50MHz 40A AC/DC current probe for 800USD (ETA5540B) - Page 1
Forum section: electronics / testgear

author: TopQuark
author_rank: Frequent Contributor
timestamp: February 07, 2022, 04:34:35 am
content: http://en.tw-eta.com/index.php?c=show&id=246I have always been in the market for low cost current probe for power supply development. I have bought the ICP5025 20A 25MHz before (https://www.eevblog.com/forum/testgear/tek-clone-instrance-icp-5025-current-probe/), but there's serious issues with the probe, mainly overshooting at low and high frequency, that I don't trust it.I was browsing taobao yesterday, and came across this thing. (https://item.taobao.com/item.htm?spm=a1z09.2.0.0.6c0a2e8dbLhBBy&id=642099114415&_u=71pvoq9c855a) 50MHz 40A AC/DC current probe for 800USD. ETA is a brand I have bought from before, and their product quality is quite good, so I thought I'd give it a shot. Placed an order, think it will take some time for the probe to arrive as China is still recovering from Lunar New Year.They did advertise square wave measurement being square as one of the selling points, so I am hopeful it will be better than the ICP5025. There's also a 100MHz option, but I don't see myself needing the extra bandwidth.I know buying used current probes is also an option, but hey, I guess someone has to try this thing out for the forum.

author: TopQuark
author_rank: Frequent Contributor
timestamp: February 10, 2022, 05:11:44 am
content: Guess what arrived in the mail today...

author: TopQuark
author_rank: Frequent Contributor
timestamp: February 10, 2022, 05:15:41 am
content: Interestingly, the 100MHz (ETA5540C), 50Mhz (ETA5540B) and 20Mhz (ETA5540A) model has the same derating curves, meaning the sensing elements between 3 models are probably the same. i wonder if I can upgrade the bandwidth of this thing by tweaking a few passives...

author: TopQuark
author_rank: Frequent Contributor
timestamp: February 10, 2022, 05:29:28 am
content: Full BW (SDS2504X Plus 500M scope) noise is 62mA p-p, 5.5mA RMS, not bad for a single 40A range.Testing small signal response by connecting my function gen to 50 ohm terminated scope input with a piece of coax spliced in the middle. Square waves look square, no overshooting observed, very happy with that.Did a bode plot with the scope builtin AWG, results seem to indicate there's more BW past 50MHz, but unfortunately the builtin AWG can't generate >50MHz signals. Might be time to bust out the VNA...

author: TopQuark
author_rank: Frequent Contributor
timestamp: February 10, 2022, 06:14:37 am
content: VNA measurement matches with the scope bode plot, there's a 4db gain peak and phase wrap around at roughly 50 MHz, but the usable bandwidth of this thing seems to be around 100 MHz. I wonder if the internals can be compensated to give a phase boost...

author: TopQuark
author_rank: Frequent Contributor
timestamp: February 10, 2022, 06:56:00 am
content: Here's the money shot, no more testing with baby currents, taking this thing straight to its 40A limit.Hooked a Lipo battery to my dynamic load (https://www.eevblog.com/forum/projects/gt1mhz-bandwidth-electronic-load/) and generated some 40A pulse loads. Had to remove most of the input capacitance to the load, so that the load's shunt measurement matches with the current probe's reading, hence the ringing in the current waveform.Yellow trace is the control signal into the dynamic loadBlue trace is the amplified current shunt reading on the dynamic load, signal clipped by opamp supply railPink trace is the current probe output.The current probe output matches beautifully with the load's shunt reading. Can't be more happy with this probe.

author: PartialDischarge
author_rank: Super Contributor
timestamp: February 10, 2022, 07:36:30 am
content: The amplitude of the vna looks horrible.And while the last measurements look nice they are not precisely fast, usec risetime?

author: TopQuark
author_rank: Frequent Contributor
timestamp: February 10, 2022, 08:05:04 am
content: Quote from: MasterTech on February 10, 2022, 07:36:30 amYou're right, I could do better.Using the spliced coax with a TLV3501 as a fast edge source, it did reveal some overshooting. On the bright side, rise and fall times of 4.3ns indicates the bandwidth of this thing is over 50 Mhz. Aligns with the observation with the VNA I think.

