Front end for isolated probe - Page 1
Forum section: electronics / projects

author: MarkoAnte
author_rank: Regular Contributor
timestamp: December 27, 2016, 01:48:00 am
content: Hey,some time ago was asking about making an isolated probe and now I have a chance to try to make it as a project for school. Its is for a course about PCB design, so there are no specification to achieve (the grade will be form layout and schematic design) but I would like it to be a usable diy isolation probe(especially for power 3 phase applications).I am stuck with the analog front end. I will use 8 bit 100 Msps ADC (ADC08100) then isolators and a DAC, that takes the same kind of digital coded signal that the adc makes, and turn it back to analog. (so no uP or FPGA)I have been simulating the analog input circuit form page 17 of the adc datasheet (http://www.ti.com/lit/ds/symlink/adc08100.pdfor see attacked picture) and:1) Why is there a 0.33uF capacitor connected via the 47R resistor to V+ of the op amp (C2)? Simulating this gives bad bandwidth, see capacitor.jpg. If I remove it the output is better.2) Why is the "negative" half cycle ''tilted'' like shown on the picture? How to fix it?3) I picked 8 bit because the oscilloscope(MSOX3014A) has 8 bit adc. Is this ok or should I go up in resolution for an usable signal?4) Any suggestions on how to make the front end better or different? I was thinking of using the oscilloscope probes 1x for 10V input, 10x for 100V and 100x for 1kV. I will be using the probes to look at 3 phase power applications so 1kV looks about right for a range. Maybe add a switch to make 600V full scale. That is if it works ok.Best regards,Marko

author: tautech
author_rank: Super Contributor
timestamp: December 27, 2016, 02:14:42 am
content: To be honest you are dicing with death to use something that has not been properly safety rated if you want to use it on 3 ph or even just mains.Sure build something for personal LV use but if you're going to use it on dangerous voltages get a proper Differential probe:https://www.eevblog.com/forum/testgear/teardown-pintek-dp-25-differentail-probe/Some study for you:https://www.eevblog.com/forum/blog/eevblog-932-how-does-a-hv-differential-probe-work/https://www.eevblog.com/forum/projects/my-first-project-low-cost-differential-probe/Further in a forum search using "diy differential probe"

author: Someone
author_rank: Super Contributor
timestamp: December 27, 2016, 07:58:30 am
content: Being a continuation of this thread:https://www.eevblog.com/forum/testgear/isolated-voltage-measurement/Adding an ADC/DAC is a lot of complication, you'll need suitable front end filtering for the ADC and then end up with just 10-20MHz of bandwidth. Transformers can cover 6 decade ranges of frequency and would be a much easier option to achieve the same goals.Differential probes are generic tools designed to be applicable to many many applications (even then there are many different differential probes), if you narrow the applications down you can make many shortcuts in the specifications/design.

author: PeterL
author_rank: Regular Contributor
timestamp: December 27, 2016, 09:56:56 am
content: Using the ADC-Opto-DAC road is not the way to go for an application like this. You will add a huge delay to signal, that makes this probe unusable if it's combined with other kinds of probes, because it's gonna show you the past.Also your schematic looks a bit half theoretical. 60 ohm resistors for example like you use in de feedback only exists in spice, not in real life.The 100R resistor after the first stage is not needed. I see you have copied this from TI's schematic, but there it is use for proper termination of an incoming line (paralleled with 240R+47R to get 75R). In your schematic you also have the 60+20 ohm voltage divider in parallel with this 100 ohm.The circuit around C2 is to inject a little bit of DC voltage into the last opamp. This is so you can trim out offsets that exists in opamp. (Dave has some video's about opamp offsets, #476 and #479 for example).C2 is there to stabilise the offset adjustment. It should not affect BW in any way. If it does I think your potentiometer is not simulated properly. I don't see a resistance specified next to it, your simulator seems to not see it, and C2 suddenly act as a filter together with R4 of 2KHz. (1/(2 x PI x R x C)).Try removing the pot's all together, and connect the 3 leads.And finally don't connect your home-made circuit to mains. To many things can go wrong and bring you or others in hazardous situations.

