My first project: Low-cost differential probe - Page 1
Forum section: electronics / projects

author: hammil
author_rank: Regular Contributor
timestamp: February 05, 2013, 12:19:13 am
content: I always hated the problem with mains-earth-referenced oscilloscopes. Not only are the input grounds all shorted together, it can be bloody dangerous to probe around other earthed boards if you don't know exactly what you're doing.The solution, of course, is to use an active/differential probe, however these can cost hundreds if not thousands of pounds/dollars. So, I decided to roll my own. I honestly don't believe this has been done before, but I'm sure someone will prove me wrong.I call it: The OmniProbe! (Yes, I had to give it a wank name.)It consists of a differential amp, a little input protection, and some power ICs. That's it. Plug one end into the scope, and the other into your probe, and it puts at least 4M of isolation between the signal and mains earth (It's likely to be significantly higher, need to measure it properly), with ~6M of differential impedance.The main component in the design is the AD8130 differential amp, specifically designed to link signal paths together while keeping the grounds isolated. Input impedance isn't quite JFET-level, but it's decent. Bandwidth is good too: 290MHz with a 0.2V signal, or 175MHz with a 2V signal. Unfortunately where it suffers is the maximum differential input; the datasheet specifies only 5V before performance starts to degrade, somehow (again, I need to take proper measurements). If higher-voltage signals need to be measured, however, a x10 attenuator will work quite nicely. Noise and biasing in the chip is all extremely low.A little aside: I noticed that with op-amps, you have to choose two of these three, regardless of cost: high working voltage, high bandwidth, high input resistance. I haven't yet found a chip that has everything.The whole device runs from a CR2032 battery, which is boosted up to 6V with a switch-regulator, then converted to +/- 12V, which gives the amplifier it's best performance.Input protection is just a couple of zeners, back to back, to limit the maximum input voltage to the amplifier. Anything more would increase capacitance between the inputs. The diodes alone add around 6pf, so I included a jumper that you can remove if you want to disconnect them.I also added an adjustable isolation output resistor to serve as compensation for the scope's capacitance.The PCB is designed to be mounted in a Pomona 2391. Overkill, perhaps, but it's the easiest way to shield a prototype design like this, I find.Schematics and PCB previews are attached. Oh, the PCB images are a little old, so they're just for illustration really.

author: GK
author_rank: Super Contributor
timestamp: February 05, 2013, 01:22:44 am
content: Not bad for a 1st project, but those 500R input protection resistors are going to limit your bandwidth.That AD part has a differential input capacitance of 3pF. That alone (not including strays) with 1k works out to -3dB at a little over 50 MHz.You can fix that problem by putting small value capacitors in parallel with the 500 ohm resistors.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 05, 2013, 04:06:38 am
content: good to see there's another diff probe project popping in... here is my venture (i dont want to update every little crannies of the details until i get a firm setup, not yet).High Voltage 10-100MHz Differential Probe Investigated1) is your intention to make the "input attenuator" separate? (external?) since 0.2V or 2V differential is close to useless (for my app only sorry)2) the good thing is you are using battery (portable). i am using bipolar supply so i can concentrate on the diff amp circuit. i was considering using 2x 9V (rectangle) battery but still no luck finding the suitable enclosure.since you are using switching circuit. i will be interested looking at your circuit noise floor. and your device bandwidth profile and cmrr. cheers.

author: Marco
author_rank: Super Contributor
timestamp: February 05, 2013, 05:02:53 am
content: What do you intend to use for the probe head?

author: hammil
author_rank: Regular Contributor
timestamp: February 05, 2013, 06:26:23 am
content: Thanks for all the feedback, everyoneQuote from: GK on February 05, 2013, 01:22:44 amThat's superb, thank you - should've thought of that myselfQuote from: Mechatrommer on February 05, 2013, 04:06:38 amMax 'guaranteed' differential input is listed as ~5V, but in terms of absolute ratings, it's 17V. I need to check what kind of performance impact it has when 5 < Vdiff < 17.To keep parasitics to a minimum, I was planning to have nothing on the board in terms of attenuation, but of course one could buy a basic x10 attenuation BNC module fairly cheaply, in addition to the one on the probe. The amplifier is easily sensitive enough to handle significant attenuation, so with two x10's, one could theoretically measure up to 500V.I'm using component values over and above those specified in the datasheet, to minimise noise. One good thing about the AD8130 is that the CMRR is given as 70dB at 10MHz, which is excellent.Quote from: Marco on February 05, 2013, 05:02:53 amAnything that can connect to a BNC will fit, but I suppose ideally it would be any good quality passive probe, with at least a switchable x10 attenuator.