author: TopQuark
author_rank: Frequent Contributor
timestamp: February 10, 2022, 08:20:36 am
content: Here's some usec level 40A waveforms. I'd say that's pretty good

author: Hydron
author_rank: Super Contributor
timestamp: February 10, 2022, 09:36:53 am
content: Thanks for the write-up, good to see that we're finally getting some more affordable options besides buying an ancient tek probe and amplifier (even those are often more than what this new one costs, especially for confirmed-working examples).

author: alm
author_rank: Super Contributor
timestamp: February 10, 2022, 03:30:18 pm
content: That ancient Tek probe went down to 1mA/div, which I think is a much more useful range for electronicsFor PCB work the limiting factor for me is often the size of the head, and hence the length of wire (inductance) that you need to add to your circuit. The head of the 'ancient' Tek is 11.8mm wide and 7mm from edge of hole to the bottom of the probe so the shortest loop of wire through and then around the probe would be about 38mm. Which is gigantic compared to most SMT components.For higher bandwidth in real circuits the insertion impedance is often the limiting factor. What is the insertion impedance at low frequency and at its max frequency? I didn't see it on their website.

author: TopQuark
author_rank: Frequent Contributor
timestamp: February 10, 2022, 04:18:07 pm
content: The insertion impedance is given in the manual provided with the probe.Probe head is 14.8mm wide, 8mm from edge of hole to bottom of the probe.I think if you get creative with how you insert the probe into the circuit, you might not need to have a wire completely loop the probe head, but I agree probe head size is quite important.

author: PartialDischarge
author_rank: Super Contributor
timestamp: February 10, 2022, 04:35:22 pm
content: Quote from: alm on February 10, 2022, 03:30:18 pmAnother not so used approach is to use a shunt and a isolated probe to measure this voltage, a shunt and a small coaxial connector (mmcx, sma, smb) can be smaller overall than a current probe.

author: alm
author_rank: Super Contributor
timestamp: February 10, 2022, 10:01:09 pm
content: Quote from: TopQuark on February 10, 2022, 04:18:07 pmThanks for the graphs! So the 100 MHz bandwidth figure must be very optimistic only achieved under carefully controlled condition. Especially at higher currents there will easily be a couple of volts drop across the probe at higher frequencies. Or when you wrap a wire around the probe N times: sensitivity goes up N times, insertion impedance goes up N^2 times.I wonder why the insertion impedance graph only goes up to 50 MHz even for the ETA5540C. Funny how the derating graph goes to 0A at 100 MHz. Very useful for a current probe.Quote from: TopQuark on February 10, 2022, 04:18:07 pmThanks for the measurements! I agree that you there are often ways to get somewhat shorter wire, but unless you have a circuit that already has long wires, like in your case with the battery, it's usually that the smaller the head, the less intrusive the measurement. I'd love to have an AC/DC probe the size of the Tek P6022 (6mm wide, 4mm from edge of hole to bottom), but it seems that the probe Tektronix designed in the sixties is still the smallest DC - dozens of MHz probe head. Clearly the physics do not allow a smaller ferrite core with a good enough low-frequency response so the transition from the hall effect sensor to the current transformer is flat across the bandwidth.Quote from: MasterTech on February 10, 2022, 04:35:22 pmThat's true. The so-called "non-intrusiveness" of a clamp-on current probe went away when we started running current over PCB traces instead of through long wires. So you might as well consider soldering in a shunt if the voltage drop and parasitics match your requirements.

author: TopQuark
author_rank: Frequent Contributor
timestamp: February 11, 2022, 06:42:33 am
content: Quote from: MasterTech on February 10, 2022, 04:35:22 pmWhat is a good probe for doing this? Your usual HV diff probe probably is too noisy for that.Btw, revisited the frequency response measurement with a spectrum analyser and TG, here's the result.

author: Martin72
author_rank: Super Contributor
timestamp: February 13, 2022, 12:19:10 am
content: Quote from: Hydron on February 10, 2022, 09:36:53 amThis "ancient" tek system is still the benchmark what measures in very low mA ranges concerns.The big minuspoint of the tek is the "horrible" space it needs on the desk.For a long, long time there are no alternatives to it.But with the instrance ICP I got and now with the ETA, hopes are high to get a usable space-saving probe for relative less money.

author: Hydron
author_rank: Super Contributor
timestamp: February 13, 2022, 12:49:33 am
content: Yeah still not a lot of alternatives to the Tek when you need mA level resolution (though not everyone does, and something is better than nothing). Even the Instrance is only 100mV/A.The old tek probes could really do with a more modern amplifier solution - no need to build half a scope front-end into the thing (1:1 and 10:1 attenuation ranges, directly driving a 50 ohm termination at the scope would probably be enough), nor do it as expensively as Tek seems to have managed with the TCPA300. I've toyed with the idea of a DIY solution (sick of the warm-up drift, space requirement and unreliable range switch of my AM503 setup), but as always it needs time to design something, and that's in short supply.