author: MarkoAnte
author_rank: Regular Contributor
timestamp: December 27, 2016, 11:40:01 am
content: Quote from: tautech on December 27, 2016, 02:14:42 amEvery single time I mention 3 phase there is somebody that not answer even 1 question but feels the need to say that its dangerous.Yes it is dangerous. Yes I know how to handle it. Yes I will test the finish device. Calm down.Quote from: Someone on December 27, 2016, 07:58:30 amWhat would be suitable front end filtering for the ADC? 10-20MHz is ok for what I need. With transformers you can't get lower frequency. I have looked at AM503 and how it works (for current ofc). It uses hall(LF) + transformer(HF).Quote from: PeterL on December 27, 2016, 09:56:56 amADC – ISO – DAC gives 25ns of delay. If we double that we get 50ns and for 30KHz (the switching frequency I’m interested in) gives a delay of 0.54 deg. And this is ok for me.Quote from: PeterL on December 27, 2016, 09:56:56 amDo you mean the low values? I’m used that this values are in kohms not in ohmes. But if you meant that there are no 60.000 ohm resistors, yes I know that. There should be a trim pot there to set the gain, but I didn’t want to mess with that for simulation so I put in 60 and 20 ohm for a gain of 4 so I can see it batter on the results. It should be 60.4 and a pot to trim in the gain.Quote from: PeterL on December 27, 2016, 09:56:56 amThe pot has a 1K value and I replace the pot with 2 resistors of the correct value (to replace the pot) and I get the exact same result.

author: PeterL
author_rank: Regular Contributor
timestamp: December 27, 2016, 01:07:11 pm
content: Quote from: MarkoAnte on December 27, 2016, 11:40:01 amThere is definitely something with the resistor values. Try to understand which gain this circuit should give. Look at your bode plot, and notice two bands with a near constant gain. I leave it up to you to figure out which of those gives the proper result.Quote from: MarkoAnte on December 27, 2016, 11:40:01 amThat's not the proper reaction to Tautech's response. Especially for someone who is still a student. You obviously don't understand the hazard here, because if you did, you'd understand this response.You cannot test your own made equipment, as you simply don't know what you don't know. And having this thing properly tested on safety by a third party will cost you much more then a decent diff-probe.So try to understand why your circuit works as it does: you can learn a lot there, then DON'T build it. Just pick something else as a school project

author: Brutte
author_rank: Frequent Contributor
timestamp: December 27, 2016, 01:56:07 pm
content: Why would you need an isolated probe? The mains is earth referenced and scope is also mains referenced (should be). The only problem is with dealing with voltage that is higher than the scope can accept directly.So, as this is mains then I guess loading the source with some resistive load (1W) is not a problem. The scope has a constant input impedance of 1M||15pF. A resistive voltage divider (a small load) should do the trick. You could also use an off the shelf 100:1 probe.I think an isolated probe is only useful when the reference voltages are not the same on both sides (like an isolation transformer with secondary winding biassed 1kV above earth).

author: MarkoAnte
author_rank: Regular Contributor
timestamp: December 27, 2016, 01:57:20 pm
content: Gezz. I understand the hazards here.The plan is to test the pcb on low voltage first. Then test the insulation with an insulation tester on 4KV for 1 minute and then some transient 6KV spikes and then when that is done test the performance on a floating oscilloscope and have the probe grounded and then put it into a plastic, metal lined box (metal erthed via osciloscope) and fill the enclosure with potting compound(and test the isolation again). That way IEC 60950-1 is upheld for electrical enclosure and reinforced insulation.+ move the resistor divider into 2 peaces so there is high-Z form both connections to the ''gnd'' or the insulated device.I have IGBT drivers working at 1.5KV done in this fashion (just that i removed the enclosure after potting) because functional insulation is enough for them as they are located in an electrical cabinet.