author: codeboy2k
author_rank: Super Contributor
timestamp: February 05, 2013, 07:08:36 am
content: nice design. I like the battery operation.Quote from: hammil on February 05, 2013, 06:26:23 amAlthough that CMRR is good, since mechatrommer specifically asked about the switcher, I'll talk about that here. What you really care about is the op-amp PSRR when dealing with switching noise on the power rails, and that op-amp has about 55dB PSRR at your switching frequency, which for the MCP1624 is 500Khz. The voltage doubler operates at 24Khz.With your MCP1624 circuit, and a 10uF output cap, assuming that your circuit draws 30mA total (just my guess), the worst case switching ripple will be at the high end of the the switcher duty cycle, say at 80%, it's going to be on for 900ns, thus, 30ma = Cout * dV/dt, dV is the ripple, dt is the FET on time.Heck, lets go worst case and assume 100mA draw from the switcher(!) and 80% duty cycle.. then 100ma = 10uF * dV/900ns , dV =9mV power supply ripple.9mV power supply ripple at 500 KHz, and the op-amps PSRR at 500 KHz is 55dB, then that 9mV power supply ripply will result in an output voltage of 10^(log(9mv) - (55/20)) =16 uV ripple at the op-amp output(due to the power supply ripple).That's a worst case guess I just did, and it depends if 16uV of error is acceptable to the designerChances are the circuit is not drawing more than 50ma, and maybe even just a few 10s of mA, so that power supply ripple will be even less and that error at the output will be even less still.The PSRR at 24 KHz is about 85 dB, but I didn't do the math for its ripple and noise contributions. But 85dB is much better, and the noise contribution will be much lower.If it was me, after building the first cut of this circuit, I'd certainly want to measure its current draw, know that value, and figure out what its ripple should be, then measure that to know its true value, and decide if the output error due to power supply ripple after rejection is OK for me and my design goals.Cheers!

author: Marco
author_rank: Super Contributor
timestamp: February 05, 2013, 08:20:10 am
content: Quote from: hammil on February 05, 2013, 06:26:23 amIn that case you don't really need the input resistors, you don't need to worry about input capacitance (in fact you need a little more probably) and you should have a 1 meg transistor parallel with the input I think.

author: poorchava
author_rank: Super Contributor
timestamp: February 05, 2013, 08:49:47 am
content: I would add a possibility to switch between 1M and 50R modes. 50R is useful and many lower-range scopes don't have that.

author: sorin
author_rank: Frequent Contributor
timestamp: February 05, 2013, 09:05:06 am
content: In my opinion this don't offer any type of protection, or solution for your problem.As I know, you need a isolated amplifier(optical or magnetic isolation).

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 05, 2013, 09:22:00 am
content: i've downloaded the AD8130 diff opamp, quite good candidate as a replacement part for my device thanks for the link. the output swing will depend on the gain, Vo = GAIN X Vi, the OP indicating he's using unity gain, i wonder if that 70dB CMRR @ 10MHz is also valid at unity gain? i havent checked the DS gotta pick the kid now. chiaow.

author: poorchava
author_rank: Super Contributor
timestamp: February 05, 2013, 09:59:32 am
content: QuoteTwo problems:-isolation amps are VERY expensive-they have really modest bandwidth (ie. AD210 , -3dB@20kHz)In some expensive scopes that have isolated channels (Tek used to do that) the channels are isolated on digital level, after the analog frontend and conversion. High speed (XX MHz) analog isolation is 'a bit' hard to do.

author: sorin
author_rank: Frequent Contributor
timestamp: February 05, 2013, 10:26:52 am
content: this circuit is useless, for what you want to use it.suppose that you want to measure two points, 40 v and 41 V respectively potential referred to ground.the differential voltage is just 1Volt, but what will happen with your circuit?