author: Martin72
author_rank: Super Contributor
timestamp: February 13, 2022, 10:07:42 pm
content: Funny, same here..Playing with the thoughts to build a housing aorund my AM503A module...QuoteAnd the mentioned ETA probe got 50mV/A....Noise is 1mV, you can calculate by yourself what that means.But apart from that, the instrance and now the eta probes are the only ones with degaussing function on the market for under appx 3000 bucks.Or to say it more precisely, the only ones who are using the current compensation method with all its benefits.That makes them so unique, I don´t get it why they´re so hard to get and very unknown here.

author: Hydron
author_rank: Super Contributor
timestamp: February 13, 2022, 10:59:15 pm
content: Quote from: Martin72 on February 13, 2022, 10:07:42 pmIf you get serious about doing more than just making a housing let me know - wouldn't hurt to share some ideas and design effort.

author: Martin72
author_rank: Super Contributor
timestamp: February 13, 2022, 11:34:04 pm
content: I´ll send you a pm in the next days.

author: alm
author_rank: Super Contributor
timestamp: February 14, 2022, 12:17:47 pm
content: Are you talking about building a TM501? Seems a bit of a hassle given the AC voltages they need, but maybe the AM503A doesn't use all of them. That should be easy to check from inspecting the PCB. Please post the project on this forum, probably in a separate topic, if you do!If you're in the US, I'd probably buy a used TM501 on eBay. Though that will be more expensive in the rest of the world.

author: Hydron
author_rank: Super Contributor
timestamp: February 14, 2022, 10:31:33 pm
content: No, I was thinking about building a AM503 replacement. The originals (even the A and B models) all basically have a scope front-end integrated in them - a big stepped attenuator with 12 ranges (in the non-A/B version this is mechanical and thus unreliable) then HF amplifier, offset etc. You're meant to feed them into a scope set at 10mv/div, and do all the A/div and offset stuff on the AM503.This is all totally unnecessary these days - it would be _much_ handier to let the scope itself do all/most of the range/offset, that way you don't need to constantly change the probe ratio on the scope to keep the measurements correct etc. It also introduces substantial amounts of warmup drift from the extra amplifier after the attenuators, especially on the earlier amplifiers (they are using mid-70s tech after all!), and you can't just use a single attenuator setting for a wide range of current as the output amplifier clips at ~+/-100mV or something.One way to improve the setup would be to feed the probe output straight into a 50 ohm scope input, bypassing the attenuator and HF amplifier sections of the AM503. This straight away gives you 1V/A with the small probes, 0.2V/A with the large ones. The main drawbacks are a limited max current range when doing this (the probes only normally see 25 ohms at DC, and thus the DC/LF nulling current amplifier would run out of voltage headroom earlier), needing to be careful not to blow the shit out of a 50R scope input, possibly not being able to AC couple the signal, and needing a decent 1mV/div or better range on your scope for low current use.Given the limited (50-100MHz max) frequency response of the probes some of these issues (risk of popping the input, or not being able to do AC coupling) can simply be solved by using an external 50R terminator on the scope end.The first issue would probably need at least one extra attenuation range (probably 2, so that you can give the large probes a power-of-10 attenuation factor and some ability to tweak DC gain, which seems to be a touch low on these). With a T-configuration 10:1 attenuator you can bypass the first series resistive element at DC enough that the nulling amplifier doesn't need to work too hard (it would be seeing more like 10 ohms than 50), and it would be a damn sight better having only two attenuations to switch between (each of which has a substantial usable range) when you need higher/lower current than a dozen different (much more limited) ones.In addition to the attenuator you'd also need to re-create the hall sensor and DC/LF amplifiers, degaussing and auto-zero circuits (the latter potentially being do-able with a single DAC and a couple of ranges to switch between, plus a sensitive zero-crossing detector). If you want to complicate matters you could also build a 10x output amplifier for getting high sensitivity on a crappy scope, but that's probably not worth bothering with for most and would bring back some of the drawbacks of the AM503.I believe that this method is how Tek is doing it with the TCPA300, but those are insanely priced, even second hand, and also needs the newer probes with lemo connectors (I bet the old ones would work if adapted though!). And it's not unusual to want to use more than one probe at once (I have more than one, but only a single working amplifier/TM501), so a DIY solution to use the older style probes seems like it would be pretty useful. If anyone has pics of the inside of the TCPA300 I'd be very keen to see them to confirm!

