
timestamp: February 03, 2018, 02:58:39 am
content: If the probe electronics is running off a 9 volt battery and DC to 200 KHz conversion driving a toslink TTL to fiber converter with the other end of the fiber cable plugged into a fiber to TTL converter. I have used a 20 foot toslink fiber cable to isolate from 10 K volts. From what I can tell a 3 foot toslink fiber will isolate 10 K volts as touching the fiber cable 1 foot away from 10 K volt did not lead to any unwelcome surprises. The good news is todlink fiber is cheap at most stereo or electronic stores. The whole system with TTL to fiber , fiber to TTL and fibert cables would be 100 bucks tops.

author: Insatman
author_rank: Frequent Contributor
timestamp: February 03, 2018, 03:12:23 am
content: Quote from: John Heath on February 03, 2018, 02:58:39 amIn my working life we used fibre optics for high voltage isolation. Depending on the fibre type you can get 5kV per inch in dielectric fluids easily. In air I'd keep at 1kV per inch minimum to be safe. Avoid fibre reinforcement on the cable as these trap stagnant air. What ICs did you use for the Analog to Digital conversion and back? I was thinking of doing something similar for quite some time.

author: John Heath
author_rank: Frequent Contributor
timestamp: February 03, 2018, 03:24:41 am
content: Quote from: Insatman on February 03, 2018, 03:12:23 amThe IC was an off the self hex inverter receiving TTL or cmos driving a standard red LED. The good new is this comes pre done when you buy a toslink TTL to fiber and it's complement fiber to TTL . They are 30 bucks each and a 10 foot fiber cable maybe 10 bucks. Band width is 10 MHz.

author: John Heath
author_rank: Frequent Contributor
timestamp: February 03, 2018, 04:01:56 am
content: I just now realized that you need analog not digital. Hmm . Maybe one of the Toslink analog audio to digital to fiber converters. I can see the fiber to analog but I do not see the analog to fiber. And I am not sure how many bits the a/d converter is and they more than likelythey block DC. I think you are better off with what you have built with your own hands. You can only go so far with off the self especially if using it in a way that was not intended. However I have a picture of a toslink fiber to analog if you want to give it a run.

author: PartialDischarge
author_rank: Super Contributor
timestamp: February 03, 2018, 04:52:04 am
content: Quote from: Insatman on February 03, 2018, 03:12:23 amWhat was the particular application for this if I may ask?

author: Insatman
author_rank: Frequent Contributor
timestamp: February 04, 2018, 04:18:35 am
content: Quote from: MasterTech on February 03, 2018, 04:52:04 amI worked for Pulse Sciences which absorbed Physics International. We built custom Pulsed Power hardware, such as lighting simulators, laser drivers, EMP machines, microwave sources, pulsed high-fluence X-ray sources to name a few. Most machines were custom built to customer specifications with a pricetag to boot. I started out in control electronics as a EE and eventually gravitated to Pulsed Power design as a Staff Physicist. I spent 33 years in the industry. Mostly we used fibre-optics for fast low-jitter triggers and later for isolation of control signals and diagnostics. We built machines up to several Megavolts and some with peak (single-pulse) power into the TW range. Most machines were smaller however. These days most applications are rep-rate with smaller peak powers and higher average power. I retired about 5 years ago.

author: John Heath
author_rank: Frequent Contributor
timestamp: February 04, 2018, 07:46:26 pm
content: Quote from: Insatman on February 04, 2018, 04:18:35 amI would like to take advantage of your experience.Can two red , hundreds of THz , laser beams be mixed together to find a beat frequency in the MHz range for a small difference between them ? Is it doable?Also did they make any lithium lenses. Little lithium bar with 1 M amp going through to focus anti protons. Would like to know more about this lithium lens. Why did it not explode with 1 M amp going through it?

author: Insatman
author_rank: Frequent Contributor
timestamp: February 05, 2018, 02:14:15 am
content: Quote from: John Heath on February 04, 2018, 07:46:26 pmMy specialty was in high voltage / High current pulser design, so both questions are out of my area. On the Laser question I really have no idea. While I worked with lasers a lot it was mostly pulsed lasers at a specific frequency. On the lithium lens question however I do know something. The lens probably does explode, but it does so long after the experiment is over. These experiments take place in nanoseconds or microseconds at most. The "front end" hardware is usually destroyed or damaged on each shot. In some experiments the pulse is so short the front end survives and can be reused because while the instantaneous power is huge but the average power is small.

author: John Heath
author_rank: Frequent Contributor
timestamp: February 05, 2018, 03:25:39 am
content: It sounds like it was interest work. Especially with energies in the T watt range , wow. Other than a lightening strike you do not get to see those types of energies.

author: Insatman
author_rank: Frequent Contributor
timestamp: February 05, 2018, 04:07:22 am
content: OFF TOPICQuote from: John Heath on February 05, 2018, 03:25:39 amActually the work was very interesting up until the last couple of years where a management change made the job a living hell. So I retired eariler than I would have otherwise. There were few TW machines, the largest one is still in operation at Sandia Labs. It's callled the Z machine. It consists of 36 large paralell pulse generators in a single layer circle. The picture shows the front 1/2 of about 4 of them during a shot. The liquid is high-purity water. the sparks are surface discharge across the water well after the main event is over. The rear part of the machine you cannot see is filled with insulating oil. My company made contributions to both the design and construction of this machine and well as many others. Most of the TW level machines have been retired because computer simulation can do much of the work previously done on them.

author: John Heath
author_rank: Frequent Contributor
timestamp: February 05, 2018, 04:24:56 am
content: Found a video on it. Mr Z. You were not kidding. That is a big machine.

