Help with nanoamp meter - Page 1
Forum section: electronics / beginners

author: Atomillo
author_rank: Regular Contributor
timestamp: February 14, 2021, 12:40:29 pm
content: Hello:First of all, this my first post on this forum, so if this is posted in the wrong spot, please tell and I will inmediately try and move it to the appropiate spot.I tried to create a pico ammeter, similar to the ones already discussed in this forum, using a low leakage op amp and a high valued resistor in a TIA configuration. I used a LMC6082, a 1G 10% resistor and a 680pF polystyrine cap to avoid oscillations. I connected the power supplies and such in a protoboard, but lifted the input pin and soldered the resistor and capacitor in air. I then cleaned it all using 98 alcohol and dried it.Because I dont have any of the specialized test equipment, I was unsure of how to test the thing. What I did at the end was to connect another 1G resistor to a variable power supply and to the input. To measure the current, I connected a Fluke 87V in the mV range in series. I then measured the output voltage, and estimated the gain in V/nA.What surprised me the most is that the gain is not always the same! In fact, it always decreases with increasing current. I repeated the measurement several times and the result was always consistent. The left column is the input current, and the right one the measured gain in V/nA:I(nA) G(V/nA)3.5 0.94.4 0.835.6 0.86.6 0.788.7 0.7610.8 0.73What could cause this? The whole thing is also inside a cookie tin, and the noise floor is around 20mV).Many thanks!

author: RoGeorge
author_rank: Super Contributor
timestamp: February 14, 2021, 12:59:14 pm
content: You may want to attach a hand-drawn schematic and a picture of the physical setup.

author: Atomillo
author_rank: Regular Contributor
timestamp: February 14, 2021, 01:07:30 pm
content: This is the schematic, and as soon as I get home I will post pictures of the physical set up as well.At first I suspected leakage, but you would expect the gain to go up with higher currents or remain constant, not actually go lower.

author: exe
author_rank: Unknown
timestamp: February 14, 2021, 01:15:43 pm
content: Quote from: Atomillo on February 14, 2021, 12:40:29 pmI'm not confident this is an accurate measurement. I'd vary power supply voltage (in series with a resistor) and measured output voltage. This way we eliminate any potential non-linearity from Fluke.

author: Gyro
author_rank: Super Contributor
timestamp: February 14, 2021, 01:28:22 pm
content: You may want to look at this thread regarding washing Polystyrene capacitors in IPA (alcohol) - It may take a few days for its leakage properties to return. The thread also contains a link to the the Picoammeter-design thread...https://www.eevblog.com/forum/beginners/polystyrene-caps/

author: Atomillo
author_rank: Regular Contributor
timestamp: February 14, 2021, 02:02:51 pm
content: Exe:I did another run measuring the voltages at the input and output. There is a still a trend towards lower gain at higher currents (calculated assuming that the input resistor was 1G) :Input voltage Output Voltage Gain1,3 1,075 0,8269230772,09 1,643 0,7861244023,7 2,675 0,7229729734,66 3,3 0,7081545065,87 4,04 0,6882453156,85 4,7 0,6861313877,34 5,037 0,6862397829,3 6,5 0,69892473111,72 8,13 0,69368600712,43 8,69 0,699115044

author: Atomillo
author_rank: Regular Contributor
timestamp: February 14, 2021, 02:25:29 pm
content: Gyro:That was the thread I was refering in my first post! The thing is that the circuit was soldered and cleaned yesterday, so I would expect the leakage to have gone down already. I'm quite at a loss right now.

author: exe
author_rank: Unknown
timestamp: February 14, 2021, 04:11:35 pm
content: Does the output drift? Esp. at higher input voltages.