author: hammil
author_rank: Regular Contributor
timestamp: February 05, 2013, 10:54:48 am
content: Quote from: codeboy2k on February 05, 2013, 07:08:36 amThank you very much for the analysisThe standby currents for all the ICs is very low, and by my estimate it would only draw a few mA from the supply in operation. I need to test that, however. Plus, 16uV is almost invisible even at 500uV/div on a scope, so I think it's well within the acceptable limit.Again, really appreciate the time you took with this. I had no idea I'd get this much feedback!Quote from: Marco on February 05, 2013, 08:20:10 amI'll have to test that out - I'll find some more information about passive probes. However, I'd really like a higher differential impedance than 1M, and I'd like to take advantage of the low input capacitance (in terms of effect on the circuit-under-test).Regarding the input resistors, I would definitely like the option to use the clamping diodes, as a differential voltage of 18V can easily damage the AD8130. If the resistors weren't there, a voltage in excess of around 12V would effectively short out the inputs, possibly destroying the diodes and/or causing problems with the circuit. Of course, if you didn't need the device to probe around devices with unknown voltages, one could just short out the pads.Quote from: sorin on February 05, 2013, 10:26:52 amI'm sorry you feel that way, however it seems it is quite clear to me and other users why a device such as this would be useful.

author: hammil
author_rank: Regular Contributor
timestamp: February 05, 2013, 10:58:45 am
content: Quote from: poorchava on February 05, 2013, 08:49:47 amI actually considered this to begin with, however adding a switch in series would increase the dreaded input capacitance by around 6pf. I could use a jumper, however... perhaps a later version of the board might have somethingOne could of course use a 50R BNC terminator, but it would be nice to have everything on one board.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 05, 2013, 11:05:02 am
content: Quote from: sorin on February 05, 2013, 10:26:52 amit will depends on what the OP want to measure (low vs hi volt CM). if its within the spec values (diff and CM voltage) then the OP is good to go even without an attenuator, i think. if he has to provide an attenuator, then thats the biggest part, every aspects esp CMRR will rely on the attenuator (front end) matching. there's nothing worst than a low CMRR (i can live with low PSRR) ie an error voltage that follows signal shape and frequency. and that is so far what i figured out from my short experimentation.Quotea "not so good" "random noisy output" can be averaged in dso, but not the non-rejected CM voltage. i was just asking for output noise level (since my circuit's noise is coming from another aspect, ie opamp gain), but that is not so much an issue compared to CMRR.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 05, 2013, 11:12:58 am
content: Quote from: hammil on February 05, 2013, 10:58:45 ami believe what he meant is that an opamp's output that capable of driving 50ohm load, it has nothing to do with the input stage. i checked the AD8130 it only specified up to 100ohm load,hence it cannot properly show a "halved magnitude" on 50ohm terminated coax. if you dont terminate your coax, then your signal reaching the scope will be out of proportion (due to coax capacitance). terminating it with 50ohm will show you 33% of the signal, but i'm not sure about the flatness with this mismatched characteristic impedance 100ohm opamp output - 50ohm coax - 50ohm terminator setup. the easy fix for me is we need another buffer on the output that is capable to drive 50ohm load.all opamps range that i used are capable except they all combined cost alot more than your AD8130.edit: sorry i re-checked the datasheet. 100ohm load is actually the 50ohm + 50ohm terminated coax. so yes, this amp alone can be used without any buffer..."The AD8129/AD8130 can directly drive loads of as low as 100 ?, such as a terminated 50 ? cable."i always got mixed up with this 50ohm cable and 100ohm loadbut still you cannot make the 50ohm terminator to be "on the board" with long coax hanging after that to dso. a terminator must be on the dso side.