author: Martin72
author_rank: Super Contributor
timestamp: February 14, 2022, 11:04:49 pm
content: Hi,Me I was indeed thinking about to build a "smaller" TM501, leave my AM503A as it is..Reduce the AM/TM thing will lead us to the instrance/ETA probes with their known problems concerning noise level.QuoteNot really in ebay germany:https://www.ebay.de/itm/TCPA300-Amplifier-AC-DC-Current-Probe-/203786269164?hash=item2f729bb1ec%3Ag%3ABcgAAOSwCQxh2vRa&nma=true&si=kwIkFloXV0sfg0JnYV2sAw25yp0%253D&orig_cvip=true&nordt=true&rt=nc&_trksid=p2047675.l2557

author: alm
author_rank: Super Contributor
timestamp: February 14, 2022, 11:37:07 pm
content: Without any attenuation would be like the TCP202, which had much worse noise on the lower ranges than the AM503 variants because the built-in amplifier only had a single gain setting. But you wouldn't need a 1-2-5 sequence either. That was useful for pre-readout scopes where you'd only have to look at the AM503 to know the vertical setting and would ignore the setting on the scope.I wonder how much you're going to improve drift, however. My impression is that the probe itself is responsible for a large part of the drift.

author: Hydron
author_rank: Super Contributor
timestamp: February 15, 2022, 10:21:45 am
content: Fair point about the drift, I should probably repeat my tests with a directly-coupled probe, and have a look at the drift of the probe only vs probe-plus-amp.TCP202 is not a fair comparison, as it's always running at 100mV/A due to the termination scheme used in the probe. This allows for full rated current (due to a smaller driving voltage required from the DC/LF current nulling amplifier when driving the lower impedance termination) at the cost of noise. What I'd like to do is what the TCPA300 does - offer a directly-coupled range with 1V/A (for lower noise at the cost of max current capability) in addition to 100mV/A (for higher current use).As for noise, I actually did some tests a while back, and yes using the amplifier does offer somewhat lower noise but it's mainly due to the fact that at 1mV/div a high bandwidth scope is seeing a lot of crap coming in beyond the ~50MHz limit of the probe. When using the 20MHz scope BW limit, I'm seeing only about ~15% higher noise from the directly coupled probe (1V/A with 1mV/div on scope) vs the amplified probe (10V/A with 10mV/div on scope). If that could be kept at a 15% noise penalty in exchange for not building another amplifier (but maybe putting a bit of HF roll-off in there) it's probably not worth adding the complexity. This is all with a decent scope mind (RTB2004), if you have a low-end rigol or something then good luck with that 1mV/div range with it's software magnification and missing codes etc.

author: alm
author_rank: Super Contributor
timestamp: February 15, 2022, 10:05:41 pm
content: I did some testing on an AM503 with P6302 connected and the head locked a long time ago, and back then I observed that the unit needed a long (1+h) warm up before it would be stable at the lower settings. On 1 mA/div, I observed about 1 div (10 mV) drift over 40 minutes after an hour of warm up, which I considered usable: at least the trace will stay on screen. Eventually it would stabilize, as seen in the attached graph, showing three series of measurements with a DMM (set to DCV) hooked to the AM503 output. The large jumps at t=6h and t=12h were me manually adjusting the dc offset on the AM503 to see if it would start drifting again, which it didn't. I don't think I tested conclusively if it was the probe or the amplifier, but I strongly suspected the probe, which may have been abused (dropped) in the past. I've read that the hall sensor is (almost) as good a strain and temperature sensor as it is a sensor for magnetic fields, so this drift does not surprise me.Have you checked if there's any frequency response compensation in the amplifier that you would need to replicate? I would imagine they are adding the signal from the DC bucking current source to the higher frequency signal from the transformer and maybe doing some corrections to make the transition flatter, but I have not studied the circuit in detail. I do know that they increased the bandwidth for the AM503A to support higher frequency probes that never materialized, at the expense of higher noise. And then subsequently reduced the bandwidth again in the AM503B. So some HF roll-off may be helpful indeed, although probably not as necessary if there is no gain circuit.