author: penfold
author_rank: Frequent Contributor
timestamp: February 14, 2021, 04:39:05 pm
content: Strange that its so non-linear, double check any voltage coefficient of resistance, I wouldn't typically imagine it to be significant at these voltages, but worth considering none the less

author: Atomillo
author_rank: Regular Contributor
timestamp: February 14, 2021, 04:56:05 pm
content: I think I've found the solution. In the attached photos I show my current set up. The 1G resistor connected with the yellow alligator clip is my input. Previously, I connected the other end using another alligator clip to the input pin of the op amp. I realized that whenever I moved this wire, the readings would change. Also, as exe suggests, sometimes at higher voltages the output would not be stable at all. Also, by letting it's insulation touch the black cable I use for ground the readings would change rapidly.By connecting it directly this way, I'm getting much much better results:Vin: Vout: Gain:1,077 1,094 1,0162,232 2,215 0,9923,214 3,166 0,9854,260 4,192 0,9845,440 5,341 0,9826,230 6,100 0,9797,380 7,220 0,9788,160 7,980 0,9789,490 9,260 0,97610,410 10,150 0,975I feel like I start to start Dave "hold your tongue at the right angle". Next step is to get proper connectors, PCB and shielded box.Thanks for all the help!!

author: Kleinstein
author_rank: Super Contributor
timestamp: February 14, 2021, 05:49:11 pm
content: Many OPs don't like capacitive loading to the output. Directly driving an output from an OP is usually not that great - just a DMM or 2 m of coax max be enough capacitance to make some OPs oscillate. So it may be a good idea to have something like 100 Ohms in series with the output - this usually solved the capacitive loading problem.Edit:One may have superimposed mains hum, and this may drive the OP into saturation at some times. This can cause a nonlinear relation.

author: penfold
author_rank: Frequent Contributor
timestamp: February 14, 2021, 06:10:02 pm
content: Quote from: Kleinstein on February 14, 2021, 05:49:11 pm+1 to that, also also noticing no supply decoupling, so that's another route for PSU noise and stability issues

author: Atomillo
author_rank: Regular Contributor
timestamp: February 14, 2021, 06:18:14 pm
content: Hi:Adding a 100 ohm didn't change anything. Scoping the output didn't reveal any oscillation besides down in the 20mV range due to my poor shielding.The results In showed before are reapetable to a good degree of accuracy. I'm starting to suspect that the only reliable way to test circuits in this current range is to solver everything in a shielded PCB.EDIT: I said the measuremts showed before were not respetable. They ARE respetable. Sorry for any confusion cause by my poor use of the english language

author: Atomillo
author_rank: Regular Contributor
timestamp: February 14, 2021, 06:21:32 pm
content: Quote from: penfold on February 14, 2021, 06:10:02 pmAdded a 10pF decoupling cap. No difference, because the supply I'm using is quite clean (noise in the 10mV range)

author: penfold
author_rank: Frequent Contributor
timestamp: February 14, 2021, 06:42:41 pm
content: Quote from: Atomillo on February 14, 2021, 06:21:32 pmIs 10pF a typo? Whilst its probably a little overkill I'd put 10uF or so where the power comes into the breadboard and 100nF as close as possible to the opamp's power pins.Have you tried checking the output with an oscilloscope? as Kleinstein suggested, oscillation is possible... also, it would be wise to terminate the inputs of the spare amp in the package, it could be doing something unexpected and coupling to the amp under test via the PSU lineOne note about the breadboard... you've clearly gone to some efforts to minimise its problems, kudos!

author: Atomillo
author_rank: Regular Contributor
timestamp: February 14, 2021, 07:01:07 pm
content: Oops I'm used to using small capacitors for high frequency decoupling sorry. Used a 10uF and 100nF as you described and grounded the unused inputs. It worked the same.Probing the output, it is a clean DC signal. The only noise is down into the 20mV region, and is electromagnetic noise (I suspect coming from one of my lights) caused by my poor and improvised shielding.Regarding the breaboard: before embarking in this project, I read a bit about low level measurements (Keithley handbook is a must) and drooled quite a bit at the sights of input stages of electrometers (I specially liked the 617). In my next order to Mouser I will buy some Teflon standoffs to make the final version tidier (along with 1G resistor with better precision than 10%...)