author: sorin
author_rank: Frequent Contributor
timestamp: February 05, 2013, 11:31:07 am
content: Quote from: hammil on February 05, 2013, 10:54:48 amWith "is useless" I mean that , dont resolve this problem:Quote from: hammil on February 05, 2013, 12:19:13 amDon't that is useless in general.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 05, 2013, 12:01:22 pm
content: Quote from: sorin on February 05, 2013, 11:31:07 amok i missed that statement, actually its already in the OP. if dangerous voltage is the concern, then its uselessAD8130 is only speced to ±10.5V CM, nothing dangerous about it.and how you are going to use Pomona 2391 for 2 input and 1 output device? it only got 1 input 1 output?http://www.pomonaelectronics.com/pdf/d2391_1_01.pdf

author: codeboy2k
author_rank: Super Contributor
timestamp: February 05, 2013, 12:28:56 pm
content: Quote from: hammil on February 05, 2013, 10:54:48 amYou're welcome. Even though the quiescent currents are very small, when it's driving your scope, especially if you use the 50ohm input setting on your scope, it might draw a few mA or more. It will never draw 100mA or even 50 or 30 mA like I said, I was just trying to pick a worst-worst case. And thus 16uV (worst, worst case!) of noise is insignificant, at 500uV/division on your scope, so your switcher will work just fine.For a more realistic 10mA , just plug it in:ripple = (Ton*I)/Cout = (900ns*10ma) / (10uF) = 900uV power supply ripple900 uV @ 55 dB PSRR = 10^(log(900uV) - (55/20)) = 1.6uV at the output of the opampCheers!

author: fcb
author_rank: Super Contributor
timestamp: February 05, 2013, 12:38:52 pm
content: Nice idea. A few comments though.1. Assuming you want to use the pomona case, you'll have insulate the input bulkhead BNC chassis from the output.2. Why bother with an internal supply? You're going to compromise the noise performance probably by doing so.3. Did you build a prototype to check what the CM range actually turned out to be?4. Most scope probes are x10 attenuation and most differential probes are x10/x25/x50/x100 at least - and you pay mega$$$ for highBW differential, even modest tek 100MHz probes are $1K.5. High Z is easier with attenuation...Why not build a differential probe with a much higher (useful) range and a lower -3dB, now that would be useful for SMPSU work - have a peek at the PICO TA041 for sensible specs.

author: codeboy2k
author_rank: Super Contributor
timestamp: February 05, 2013, 12:44:03 pm
content: Quote from: Mechatrommer on February 05, 2013, 12:01:22 pmThe OP said he would use a divider on the input, i.e. 1 10:1 probe.But that has it's own problems, like then you don't need the 500 ohms at the input of the opamp, and the probe adds capacitance that might need compensation at the opamp inputs, plus the probe needs a 1M at the input to work as a proper divider...but..I don't think OP can do this, since they are differential inputs, you need to divide them both down before the input, not divide them relative to each other like a 10:1 probe would do it.Best probe is just the bare end of the coax. divide both inputs at the op-amp.QuoteHe's using one BNC center pin and shield as the two inputs. There's nothing wrong with this approach, but care has to be taken to isolate the BNC shield on the input side from the metal case of the Pomona box and the output BNC shield. (oops fcb already said it !)

author: hammil
author_rank: Regular Contributor
timestamp: February 05, 2013, 12:52:30 pm
content: Quote from: Mechatrommer on February 05, 2013, 12:01:22 pmNot dangerous voltage as such, but dangerous current. It's not uncommon to have a logic rail capable of 10A or more, and it would be very bad news if you managed to short that out. And of course, with a x10 attenuator one could measure up to +/- 100V common mode, with 50V differential, which will suffice for a lot of power supplies.Of course, this is just one application. I tried to design it to have low noise, and good signal integrity. And to be honest, I'm learning a lot of this as I go.Quote from: codeboy2k on February 05, 2013, 12:44:03 pmDamn, that's a good point. Should be possible, though, with some isolation materials and a little bodging.Again, thanks to everyone for the feedback

author: hammil
author_rank: Regular Contributor
timestamp: February 05, 2013, 12:58:29 pm
content: Quote from: fcb on February 05, 2013, 12:38:52 pm1. Yeah, that's a very good point. Should be possible to do, though.2. Need to get power from somewhere, and using a battery with a couple of ICs seemed the easiest way. The noise should be pretty low as well - 16uV at the worst worst case, as codeboy2k calculated.3. Working on it4. 5. Yeah, that's definitely something I'll consider, but I reckoned an external attenuator would do a better job.Thanks

author: BravoV
author_rank: Super Contributor
timestamp: February 05, 2013, 01:02:33 pm
content: Interesting thread, thanks for sharing.Noob questions :- So this thing can not be used on probing high voltage like mains powered AC to DC power supply circuit ?- How good is this probe's performance compared to using the common two probes trick with both ground clips connected to probe differential signal ?