author: cat87
author_rank: Regular Contributor
timestamp: December 27, 2016, 02:05:23 pm
content: So, if you want tu build this thing and have it working just for you school project, then go ahead. BUT, if you really want to use it on 3-phase power, you'll most likely end up blowing you scope. And for a student, that may be a fate worse than death.When designing an insolated probe, the key word is ISOLATION. You want to have a probe that can measure a voltage differentially, while being capable of withstanding large ammounts of common mode voltage. I.e. maybe you want to measure a differential signal that is only 20 Vpp let's say, but that signal may be riding on commom mode voltage of 300V. This is just some example off the top of my head.The ADC-DAC combo is verry nice to play with. If you're going to go down this road, do yourself a favor and lay down some basic parameters for your project:- What is the bandwidth that yo want to measure? 100 KHz? 1 MHz? Some modest diff. probes go to 20-25 MHz bandwith without much fuss- What is the maximum differential voltage you want to measure and what is the maximum commom mode voltage? If you'll be using it on the 3-phase, Something like 750 V commom mode voltage and about the same in differential voltage would be more that adequate.- Think about how you will power the front end of you probe. Will it be from batteries, or from a wall-wart? (Personally, I'd go with a 9V battery or 4 AA batteries)Also, just so you don'tand also to answer your questionQuoteI'd put as much MegOhms in front of that thing as possible.Something lke this:https://www.eevblog.com/forum/blog/eevblog-932-how-does-a-hv-differential-probe-work/and look at the image in the second postThen, after that stage, you need to buffer you differential signal that will be going into your ADC. Of course, the ADC would preferably take the input as a differential voltage (i.e. the VinGND of the ADC is not connected directly to the AGND or DGND) From here on, you can do whatever you hear desires.Just as a disclaimer, this would not be a proper way to build such a probe, but we're here to help, solet's see how it turns out.Oh, and try not to create any weird groud loops between the scope and probe.Some interesting reading might be:https://www.eevblog.com/forum/projects/oshw-diy-1kv-100mhz-differential-probe-(dilemma-vs-hope)/Also, I have a little article I wrote about building a proper Diff Probe. Just check out the link in my signature wink wink

author: Someone
author_rank: Super Contributor
timestamp: December 27, 2016, 09:41:54 pm
content: Quote from: cat87 on December 27, 2016, 02:05:23 pmIf you refer to their previous thread (which is where this entire thread should be for the context) they didn't like the noise pickup from the existing high voltage differential probes, this may be why they're investigating isolated solutions instead.Quote from: MarkoAnte on December 27, 2016, 11:40:01 amSome of us have done this sort of thing before, we're happy to share information but I'm not going to slowly work to your drip feed of information. If you want to lay out all the requirements and constraints we can talk, but you're just doing your own thing and none of us understand yourmotivationfor an ADC-DAC isolation. Mains monitoring rarely needs a DC measurement so I've done well with just transformers in the past, if you want DC there are ways to bridge that over and its much simpler than trying to run high speed ADCs across an isolating link, but you haven't explained any of the constraints.