author: exe
author_rank: Unknown
timestamp: February 14, 2021, 07:46:06 pm
content: Well, lets assume the noise is 20-30mV p-p. Looking at your table, in lower range I see offset at around ~20mV or so. Which might be because of noise rectification in IC. Of course, this is very vague calculation, but it seems that overall gain around ~0,975. It is higher at low voltages because of the noise.Breadboard circuits don't always behave as expected due to parasitic inductances, noise pickup, leakage, etc. The clue is how wiggling wires affect output.

author: penfold
author_rank: Frequent Contributor
timestamp: February 14, 2021, 08:02:05 pm
content: Quote from: Atomillo on February 14, 2021, 07:01:07 pmWhilst breadboard is a terrible choice for anything more than a 555 timer, there shouldn't be any major problems with what you've done. That being said, it is far easier to debug something from pictures when its "solidly" built, there isn't anything wrong with the circuit per say, but there's a lot of unknown (remember there's also a black rubber adhesive strip connecting all those breadboard wire clip thingies that will have some resistance, its just very hard to judge from here how much that is and whether its affecting your result).I would recommend building on fibreglass perf. board, its a little more expensive than the SRBP but it can be more "predictable". TE Connectivity HBA series resistors are good for high values.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 15, 2021, 09:49:58 am
content: For a slow circuit a breadboard is not that bad for a first test. A main problem is quite some parasitic capacitance.For shielding a circuit on a bread board, I like an upside down cooky box (metal). One has good access and when done putting the circuit together just cover with the shied (main part of the box) - thin cables can sitll go out to the side.Decoupling is not so much about reducing the supply ripple, but to provide a low supply impedance at higher frequency. With a circuit that is essentially DC one would not see too high a supply impedance as ripple, but it can still make the circuit more prone to oscillation and RF pic-up. In a linear circuit it is more the GBW of the OPs that matters not the actual frequency used in the circuit.It is only with a circuit that is actually switching and this has significant AC current to the supply that one would see high impedance as supply ripple.

author: Terry Bites
author_rank: Super Contributor
timestamp: February 15, 2021, 11:28:46 am
content: The dark forces of dielectric adsorption creeping up on you. Seehttps://www.edn.com/wp-content/uploads/2016/11/1982-10-13_EDN_Pease-capacitor-soakage.pdffrom Mr Pease. The input wiring may be charging and leaking in uncontrolled way.Guarding will help kill stray input capacitance and leakage. Make sure you're breadboard is bone dry.Give the op-amp some input protection before it gets zapped.You can reconfigure the feedback (at the expense of some increased noise) With a tee network. This allows you to use less exotic resistors and lower the charging voltage across the feedback capacitor.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 15, 2021, 01:34:00 pm
content: Extra diode for input protection are not that practical for an pA meter. The more practical way is to increase the resistor just before the OPs input to some 100K to 1 M, so that the OP internal diodes can stand the current from a more usual ESD event.2N4393 does not make a good low leakage diode - of at all a 2N4117 or a purpose made low leakage diode like BAV199.

author: RoGeorge
author_rank: Super Contributor
timestamp: February 15, 2021, 01:46:59 pm
content: Could be the capacitor, but I would rather suspect the breadboard.Before debugging, I would remove the breadboard completely. Use a copper clad PCB instead, with islands to solder and the remaining copper grounded, all well cleaned before tests, preferably in an ultrasonic bath.Put everything in a shielded box, or at least put the board inside a grounded metal food-cane while measuring.

author: Atomillo
author_rank: Regular Contributor
timestamp: February 15, 2021, 03:38:39 pm
content: Hello all:I have great news!!! I did what you all told me and soldered the circuit in a small piece of perfboard (sadly I don't have copper clad) and shielded it using an upside down metal cookie box. After cleaning and drying the results are great! The output follows closely the input voltage with a small difference (as stated previously, the 1G resistors have quite an error margin) that seems to always be the same. I will wait until tomorrow to make sure the measurements are repeteable before reporting them, but so far (three measurements with around 1h in between) everything is looking golden.Terry and Kleinstein:This is just a prototype, in the final version I intended to use input protection. In the X Chapters, image 1x.124 there is a very interesting plot of leakage current of FETs and BJTs used as diodes (is it ok for me to post a photo of the graph? Am I breaking any copyright or rule?). What I didn't know was that the IC had protection diodes incorporated.... That certainly makes it easier to work with.I also will look at the tee network. Also in the Art (that book sure is a gold mine!) they refer to it as the Resistance Multiplier (in page 259 of the third edition). What compelled me to not use it is the increased offset voltage. If I switch to an op amp that has external compensation pins available or as in the already linked pico ammeter thread add an offset correction, then I would consider it as a pretty good option for pA/nA level measurements. I would imagine that the increased noise make it unsuitable for fA measurements, although that level of precision is well beyond me!

