1:1 differential probe or isolation amplifier - Page 1
Forum section: electronics / testgear

author: nctnico
author_rank: Super Contributor
timestamp: December 21, 2015, 10:09:40 am
content: Does anyone know a 1:1 differential probe which can handle a few volts of DC offset, a signal of +/- 2V and has a bandwidth of >1MHz (preferably more)? This sounds simple but so far I have not found anything which matches. The 1:1 ratio is important because the signals can also be very small but the peaks can be very high.

author: daqq
author_rank: Super Contributor
timestamp: December 21, 2015, 10:15:18 am
content: If you don't need the DC value, you could get away with an (AC coupled) RF transformer, with an output sunk into a load resistor. This isolates the hell out of a signal and you get as much AC bandwidth as you need. You can even use a 2:1 for amplification.The drawback is that this loads the circuit pretty heavily.

author: Marco
author_rank: Super Contributor
timestamp: December 21, 2015, 10:20:57 am
content: How about the Chipwhisperer differential probe? Easy enough to mod for unity gain.

author: tautech
author_rank: Super Contributor
timestamp: December 21, 2015, 10:45:11 am
content: Quote from: nctnico on December 21, 2015, 10:09:40 amAnd a guy with your experience wouldn't float your scope for this? ^^^

author: nctnico
author_rank: Super Contributor
timestamp: December 21, 2015, 10:58:35 am
content: Quote from: tautech on December 21, 2015, 10:45:11 amWell... I can't and I need DC coupling. I have to measure several other points in the same circuit. The 'thing' I want to float is a current shunt resistor which doesn't sit at the circuit's ground level.Building something like the chipswhisperer has crossed my mind but if there is something out there to buy off the shelve life is easier.

author: Marco
author_rank: Super Contributor
timestamp: December 21, 2015, 11:03:57 am
content: Quote from: tautech on December 21, 2015, 10:45:11 amWith some ferrite on the probe cable.

author: tautech
author_rank: Super Contributor
timestamp: December 21, 2015, 11:05:17 am
content: Quote from: nctnico on December 21, 2015, 10:58:35 amOK, nevermind, seems like you don't know how to do it anyway and via the forum I'm not about to tell you.

author: daqq
author_rank: Super Contributor
timestamp: December 21, 2015, 11:10:28 am
content: Maybe measure the DC offset by a normal probe, use AC coupling with high gain for the fine 1MHz+ signal?

author: alsetalokin4017
author_rank: Super Contributor
timestamp: December 21, 2015, 12:47:36 pm
content: I suppose taking a differential measurement with two ordinary passive 1x or 10x probes is too obvious a solution.

author: nctnico
author_rank: Super Contributor
timestamp: December 21, 2015, 01:02:25 pm
content: Quote from: alsetalokin4017 on December 21, 2015, 12:47:36 pmYes. I don't want to sacrifice 2 channels, the CMRR isn't that good, math updates aren't very fast and catching a tens of mV signal riding on a few V offset is likely going to kill accuracy.

author: macboy
author_rank: Super Contributor
timestamp: December 21, 2015, 01:18:03 pm
content: Why not build an instrumentation amplifier? With a simple 3 op-amp circuit you get very high input impedance, very high CMRR, and you can even build in all the gain you want. At 1 MHz this is easy, you could even breadboard it with decent results. This all assumes that the "few volts" of DC offset still keeps the signals within the rails of the op-amps. Another option is a special purpose high side current sense amplifier IC.

author: owiecc
author_rank: Frequent Contributor
timestamp: December 21, 2015, 01:20:56 pm
content: You can just buy an instrumentation amp IC.

author: nctnico
author_rank: Super Contributor
timestamp: December 21, 2015, 01:47:18 pm
content: Quote from: macboy on December 21, 2015, 01:18:03 pmI'm thinking across these lines as well. But I thought I'd ask first before building something I can also buy off-the-shelve. Current sense amplifiers would have been an option too if they didn't had so much gain. I also need to be able to measure 1us spikes up to 1kA so an instrumentation amplifier with a gain of 1 is the best option due to the required slew rate. So far I have found the AD8421 (15Mhz / 35V/us) which can be powered from +/-15V.