author: MarkoAnte
author_rank: Regular Contributor
timestamp: December 27, 2016, 10:31:05 pm
content: @ Someone: Sorry for the poor project definition. As you said I didn't like the noise pickup of the existing high voltage differential probes. I was thinking how would I make it better and I looked at how the amc1200 works as I use 8 of them already (3 for phase current, 3 for phase voltage and 2 for dc voltage and current) and they seem to work excellent.Then I had a lot of stuff to do for university and that is were that ended. Then a opportunity opened to make a circuit for a class focusing on EM hardens & compatibility and circuit design. So I thought of using this for the project.For the specifications i started by looking at what digital insulation I could reasonably get and found ISO7640 @ 150Mbps with 1400V working voltage and 6kV transient voltage. Then as I wanted to keep the design simple (no micro processors or FPGAs) I looked for a adc with around 100Msps paralel communication and simple implementation. I found ADC08100 that has all of that and DAC THS5641A that I should be able to connect to the output of the adc and get analog out. (same protocol) I chose 8 bit adc because my scope is 8bit, so with some minimal filtering after the DAC it should be an ok resolution signal. I want to make a prototype to see if it works as intended and if it does, i would like to make a couple so I could look at stuff in my project. I mostly make power stuff: 3-phase PFC, multyphase buck converters, resonant converters itd. The switching frequency is usually around 20 - 30 Khz, but it is a "square" wave. So I figure about 20-30 Mhz bandwidth should be enough. If I will need some more difficult measurements I will just float a battery operated scope. (But for safety this is not preferd+ the scope sucks). For range select I will just use different oscilloscope probes (1x,10x,100x) with maybe a switch to half that so I would get 0.5x 5x and 50x. As this looks like it would be easy to do and I have a lot of probes to select form.So:- bandwidth 20-30 Mhz (more will not hurt but I do need DC)- 100Msps is the upper limit because then it will be hard to get good insulators- with a 100x probe 1Kv gives full scale on ADC- input impedance 1M with 20pF (but 20pF is a best case It will be more)- for immunity I will probably make the output 50ohm. So the output will be scaled acordingly (but this is not what I'm looking into now)- Power: battery or switcher, maybe option for both because some noise will be coupled true the switching transformer. So I will probably make and option to connect a 9 V baterry if the noise coupling will be to great with the switcherI am currently looking at what kind of front end do I need. For this adc I need some shifting because the adc is form 0 - 3V. And I will need some buffering between the voltage divider and the shifter because the shifter(adder) has a low input impedance and it will mess up the voltage divider. I will also need some protection that the voltage does not drop to - Vcc, will try to do with a diode in the opamp part (like the super diode circuit). This would have adverse side effects on the bandwidth if it was to be use repeatably, but this is only for protection. I would probably need a 30 Mhz filter in between right?

author: tautech
author_rank: Super Contributor
timestamp: December 27, 2016, 11:03:01 pm
content: For the PSU have you considered USB supply and maybe a USB hub for multiple units ?

author: MarkoAnte
author_rank: Regular Contributor
timestamp: December 27, 2016, 11:05:14 pm
content: Hm interesting idea.

author: Someone
author_rank: Super Contributor
timestamp: December 27, 2016, 11:24:03 pm
content: Quote from: MarkoAnte on December 27, 2016, 10:31:05 pmYou talk about mains and then make it a requirement to do DC, so you're back to a general purpose design. But none of which addresses your issue with the noise level and pickup as far as I can see, now you're taking on the task of building a 1M ohm scope front end to coupled to an ADC, power all of this isolated, and the run a DAC into the scope. There are many examples of working projects here on the forum to do this front end amplifier but you're asking us for help starting from scratch with the app notes on the ADC.You've got the start of a specification but they don't address your original problems. It might be a fun project to fill time with but it will be more expensive and lower performance than the "default" differential probes with high divider ratios. Even if it needed isolation I can't see themotivationfor digital isolation when transformers would be able to do the job passively. But just by designing better front end protection and suitable impedances you could already get what you want from a differential probe, there is nomotivationfor isolation other than your dreams.Feel free to reinvent the world and ignore all the resources on this forum, but don't then ask for help.

author: PartialDischarge
author_rank: Super Contributor
timestamp: December 28, 2016, 04:11:52 pm
content: Let him do it his way. It may be more complex but has its advantages too, not everyone has to do things the same old known trusted way. I consider his approach an interesting project, and he did well in asking for help here.Optical isolation beats transformers at isolation and thats something appreciated in the HV world, besides beign able to measure DC.My only concern is that thte PS has to come from a battery or youll lose the goodies of infinite isolation. No point in having optical isolation from one side and less than 1kv isol from a usb power supply.

author: MarkoAnte
author_rank: Regular Contributor
timestamp: December 28, 2016, 04:24:48 pm
content: TY MasterTech.I think he meant to power the whole thing form the USB port on the osciloskope and then make an insulation switcher to get the the HV side.