author: RoGeorge
author_rank: Super Contributor
timestamp: February 15, 2021, 03:51:25 pm
content: Great!No copyright problem with posting just a photo of a page, and state the source and say all the copyrighted materials are not yours, and belonging to their legal owners.

author: Atomillo
author_rank: Regular Contributor
timestamp: February 15, 2021, 09:47:13 pm
content: I have good and bad news.The bad news is that the circuit worked previously because I had not connected the capacitor. Thus, scoping the output showed oscillation at around 255Khz and 800mVpkpk.Getting rather demoralized at the thought of complex things I have no idea about like dielectric absorption and such, I realized that my capacitor was a lot bigger than that used by Alan Yates and other (680pF).Connecting another capacitor in series (thus a total capacitance of around 340pF) immediately solved the issue, reducing the aforementioned oscillation to less than 50mV pk pk, and also gets me a perfect dc response. It seems like I had reduced the bandwidth way too much and made a poor integrator without realizing!Oops! I will let this rest for the night and tomorrow I will make sure everything is right before closing this case. In any case, many thanks to everyone!!!!

author: exe
author_rank: Unknown
timestamp: February 15, 2021, 09:55:09 pm
content: That's weird, the bigger the cap, the less bandwidth the circuit has, thus it is more stable. Can you please share the waveform of that 50mV p-p? I wonder what the frequency is. If it's 50/60Hz (or multiple of it) it's mains hum. Otherwise it is oscillating, or it's a noise from somewhere.

author: Atomillo
author_rank: Regular Contributor
timestamp: February 16, 2021, 01:21:10 pm
content: I attach both the waveform obtained with the two capacitors in series as well as the oscillation present when there is no capacitor whatsoever.With two capacitors in series, the circuit works perfectly now, no problem whatsoever. It might very well be that the oscillation in the 20mV range is noise of some kind.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 16, 2021, 01:34:04 pm
content: The signal with the 2 caps does not look much like noise, though it does not look like an oscillation either. It looks more like EMI, like a radio signal, SMP ripple or some other relatively strong interference. Ocillation would normally be a more repeatable waveform and noise would be more random, except if the circuit is very close to the edge of oscillation.

author: Atomillo
author_rank: Regular Contributor
timestamp: February 16, 2021, 02:11:28 pm
content: It definitely is not oscillation. Adding another capacitor in series doesn't change the amplitude or frequency of the oscillation at all. The circuit sits in a shielded box, without any trace of hum. I will follow previous advice and solder a 100nF directly to the board and see if it is a PS problem.

author: Atomillo
author_rank: Regular Contributor
timestamp: February 16, 2021, 02:42:12 pm
content: Okey great news a 100nF helped tremendously. The remained noise seems to be pretty much EMI. However, I already can report great results. With the input connected to the turned off power supply, a Fluke87V measures 0.3mV (presumably from the alredy mentioned interference). Preeliminary tests with mV signals reveal great perfomance, even though the offset is noticeable. Note how the output cable goes outside the shielded box. It is interesting how just covering it with sticky piece of aluminum tape (and connecting this "shield" to ground) reduces the signal measured in the oscilloscope by a noticeable degree. If at the end I want to display the signal on an oscilloscope, I think I should just add a low pass filter (which it seems like the Fluke already has incorporated).