author: John Heath
author_rank: Frequent Contributor
timestamp: February 05, 2018, 04:46:52 am
content: Back to your problem . I did have some after thoughts on it. The cheap toslink TTL to fiber and fiber back to TTL adapters. For analog you could modulate the duty cycle. Second option frequency mod using a voltage to frequency and frequency to voltage standard ICs. Can not remember off the top of my head what the name of those standard ICs are but it will google up easy. I like the duty cycle modulation better for greater band width ,, maybe 2 MHz if lucky but .5 to 1 MHz for sure. With a 20 foot toslink fiber 100 K volt isolation.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 05, 2018, 07:59:56 am
content: Quote from: Insatman on February 05, 2018, 02:14:15 amI did some math in a previous Heath-asks-questions thread; it seems reasonable that a sub-100us pulse (fired from lots of electrolytic capacitors, nothing special) raises the temperature only 100-200C. Less if the beam line is cold, which it probably would be. Skin depth is probably the limiting factor to pulse duration (takes time for current to reach the center, and then the antiproton beam can fire).I think that was one of those too-long replies of mine, which was shelved and not returned to for additional insight/questions. Oh wellTim

author: John Heath
author_rank: Frequent Contributor
timestamp: February 05, 2018, 02:14:43 pm
content: Long replies are good and yours was. The only reason it fizzled out is you went a little over my head. You have to go easy with us simple folk. The cern lithium lens diagram I have was intended to run for a long time supplying lots a anti protons to keep the bean counters happy for months. I find it fascinating that a bar of lithium would be the way to focus anti protons. What was the underlining philosophy behind the design? The magnetic field can be understand but what is lithium doing there? Copper would be a better conductor of all that current. Why lithium?

author: Insatman
author_rank: Frequent Contributor
timestamp: February 06, 2018, 02:40:05 am
content: Quote from: John Heath on February 05, 2018, 02:14:43 pmFrom a CERN pubication..."Lithium is chosen to minimize nuclear interaction losses of antiprotons. A gaseous plasma would be even better and such lenses are being investigated."Lithium is a light metal, so it's density is much less than copper and so the antiprotons are more likely to pass thru without hitting any atoms.Note this publication is from 1989, so perhaps they are already using a plasma.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 06, 2018, 05:17:22 am
content: Yeah, somewhere in there I said lithium is low-Z --> low cross section.If it's over your head, perhaps you should concentrate your study on the fundamentals; fantasizing about high energy physics experiements is fun, but its practitioners spend many years indeed to reach that level.Tim

author: John Heath
author_rank: Frequent Contributor
timestamp: February 06, 2018, 08:39:39 am
content: Quote from: Insatman on February 06, 2018, 02:40:05 amPlasma , that would make sense. Low resistance. I can remember blowing a 2 amp fuse with a tiny 1/4 inch neon bulb. I would guess a plasma would be less dense for less annihilation with anti protons.I know what you mean about a change in management. You get settled in and then everything changes. I was not close enough to retirement at the time so my solution was the private sector. Self employment. You always have 100 percent control. Take a day off to play gulf just cause you feel like it. The down side is work does not end at 5:00 PM. It winds down a bit but it is always there to some degree. Even when playing gulf it is still on your mind.

author: John Heath
author_rank: Frequent Contributor
timestamp: February 06, 2018, 09:57:53 am
content: Quote from: T3sl4co1l on February 06, 2018, 05:17:22 amNeed fundamentals , fantasizing. There is no science here. What I see is a vulgar display of a human ego. You are better than that. If I said something you did not like I am sorry.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 06, 2018, 08:53:47 pm
content: Apologies, I didn't mean that in a belittling way -- take it as encouragement. I appreciate your inquisitive bent.Tim

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: February 07, 2018, 12:04:12 am
content: Worth looking at for this topic:AN-285 and AN-298 from NS,AN-62, -87, -105, -106, -113 from LTthere was also an Application for the UC1901 'simple versatile isolation amplifier', but I can't find it right now

author: John Heath
author_rank: Frequent Contributor
timestamp: February 07, 2018, 12:09:41 am
content: Cool , that turned out the way I wanted. And you are right I do tend to run ahead without doing the home work required.

author: John Heath
author_rank: Frequent Contributor
timestamp: February 07, 2018, 12:46:33 am
content: Quote from: Neomys Sapiens on February 07, 2018, 12:04:12 am¿Eres de Mannheim Alemania o Mannheim Chile?

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: February 07, 2018, 02:51:55 am
content: Quote from: John Heath on February 07, 2018, 12:46:33 amWas?Eres is somewhere else (insert Kepler elements here)And the remainder may be deduced from the ID!

author: John Heath
author_rank: Frequent Contributor
timestamp: February 07, 2018, 01:41:30 pm
content: Quote from: Neomys Sapiens on February 07, 2018, 02:51:55 amThere is a Mannheim in Germany as well as Chile. I was thinking you were assigned the wrong flag in your ID. Anyways back to your UC1901 isolation.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: February 13, 2018, 11:26:21 pm
content: On the UC1901 application:It is DN-19 in the 1987-1988 Unitrode Applications Handbook.When I try to print the two pages into a new PDF, freePDF collapses, so I can't post it.But the manual is available in a few places.

author: Insatman
author_rank: Frequent Contributor
timestamp: February 14, 2018, 01:31:51 am
content: Quote from: Neomys Sapiens on February 13, 2018, 11:26:21 pmSee attached PDFThe UC1901 is available on Ebay but it's a bit pricey...plus there is the problem of the isolation transformer

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: February 14, 2018, 02:28:27 am
content: You could of course use a 2901/3901, which should be cheaper.I just wanted to point out circuits that I have seen.On the other hand, I just have one rail of UC-1901.