author: fcb
author_rank: Super Contributor
timestamp: February 05, 2013, 01:10:27 pm
content: FWIW Hammil, I would deal with the attenuation inside the case and PSU outside the case (if you've run out of space).Conducted noise might only be 16uV, radiated noise (L1 & tracks) is another matter altogether.

author: cyr
author_rank: Frequent Contributor
timestamp: February 05, 2013, 01:50:25 pm
content: Using a 10x passive probe won't help with common-mode range if you do it like this, I would change the design so that it has two probe inputs instead (something like the Tek AM502)http://www.barrytech.com/tektronix/tektm500/tekam502.htmlHowever, I'm not sure how well a pair of cheap passive probes will match (probably not very well, destroying your CMRR).Still it would be an improvement on using the math function on the scope, better usable range and only one scope channel used...

author: poorchava
author_rank: Super Contributor
timestamp: February 05, 2013, 02:11:14 pm
content: I think you still need to use insulation 1:1 mains transformer. In case your scope is powered from different phase than DUT, you can have like 400V between scope ground and either input of the probe. It's even worse if you need 2 or more channels at the same time: common mode may be a problem. Not because attenuation, but rather plain semiconductor damage because of high voltage.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 05, 2013, 02:48:07 pm
content: Quote from: poorchava on February 05, 2013, 02:11:14 pmare you saying 2 very far apart isolated system? if yes then lets assume the worst, like what? 1K, 5K, 10K, 100KV ground differential?. no differential probe can survive that AFAIK if its beyond their rated limit, arching is another issue.its beyond the realm of engineeringwe are talking something similar to ESD. we have to limit our scope of discussion, ie assuming we can bring both ground potential to equal (or within limit) then all the specs (CM) will become make sense.Quote from: BravoV on February 05, 2013, 01:02:33 pmhe is still lacking for the attenuator or coupler there.Quote from: BravoV on February 05, 2013, 01:02:33 pmby using external diff probe:1) you save one channel of your dso, ie you can use 2 diff probes to measure 2 separate diff signals using your 2 channel dso. and also using math function of your dso will reduce its performance (ds1052e tested)2) theoritically... you can tune the diff probe's circuit to get better CMRR compared to 2 passive probes trick. but... thats the "most fun" part if you want to beat the aftermarket products. opamp's CMRR spec is just the limiting factor of what you can achieved (i'm yet to get any closer). once you put your attenuator, you are on your own. in my case, moving my finger near and around the attenuator changes the CMRR, magic!but i have to admit, its my limitation in knowledge and experience and i'm restricting myself to jellybeans parts (for a reason).

author: BravoV
author_rank: Super Contributor
timestamp: February 05, 2013, 03:17:10 pm
content: Quote from: Mechatrommer on February 05, 2013, 02:48:07 pmAny idea what is the ball park number (quantitatively) for best case scenario ?Honestly I'm actually interested to build one too, its just I need to know what should I expecting, worst and best case scenarios to judge if its worth it.PS : I do have a 1000 VA isolation transformer.