author: penfold
author_rank: Frequent Contributor
timestamp: February 16, 2021, 04:14:42 pm
content: Is the box painted on the inside? Its clearly not presenting a problem to you now, but its worth keeping in mind that you can get strange effects due to static charges and things I can't remember the name of, so sometimes coating it with a relatively high resistivity coating like Aquadag or nickel loaded spray paint can give avoid some problems particularity if you're chasing down to fA's or have high voltages involved

author: Atomillo
author_rank: Regular Contributor
timestamp: February 16, 2021, 04:53:34 pm
content: Well penfold, it very well might have been the paint (it is indeed painting in the inside), or charge building in my vinyl gloves, anything else.I think that the op amp is dead. When turned on, it saturates or gets very close to doing so, then after a minute or two it briefly goes down to the correct value and then goes to saturation again. In the next step, I will add the protection resistor, and also a more robust input connector.It will be a while because I just started the memorizing process of informations for exams known as university, but will report in as soon as everything is set in.Once again, thanks to everyone. This forum has been extremely welcoming and helpful, unlike most internet places I've visited, and will do my best to contribute to it and make it even better.

author: penfold
author_rank: Frequent Contributor
timestamp: February 16, 2021, 05:08:28 pm
content: Quote from: Atomillo on February 16, 2021, 04:53:34 pmI wasn't really suggesting it could be what killed it, just maybe in the future if you have any strange inconsistent offsets or strange non-linearities,Best of luck in your exams

author: Terry Bites
author_rank: Super Contributor
timestamp: February 17, 2021, 01:49:08 pm
content: The BAV199 has a reverse leakage of at least 100pA at zero voltsThe 2N4393 has a maximum reverse leakeage of 0.1nA2N4117A less than 10pA.For ultimate low leakage use PAD1 or PAD5 type diodes.https://www.mouser.co.uk/ProductDetail/Vishay-Semiconductors/PAD1/?qs=%2Fha2pyFadui1ghIWWxBShCxeDO3JbvBERememeber that under normal operating condions there is zero volts across the diodes or jfets and in practice this means the leakage is extremely low.

author: exe
author_rank: Unknown
timestamp: February 17, 2021, 05:57:20 pm
content: Quote from: Terry Bites on February 17, 2021, 01:49:08 pmHow do you know this? The datasheet (from nexperia) promises much better performance.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 17, 2021, 08:58:16 pm
content: The datasheets have quite a large spread between the typical and maximum leakage specs. This is because testing to low leakage takes time and is expensive.The numbers, especiall for the test limits are different between manufactures, so the number can vary somewhat From the DS I have, I getPhilips BAV199: 3 pA typical and 5 nA max at 75 V (rather high voltage)Interfest 2N4391 100 pA max at 20 VFairchild An6609 process 51: typical current near 15 pA for 20 VThe BAV199 has a low typical leakage, but a rather crude test. The main difference for the way more expensive PAD1-5 is that they are actually tested to lower currents.So chances are that at some 10 V the leakage of the PAD , BAV199 and 2N4117 JFET gate is similar, just the price is different (some $20, $0.20, $5 ), refelcting the test limits of some 2-5 pA , 5 nA and 10 pA. I would be surprized if the lower cost MMBF4117 are actually tested to 10 pA - more like they accidentially coppied the data from the 2N4117.Diode leakage may be an interresting thing to measure with the DIY nV meter. Just make sure to keep RF out.Edit:For testing the TIA type meter, the offset / bias current is measurend with an open input. The TIA does not like the input shorted and than gives erratic errors from the OPs offset voltage. So the test case is an open input. The capacitor in Fb should be relatively small, maybe some 10 pF (preferrably PS type) or the like.

author: Gyro
author_rank: Super Contributor
timestamp: February 17, 2021, 09:57:54 pm
content: With CMOS opamps, I found it better to include a decent package size series resistor on the opamp input (inside the feedback loop) and use the on-chip protection diodes for minimum leakage current.Eg. A 1M 2W resistor will limit the protection diode current to 1mA at 1kV - well within ratings. It works particularly well with opamps that have bootstrapped protection diodes such as the LMC662 or LMP7721 which givetypicalinput bias currents of 3fA and have internal protection diode rating of 5mA (10mA on the LMP7721).It is a cheaper approach too.