author: MarkL
author_rank: Unknown
timestamp: December 21, 2015, 02:01:12 pm
content: If the signal is small, how about a differential amp to increase the gain too? The Tek AM502 starts at x1 and goes up to x100k.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 21, 2015, 02:17:22 pm
content: Quote from: nctnico on December 21, 2015, 01:02:25 pmWell, you opened with a spec of 2V. Your SNR spec just went in the shitterby 40dB, so thanks for telling us about that in the first place...I suppose your CMRR needs to be good well beyond 1MHz as well?? (Ex: current sense on noisy SMPS, let alone the switching node itself)Tim

author: nctnico
author_rank: Super Contributor
timestamp: December 21, 2015, 02:29:39 pm
content: Quote from: T3sl4co1l on December 21, 2015, 02:17:22 pmFortunately this isn't for measuring in an SMPS. For a chance the circuit I'm working on is mostly DC which in this case is challenging enough. There are 2 things I want to measure: a rush-in current pulse up to 1kA and tens of Ampere during normal operation. The specs of 2V just is a realistic-ish target based on the differential probes I have found so far. The current shunts I have are 5m Ohm (5V @1kA) and 0.5m Ohm (0.5V @1kA). Measuring tens of amps would result in signals in the mV range but still with the occasional 1kA spike it should be able to survive.

author: Marco
author_rank: Super Contributor
timestamp: December 21, 2015, 04:10:31 pm
content: Classical instrumentation amplifiers are a silly architecture for high side current measurement, the source impedance is essentially 0, symmetry in the input impedance is irrelevant.

author: nctnico
author_rank: Super Contributor
timestamp: December 21, 2015, 08:08:31 pm
content: I'm planning to measure the low(est) sideA generic instrumentation amplifier box may prove useful for other purposes as well though so going that route and making the input impedance 1M would allow to use it with a standard probe and a selectable gain of 1 or 10 could turn it into a neat pre-amplifier as well.

author: tautech
author_rank: Super Contributor
timestamp: December 21, 2015, 08:36:49 pm
content: Quote from: nctnico on December 21, 2015, 08:08:31 pmIMHO this is where TE equipment manufacturers have let the industry down.We now have all manner of cheap DSO's available with ranges of features that are astounding BUT none with Isolated Channel inputs for the arguably BASIC measurements that you need.Massive marketing opportunity IMO.

author: nctnico
author_rank: Super Contributor
timestamp: December 21, 2015, 09:15:48 pm
content: I agree. I have been thinking about an isolation amplifier design which doesn't use the classic differential amplifier approach. I hope I have some time to create the layout of a first prototype during the Christmas vacation. I keep running into needing isolated oscilloscope channels but there is nothing really good in the market.

author: Someone
author_rank: Super Contributor
timestamp: December 22, 2015, 12:05:50 am
content: Quote from: tautech on December 21, 2015, 08:36:49 pmTek has the specialist TPS2000 series which is the standard for isolated measurements, since it hasn't been updated in a long time you can get a hint as to how popular that market segment is.Quote from: nctnico on December 21, 2015, 10:09:40 amStart with a Tek ADA400, from there you could easily modify it to have lower noise and input impedance.

author: tautech
author_rank: Super Contributor
timestamp: December 22, 2015, 12:30:25 am
content: Quote from: Someone on December 22, 2015, 12:05:50 amPrice?That's the reason sales are low and compare their specs to any modern DSO and prepare to beIsolated measurements per se are not unordinarily specialised, and while Tek might have had this market almost to themselves for a while, is it not time for a change?For one to set themselves up for isolated measurements there is significant cost involved, even 2 basic differential probes can cost more than a new 4 channel DSO.No wonder some people are looking for alternatives.

author: Someone
author_rank: Super Contributor
timestamp: December 22, 2015, 05:38:06 am
content: Quote from: tautech on December 22, 2015, 12:30:25 amThe TPS line is 10 years old and haven't been updated in specs or price, the market is so small they aren't interested but at the same time one of those scopes is comparable in price to a similarly specified differential probe so its what the market will bear. If its such a lucrative and competitive market perhaps you should setup shop and make your fortune.

author: nctnico
author_rank: Super Contributor
timestamp: December 22, 2015, 08:13:31 am
content: There are loads of handheld scopes with isolated inputs so people do buy them. The problem with Tektronix' TPS2000 series is their extremely poor value for money due to the 2.5kpts sample memory and lack of other options.