author: hammil
author_rank: Regular Contributor
timestamp: February 05, 2013, 03:18:14 pm
content: Quote from: BravoV on February 05, 2013, 01:02:33 pm1. Not without attenuation, no.2. I don't think that will always work, since the scope still measures the voltage referenced to mains earth. I'll look into it further, though. However, either way it still uses two channels, whereas my solution should only use the one.Quote from: fcb on February 05, 2013, 01:10:27 pmThat might be an option, yeah. I've ordered a few boards from oshpark, so I should be able to see if there's a visible effect from noise.Quote from: cyr on February 05, 2013, 01:50:25 pmThanks for the thoughts. What I'll probably end up doing is having a (selectable?) x10 or x100 attenuator inside the case, and advise that the probe be used in x1 mode.Out of curiosity, though, if I did have two probe inputs, where would I connect them, and how would this improve the common-mode range?Quote from: poorchava on February 05, 2013, 02:11:14 pmI can see why that would be a problem, however using a transformer will add a high-pass filter unless I used some switching trickery, which would vastly increase the complexity of the design. Thank you for the information, though.One solution for the common-mode problem, I believe, would be to simply have a (selectable, of course) attenuator referenced to mains earth itself.... Other than that, I can't seem to find any simple fixes. I may just have to accept this as a design limitation.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 05, 2013, 03:45:05 pm
content: Quote from: BravoV on February 05, 2013, 03:17:10 pmdepending on your luck, if you are hobbiest. somewhere around 80dB at low freq, and/or 60dB at higher freq if you are an engineer.

author: Marco
author_rank: Super Contributor
timestamp: February 05, 2013, 04:37:22 pm
content: Quote from: poorchava on February 05, 2013, 09:59:32 amWon't you be hanging a huge amount of stray capacitance on the DUT in that case?

author: codeboy2k
author_rank: Super Contributor
timestamp: February 06, 2013, 05:43:30 am
content: Quote from: fcb on February 05, 2013, 01:10:27 pmYeah I was concerned about that too, especially saw L1 so close to the output BNC. I would like to see the entire power supply capped over and shielded on the PCB; if not, at least shield L1.

author: hammil
author_rank: Regular Contributor
timestamp: February 06, 2013, 07:16:51 am
content: Quote from: codeboy2k on February 06, 2013, 05:43:30 amIt was either that, or have it closer to the amplifier, which would be much more sensitive. Looking at it now.. I could have L1 on the other side of the PCB, which would help, I'd imagine.How easy/difficult is it to make a basic metal shield can? I'm really not too 'handy', unfortunately.On an unrelated note, I have updated the schematic to include a x10 attenuator that I believe attenuates both the differential and the common-mode components of the input.In fact, it would only require a few extra resistors and jumpers to make a selectable x1/x10/x100 attenuator there.

author: fcb
author_rank: Super Contributor
timestamp: February 06, 2013, 09:52:01 am
content: Hi Hammil,I don't think I would bother with x1 option. The second design should be 'worry-free' in operation, very low risk of cooking anything. If you need x1 then I would put a second opamp on the output that has a switchable gain. If you wanted to make it really mean - make the front end switchable AND add a switched-gain opamp in the back. You can at least choose opamps based on specific places in the circuit.I would still get rid of the switcher in the unit - if you want to keep it, add some inductance in the power lines and island the switcher ground. Again though, I would definetley put the PSU outside the 'head' unit - there's another possible benefit, if you fitted something like a 9 pin D on the side of the unit (or something smaller - perhaps a six pin Hirose HR10A), you could then have a couple of control lines from the external supply - these could drive two small relays, which make up an attentuator & gain control. If your worried about current drain - there's some neat latching relays available.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 06, 2013, 11:12:13 am
content: (for no particular reason) let me highlight this, since this is the most interesting part for me. have fun doing thatthats where you will learn about matching circuit and parasitics.

author: fcb
author_rank: Super Contributor
timestamp: February 06, 2013, 03:40:56 pm
content: You might find this Bob Pease show informative:

author: Marco
author_rank: Super Contributor
timestamp: February 06, 2013, 08:44:49 pm
content: Quote from: Mechatrommer on February 05, 2013, 03:45:05 pmTektronix's universal differential probes don't even get that ...http://www.tek.com/datasheet/differential-probe-differential-probes

author: Unknown
author_rank: Unknown
timestamp: February 06, 2013, 10:45:06 pm
content: 10x probes are typically 1% accurate in attenuation (at least the expensive ones that actually come with a datasheet). This limits your CMRR to 40 dB max (at DC). 1x probes are better, but limit the BW to a few MHz. Tektronix made some special 10x probes with adjustable attenuation (P6055/P6135?). These had a variable resistor in parallel with the scope input to vary the attenuation ratio. I guess you could do something similar by designing the amplifier for >> 1 Mohm input impedance and put a 990k + 25k trimmer in parallel.60 dB requires better than 0.1 % matching between the two channels. Are you going to match the caps in the input attenuator own to better than 0.1 %? What about the voltage coefficient (the capacitance of a cap changes with applied voltage). I believe that 'cheap' differential probes (eg. the ~$300 ones) have their CMRR limited by the voltage coefficient of the attenuators.