author: rx8pilot
author_rank: Super Contributor
timestamp: December 22, 2015, 08:56:48 am
content: Quote from: Someone on December 22, 2015, 12:05:50 amI got an ADA400A for around $100 used and it works quite well.Sent from my SAMSUNG-SM-G890A using Tapatalk

author: borjam
author_rank: Unknown
timestamp: April 04, 2016, 02:11:48 pm
content: Quote from: nctnico on December 21, 2015, 10:09:40 amA bit late, but Hameg/R&S sells the HZ109. 1:1 and 1:10 with maximum input voltages of +-3.5 V/+-35 V.Seems to be what you need? The price is similar to the Pintek/Testec and other OEM differential probes around.

author: mikerj
author_rank: Super Contributor
timestamp: February 12, 2019, 06:32:53 pm
content: Quote from: borjam on April 04, 2016, 02:11:48 pmOld topic I know, but we have an HZ109 that's been incredibly useful and went a buy another one today only to find Rohde and Schwartz discontinued themDoes anyone know of any other 1:1 probes on the market with similar specifications and cost?

author: thals
author_rank: Contributor
timestamp: February 12, 2019, 07:59:18 pm
content: Quote from: mikerj on February 12, 2019, 06:32:53 pmThe Pintek DP-30HS seems to fit the bill, with up to 30MHz BW, 65V differential voltage and x1/x2/x10 modes.Cost is around 200 EUR / $230.Specs:(1) Bandwidth:DC - to 30 MHz (-3 dB) for x 2, x 10, DC - to 15 MHz (for attenuation x 1)(2) Attenuation: x 1, x 2, or x 10(3) Accuracy: +/- 2%(4) Voltage Input Ranges (DC + AC peak to peak)≦ +/- 6.5 V for x 1,(i.e about 2.3 V RMS or DC)≦ +/- 13 V for x 2, (i.e about 4.6 V RMS or DC)≦ +/- 65 V for x 10, (i.e about 23 V RMS or DC)(5) Permitted Max Input VoltageMax differential voltage: 65 V (DC + AC peak to peak)Max voltage between each input terminal and ground: 600 V RMS(6) Input Impedance:Differential: 2 MΩ / 1.7 pFBetween terminals and ground: 1 MΩ / 3.4 pF(7) Output: ≦ +/- 7.0 V(8 ) Output Impedance: 50 Ω(9) Rise Time: 12 ns for x 2, and x 10; 24 ns for x 1(10) Rejection Rate on Common Mode:60 Hz: ＞ 80 dB ; 100 Hz: ＞ 60 dB ; 1 MHz: ＞ 50 dBhttps://www.globalmediapro.com/dp/A25XX7/Pintek-DP-30HS-Differential-Probe-30MHz-65V/Edit:And there is also a 60MHz model at around $300.http://www.pintek.com.tw/product_detail/landersound/index.php?Product_SN=224408&Company_SN=6002&Product_Site_Classify_SN=17072High Sensitivity Model: x 1, x 5, x 10 Attenuation1 MΩ // 2.5 PF Input ImpedanceHigh Frequency Operation: x 1(15 MHZ), x 5(50 MHZ), x 10(60 MHz)Especially For Low Voltage: x 1(7 Vp-p), x 5(36 Vp-p), x 10(70 Vp-p)

author: mikerj
author_rank: Super Contributor
timestamp: February 12, 2019, 10:21:34 pm
content: Thank you kindly, that looks idealMy Google-fu is clearly weak.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: February 13, 2019, 01:15:00 am
content: Did you look at Knick?

author: mikerj
author_rank: Super Contributor
timestamp: February 13, 2019, 03:08:14 pm
content: Quote from: Neomys Sapiens on February 13, 2019, 01:15:00 amNo, mainly because I don't know what Knick is!

author: ArthurDent
author_rank: Super Contributor
timestamp: February 13, 2019, 04:56:15 pm
content: This test circuit I tried with parts I had on hand doesn't meet the criteria you're looking for but for my limited use on HV and low frequency it should work for me, at least for now. As others have mentioned, instrumentation amps can be used. This amp is an expensive one but I got a board with 6 of them for basically nothing so what the heck. As for interest in probes like this, I posted this and got no replies so I assume there isn't a lot of interest.https://www.eevblog.com/forum/projects/differential-probe-or-isolation-probe/msg2182481/#msg2182481

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: February 13, 2019, 09:15:11 pm
content: Quote from: mikerj on February 13, 2019, 03:08:14 pmhttps://www.knick-international.com/de/products/index.html