author: hammil
author_rank: Regular Contributor
timestamp: February 06, 2013, 11:40:50 pm
content: Quote from: fcb on February 06, 2013, 09:52:01 amThat's a pretty good idea. Especially since I'm considering making different versions of the board. ( A JFET-input and a transformer isolated one, if you were curious ). The power supply/control board would almost certainly be identical between the versions, so that might be handy. For a connector, I'll probably go with a .1" header, just to cut down the cost.Relays would be awesome, but they would cost around £3 each from what I can see, which as it stands would easily double the BOM cost.Quote from: alm on February 06, 2013, 10:45:06 pmAbsolutely - I was planning to do something very similarQuote from: alm on February 06, 2013, 10:45:06 pmI'll just have to get the best trimmer caps I can reasonably include... The main limiting factor here is cost; I'd like this to fill the gap in the market of 'entry level' differential probes.Thank you very much for the info and advice

author: robrenz
author_rank: Super Contributor
timestamp: February 07, 2013, 12:08:21 am
content: Take a look atthis DXC100A10X-100X probe manual from Lecroy. It hasschematics and calibration procedures. This gives an indication of what is involved in optimizing CMMR and response. I have one of these and the Preamble/Lecroy DA1855A 100MHz diff amp it mates with.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 07, 2013, 03:42:28 am
content: just for the "mailbag update" i've ordered few 0.1% 1Mohm resistors, varieties chocollete of var caps 4-60pF, 2-30pF, 4-10pF etc from digikey, and only 2 Bourns 1Kohm multiturn trimpot (i wish i can get more but they are expensive), i wish i can get 0.05 - 0.01% resistor but, thats what my "error budget" tells me for now. if your design space does not limit you, maybe you can build your own or buy variable butterfly capacitor, just like in robrenz's 7a22 teardown. maybe it has more resolution for matching and stability purpose, i'm not sure. pcb layout is another issue, high impedance node of the attenuator likes to couple with anything near it including pcb gnd plane and copper pour, i cant afford to keep ordering prototype pcb from itead my budget is limited

author: hammil
author_rank: Regular Contributor
timestamp: February 07, 2013, 02:08:02 pm
content: Quote from: robrenz on February 07, 2013, 12:08:21 amAwesomeThis will come in very handyQuote from: Mechatrommer on February 07, 2013, 03:42:28 amTryhttp://oshpark.com/. $5 per square inch, 3 good quality boards, 12-day turnaround.

author: Marco
author_rank: Super Contributor
timestamp: February 08, 2013, 08:31:51 pm
content: Quote from: alm on February 06, 2013, 10:45:06 pmI'm intrigued by Douglas Smith's balanced probe as a way to sidestep this problem .... he uses a pure resistive divider with coax just like DIY single ended passive high frequency probes, but uses 2 for differential measurement with a BALUN/combiner instead of a difference amplifier.I wonder if this wouldn't be superior for the HF part of the spectrum, obviously it can't work down to DC ... but you can use a separate measurement circuit (with it's own high impedance attenuators) for the LF and combine them.

author: ericjackson05
author_rank: Newbie
timestamp: May 24, 2016, 03:01:31 am
content: Sorry my English, which is not my mother tongue. I try my best to explain explictly.(-:I have been designed a differential probe according to your circuit diagram. Some problems occur in the debugging process.1. The provided current of button cell such as CR2032 is not enough for this application, the voltage of which is pulled down to below 2V. two AA batteries is OK.2.The output current of MAX865 is not enough. positive and negtive 12V pulled down to +8V and -7V or so;3. The input signal amplitude of AD8130 is only -2.5V to 2.5V according to chip manual and my test, which is not rail-to rail. Which show strong limitation.My email is ericjackson05@qq.com, any one can contact with me if you have interest. (-: