el cheapo micro-ammeter - Page 1
Forum section: electronics / projects

author: dannyf
author_rank: Super Contributor
timestamp: February 19, 2015, 10:28:40 pm
content: In a separate discussion, someone asked how to measure small current. So I thought I would experiment with a cheap ammeter that can go down to ua range -> it actually has the potential to go further down but we will not discuss that now.The design goal is:1) cheap -> utilize easy to get jfet or cmos opamps. I will start with a TL082 now but will migrate to LMC6042 later;2) easy to build -> dual opamp, minimal parts count, perfboard construction;3) work as an adapter for other meters, like a dmm.

author: dannyf
author_rank: Super Contributor
timestamp: February 19, 2015, 10:30:38 pm
content: The basic topology is simple: an opamp plus a big resistor -> virtual ground feedback ammeter.we will use another opamp as rail spliter. Battery powered -> TL082 consumes 5 ma so 60 hours of operation. LMC6042 consumes 30ua and no need for a battery switch.

author: george graves
author_rank: Super Contributor
timestamp: February 19, 2015, 10:32:11 pm
content: subscribed

author: dannyf
author_rank: Super Contributor
timestamp: February 19, 2015, 10:35:45 pm
content: I already built a prototype on a breadboard, with a 12v source.When measuring current in a 9v battery + 1.5Mohm resistor, the output is 0.666v -> fairly close to what you would expect.I will share a picture later.

author: dannyf
author_rank: Super Contributor
timestamp: February 19, 2015, 10:40:01 pm
content: Range selection will be done by wiring different value R3.10k -> 1v/100ua100k -> 1v/10ua1Meg -> 1v/1ua10Meg-> 1v/0.1uaAnything below that gets a little bit tricky as the Ibias for TL082 is 30pa+, depending on the source of the datasheet. Now, Ibias doesn't change very fast so if you just zeroed the meter, the measurement is still good.But that becomes tedious. A low Ibias opamp would be needed if you intend to go below 1v/1ua. That's where something like LMC6042 would come into play, both for its super low Ibias (2fa) and low current consumption (20ua or so if I recall right).

author: dannyf
author_rank: Super Contributor
timestamp: February 19, 2015, 10:41:40 pm
content: The construction will be a little bit tricky, if you wish to go below 1v/1ua: guard ring would be a must as you cannot trust most pcb substrates.For me, I would just use 3d / point-to-point construction on the input pins.

author: dannyf
author_rank: Super Contributor
timestamp: February 19, 2015, 11:34:52 pm
content: Some pictures.First, the micro-ammeter on a breadboard. Red arrow shows the range setting resistor R3 (=100k here). Green box shows Ibias generator.

author: dannyf
author_rank: Super Contributor
timestamp: February 19, 2015, 11:40:23 pm
content: Running the meter at ~10v.Measuring current in a loop with 9v battery + 1.5Mohm resistor -> 0.6v output expected. We measured twice, reversing the battery polarity.If the meter is perfectly zeroed, we would expect identical readings and with opposite signs.

author: dannyf
author_rank: Super Contributor
timestamp: February 19, 2015, 11:44:16 pm
content: Same meter, unzeroed, running under 5v. Swinging shouldn't be a problem but we will have offset. So the difference between the two readings should be about 1140 (571mv - (569mv) = 1140mv).We got two readings, 539mv and (599mv), for a difference of 1138mv, vs. 1140mv expected. Not bad.

author: dannyf
author_rank: Super Contributor
timestamp: February 19, 2015, 11:46:56 pm
content: The Ibias generator has an adjustment range of about +/- 17mv, at 10v power supply, vs Vos of 10mv.That range is greatly reduced at 5v so I could never zero the readings with this particular Ibias generator. However, if I were to reduce the value of R6 to 1Meg, zeroing becomes possible.So when you design for your particular implementation, keep that in mind.

author: dannyf
author_rank: Super Contributor
timestamp: February 19, 2015, 11:48:32 pm
content: You may also notice that I used 100K resistors rather than 1Meg resistor for my build, purely for convenience as I don't have 1Meg resistors handy. It also tells you how unimportant those resistors are, value-wise.I will play with other dual jfet opamps and see how that they perform.

author: dannyf
author_rank: Super Contributor
timestamp: February 20, 2015, 12:08:57 am
content: Here is the meter utilizing a KF353 (LF353 by Fairchild). LF353 has higher Vos so I was not able to quite zero the meter (1mv off) but good enough.So the meter is actually quite tolerant of the opamp choices. I am sufficiently comfident that if you build it around other jfet opamps (including the quads) it would work just as well.

author: dannyf
author_rank: Super Contributor
timestamp: February 20, 2015, 12:18:52 am
content: Voltage sensitivity:Ran the meter at 12v and then 10v on the same current.Reading: 5.71ua vs. 5.72ua.

author: dannyf
author_rank: Super Contributor
timestamp: February 20, 2015, 01:51:37 am
content: Identical implementation by someone else:http://www.vk2zay.net/article/251His unit was built around lmc662(the same family with LMC6042.Notice how the constructed the input?Absolutely necessary for a picometer.Well done.

author: Vgkid
author_rank: Super Contributor
timestamp: February 20, 2015, 07:20:38 am
content: Is the bias generator used to null out the opamp input bias currents?

author: dannyf
author_rank: Super Contributor
timestamp: February 20, 2015, 12:12:56 pm
content: Yes.The difficult with nulling that current is that its direction changes from one opamp to another. The schematic showed a Vcc connection and if you notice my breadboard construction it was actually connected to Vee.Just something to keep in mind.

author: dannyf
author_rank: Super Contributor
timestamp: February 20, 2015, 01:49:36 pm
content: Measuring the photodiode current from a led, using a 10Meg ohm feedback resistor -> 1v/0.1ua. The 9v battery is not in the circuit.6.5mv -> 0.65na.The readings would fluctuate under a led lightbulb, and as you wave your hands. So in real life, shielding would be a must.

author: dannyf
author_rank: Super Contributor
timestamp: February 20, 2015, 01:50:39 pm
content: The reading when the led is reverse biased by the 9v battery is 11mv -> 1.1na.

author: babysitter
author_rank: Frequent Contributor
timestamp: February 20, 2015, 02:21:13 pm
content: If you are on a budget and dont wan't to solder, there is something else you have to measure low currents.El-even-cheaper.Basically a voltmeter in parallel with 10 Megaohm...a voltmeter in parallel with 10 Megaohm...a voltmeter in parallel with 10 Megaohm...Yes, your garden variety dmm with 10 Megaohm impedance :-)(exact value may vary but no one prohibits you from finding out)

author: Kalvin
author_rank: Super Contributor
timestamp: February 20, 2015, 03:48:57 pm
content: Quote from: dannyf on February 20, 2015, 01:51:37 amThe 1 gigaohm resistor maybe something which is not easily found. You can split that huge resistor into three smaller ones (See R2, R3 and R4 in the schematics attached). The schematics shows a measurement circuit for 1nA using more common component values. For the simulation purposes the voltage source provides a 1V step voltage which is fed through a large (1 gigaohm) resistor into the measurement circuitry giving a 1nA to be measured.

author: DmitryL
author_rank: Regular Contributor
timestamp: February 20, 2015, 03:59:47 pm
content: Quote from: dannyf on February 19, 2015, 10:28:40 pmWhat's wrong with existing uCurrent design ? Works pretty well.

author: dannyf
author_rank: Super Contributor
timestamp: February 20, 2015, 04:18:09 pm
content: QuoteIn an I-V converter, you try your best to avoid the use of small value resistors to minimize johnson noise. If you look at commercial picometers, they all use high value resistors, some as high as 1Tohm.Yes, they are expensive, though glass resistors are a lot more available now, especially from China. The polymer types however aren't that great.

author: dannyf
author_rank: Super Contributor
timestamp: February 20, 2015, 05:23:06 pm
content: Put an electrolytic capacitor under test, for its leakage current.Feedback capacitor is 100Kohm. So gain is 1v/10ua.When the capacitor is charged under the right polarity, the leakage current is 0.34ua - picture on the right;When the capacitor is charged under the wrong polarity, the leakage current is 11.7ua - picture on the left, ~40x bigger.Really leaky capacitors, if I may say so.

author: dannyf
author_rank: Super Contributor
timestamp: February 20, 2015, 05:24:56 pm
content: QuoteI haven't seen it.

author: DmitryL
author_rank: Regular Contributor
timestamp: February 20, 2015, 05:30:51 pm
content: Quote from: dannyf on February 20, 2015, 05:24:56 pmLOL! What a funtry to search by this keyword on this site.

author: Kalvin
author_rank: Super Contributor
timestamp: February 20, 2015, 05:41:50 pm
content: Quote from: dannyf on February 20, 2015, 04:18:09 pmI didn't realize how big difference is between the circuit using only the 1 gigaohm resistor and the noisy circuit I provided until I ran the noise analysis. Very illustrative lesson learnt, thank's for pointing that out.

author: dannyf
author_rank: Super Contributor
timestamp: February 20, 2015, 09:03:23 pm
content: That feedback network was typically and mistakenly used by many: even Pease and Widlar used it in some appnotes from NatSemi.Thankfully, the test equipment manufacturers knew better - they all use large value resistors for I-V conversion.EDIT: just to be complete, the Pease / Widlar feedback network is not without merit: it has much faster response time. The bandwidth of an I-V converter with a large resistor is typically less than 1Hz, and often less than 0.1Hz. That kind of bandwidth is not a problem for typical applications for those meters - they are meant to measure DC.If you intend to measure faster signals, you have no choice but to use the feedback network Pease and Widlar used.Hope it helps.

author: dannyf
author_rank: Super Contributor
timestamp: February 20, 2015, 09:13:10 pm
content: QuoteObviously it is hard to make an objective comparison. But here is my attempt:1) ucurrent's topology is rarely used for measuring tiny current; Burden voltage would be a consideration here;2) ucurrent is easier to construct: the input impedance is low so it should work even without shielding;3) ucurrent is more complex;4) it is hard to make ucurrent to go down to lower measurements. It would be difficult to see how this can be done as a picometer.If you want to measure small current in a circuit with large voltage drops, ucurrent would work. If you want to measure really small current (reverse leakage current on a diode, leakage current on a ceramic capacitor, etc.), I-V conversion is pretty much the only game in town.

author: Zero999
author_rank: Super Contributor
timestamp: February 20, 2015, 09:48:36 pm
content: Quote from: babysitter on February 20, 2015, 02:21:13 pmThat will work but the input impedance of DVMs isn't accurately controlled. Fortunately it's easy to measure the input impedance with another meter and take it into account.At work there are some MCP602s in stock and some 100M resistors which should work quite well. I'll give this a go one lunchtime next week.

author: codeboy2k
author_rank: Super Contributor
timestamp: February 20, 2015, 10:02:45 pm
content: Cockroft and Walton say:El cheapo megger coming soon !

author: dannyf
author_rank: Super Contributor
timestamp: February 21, 2015, 02:25:14 pm
content: I did some digging on the bias generator. With my tl082, cos is 10mv with a 100k feedback resistor and 500mv with a 10meg feedback resistor.So id you are ok with manually subtracting that from your reading, or if your meter can measure relatives, you can get away from that - a path I intend to make with my build.Now I think a build around a quad would be good -one opamp as rail splitter, three as i-v converters, with 100ua/v, 10ua/v, to 1ua/v. Another would be 10ua/v, 1ua/v, 0.1ua/v.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: February 21, 2015, 03:40:01 pm
content: Quote from: dannyf on February 20, 2015, 09:24:22 pmI've heard of putting your heart and soul into your work...

author: dannyf
author_rank: Super Contributor
timestamp: February 21, 2015, 04:10:59 pm
content: Finished building the uAmmetor on a quad (TL074).The test was measuring the reverse current in a blue led under 9v battery + 1Meg resistor.The left picture shows the uAmmetor's reading using a 10Meg feedback resistor (0.1ua / v): 0.129V translates to 0.0129ua.The right picture shows the uAmmetor's reading using a 1Meg feedback resistor (1ua / v): 0.011v translates to 0.011ua.Not bad for something you can build with Radioshack parts.

author: dannyf
author_rank: Super Contributor
timestamp: February 21, 2015, 04:11:45 pm
content: This build on a quad opamp has no bias generator and the breadboard contains all the parts needed for the uAmmetor.Pretty amazing if I may say so.

author: dannyf
author_rank: Super Contributor
timestamp: February 21, 2015, 07:25:00 pm
content: Here is the uAmeeter's performance with regards to Vcc changes.TL07x is specified from +/- 3.5 to +/- 15v (7v to 30v in unipolar supply).Here, we are using the uAmmeter to measure a small current, gain is 0.1ua / v.We take measurements under 5v/12v/20v power supply voltage. When we switch supply voltage, we zero the meter.The readings for the same circuit under different power supplies are:5v: 338mv = 33.8na;12v: 325.1mv = 32.5na;20v: 320.0mv = 32.0na;At 5v, the opamp is out of spec, but not far from a 9v battery's minimum output voltage (6-6.5v).The sensitivity is the highest at 0.1ua / v and decreases as gain goes down (to 1ua / v or 10ua / v).

author: dannyf
author_rank: Super Contributor
timestamp: February 21, 2015, 10:36:15 pm
content: The smallest current I was able to measure was the reverse current on a led: 2.1mv @ 0.1u/v = 0.2na. Would be interesting to see what practical applications there are for a meter like that.

author: dannyf
author_rank: Super Contributor
timestamp: February 22, 2015, 12:39:30 pm
content: Played around the uAmmeter a little bit more. See the photos below.All measurements are taken on the 10Mohm setting (=0.1ua/v). All zeroed.First of all, very little drift in the hour or so I used it.Now, g-s leakage current for a fqu1n60c (small mosfet by Fairchild). Immeasurable at this setting, vs. max 100na per the datasheet.

author: dannyf
author_rank: Super Contributor
timestamp: February 22, 2015, 12:43:20 pm
content: The same FQU1N60c reverse leakage.36.3mv @ 0.1ua / v = 3.6na, vs. max 100na in the datasheet.

author: dannyf
author_rank: Super Contributor
timestamp: February 22, 2015, 12:45:55 pm
content: Now a p-ch mosfet: FQP3P50.(forward) Leakage current is immeasurable: 0.00mv @ 0.1ua/v = 0.00na, vs. 100na maxReverse leakage is about 3.6na, vs. 100na max.

author: dannyf
author_rank: Super Contributor
timestamp: February 22, 2015, 12:46:59 pm
content: Leakage of a 1n4148. 17.7mv @ 0.1ua/v = 1.77na.

author: dannyf
author_rank: Super Contributor
timestamp: February 22, 2015, 12:50:32 pm
content: Measured some capacitors.1. ceramics: mostly immeasurable at this setting.2. electrolytics: too much leakage for this setting.3. here is polypropylene cap. 0.2mv @ 0.1ua / v = 0.02na. Cannot complain about it,

author: dannyf
author_rank: Super Contributor
timestamp: February 22, 2015, 09:05:39 pm
content: A while back, I talked about TDA1308, my new favorite opamp. This is a 8pdip cmos opamp, rated at 5v.So I decided to try it here. Initially, I ran it at 5v and no problem - a little bit limited in terms of operating range. So I ran it at 12v, no problem still. Good for 9v battery operations, and +/- 4v output for sure.All test be low are done under 12v power supply.First, measuring reverse current in some random led, TDA1308 vs. TL074. Feedback resistor is 100k (10ua / v).Difference is only 0.001mv (0.01ua).

author: dannyf
author_rank: Super Contributor
timestamp: February 22, 2015, 09:08:44 pm
content: 2nd, Vos at 10Meg feedbakc resistor.0.000v, vs. 96mv for TL074.This really surprised me. Vos is specified at 10mv typical in the datasheet.Maybe it is my lucky day,

author: Kalvin
author_rank: Super Contributor
timestamp: February 23, 2015, 06:16:53 pm
content: Thanks for nice idea, dannyf. I built something similar just for fun.I took your design as a basis for my project. However, I used a bipolar power supply of +/-12V, and added a potentiometer to be able to set the test voltage for the DUT in range +/-10V. I also added a precision full wave rectifier to the op amp's output and hooked one of those cheap 5-digit voltage display to show the rectified leakage current/voltage. As the 5-digit volt display cannot display negative voltages, I added two leds which will indicate the polarity of the test voltage. There is also a toggle switch to select which parameter will be displayed: The test voltage or the measured leakage in uA. The op amps are LF411 with offset nulling, and a LF347 for the full wave rectifier, led polarity comparator and HI-Z buffer for the test voltage measurement. The measurement range is approximately +/- 10.000uA and the resolution is 1nA.Edit: I used this project to evaluate those cheap 5-digit volt meter/displays and Mornsun IA0515KS-2W isolated 5V in, +/-15V out dc/dc modules. The circuit is running from a +5V Wall-wart, and the op amps are running from the (unregulated) +/-15V power supply created by the dc/dc-converter. The 5-digit voltage meter/display is operated from the +5V power supply to keep the noise levels under control. Those dc/dc-converters seem to be unregulated and have some output ripple. I used a simple LC-filtering, and the power supply quality seems to be adequate for this application. However, using a linear post-regulator with a LC-fllter might be advisable in more sensitive applications.

author: dannyf
author_rank: Super Contributor
timestamp: February 23, 2015, 10:35:04 pm
content: Sounds like a fun project.Just a word of caution - because of the large value feedback resistor plus capacitance, either discrete or intentionally added, the circuit can have very limited bandwidth, in the sub-hz range with 100gohm resistors for example.If you need to measure ac, you may consider using lower value feedback resistors plus additional amplification, or the t-network in the feedback loop.You effectively trade noise for bandwidth.Hope it helps.

author: dannyf
author_rank: Super Contributor
timestamp: February 27, 2015, 01:20:57 am
content: More measurements.1) the uAmmeter vs. a multimeter.Measuring small current from a red led + 1.2Mohm resistor powered by a 9.26v battery.@ 10ua/v, the uAmmeter reads 0.615v @ 10ua/v = 6.15ua. The multimeter reads 6.1ua.I made another measurement, with a different led and got 5.16ua on the uAmmeter and 5.1ua on the multimeter.

author: dannyf
author_rank: Super Contributor
timestamp: February 27, 2015, 01:24:42 am
content: Measured the one circuit (@ 10ua / v) under Vcc=5v/12v/20v.The measurement remained the same.

author: dannyf
author_rank: Super Contributor
timestamp: February 27, 2015, 01:29:43 am
content: All the experiment was done so far with a TL074 (actually, I suspect it is a knock-off TL074).Here is a LMC6042 (Ibias=2fa and Vos = 3mv)Here is a LMC6042, with a 10Meg feedback resistor.output offset voltage with shorted input is 3.5mv. The TL074 would have an output voltage of 100mv+ in the same condition.

author: dannyf
author_rank: Super Contributor
timestamp: February 27, 2015, 01:34:02 am
content: Now reverse current in a yellow led, @ 0.1ua / v.The LMC6042 reads 0.87na and the TL074 reads 0.84na.With the light turned off, the LEd would have a reading of 0.27na. So I attributed the difference between 0.87na by LMC6042 and 0.84na by TL074 to inexact location / lighting of the led between each test. Nonetheless, they are fairly close.

author: dannyf
author_rank: Super Contributor
timestamp: February 27, 2015, 01:36:04 am
content: I am going to finish the construction next.I hoped that this has shown that measuring small current can be done fairly easily with run-off-the-mill parts.

author: Kalvin
author_rank: Super Contributor
timestamp: February 27, 2015, 10:09:50 am
content: dannyf, what do you think whether the HX711, the inexpensive 24-bit ADC you were evaluating few months ago would be usable with this micro-ammeter project?

author: dannyf
author_rank: Super Contributor
timestamp: February 27, 2015, 11:34:58 am
content: I abandoned that project. The drifting, particularly the opposite tempco for the A/B channels, is a killer for my intended application (milliohm meter). I think it would have worked if the two channels had similar or at least the same sign of their tempcos. Without temperature compensation, you are better to just treat it as a 12bit adc.The only solution I could think of was an oven and that wasn't doable in a hand-held device.I was told its sister, hx712, has solved the tempco problem but I am not about to try it. I think a good 20 - 24bit adc (external ref + 1 channel, or two differential channels) would have worked better for the milliohm meter application.

author: Kalvin
author_rank: Super Contributor
timestamp: February 27, 2015, 08:28:02 pm
content: Thanks, dannyf. I have two of those boards here and I thought to give a try. Plan is to use analog switches to route the A and B inputs to the known voltage reference and known zero voltage. The measured values would be then used for calibration and removing the offset. Here's been quite good discussion about different voltage references, so maybe I can find something usable. And need to find suitable analog switches. Of course, all this in the spirit of cheapo

author: dannyf
author_rank: Super Contributor
timestamp: February 27, 2015, 10:41:28 pm
content: Sounds fun. Would love to see how you make it work. Keep us posted, please.

author: dannyf
author_rank: Super Contributor
timestamp: February 28, 2015, 12:05:57 pm
content: I'm happy to report that for the 10ua/v setting (100K feedback resistor), a NE5532 works as well.I have not tried the 1ua/v or 0.1ua/v settings but given the significantly higher Ibias, I would suspect that offset would overwhelm a bjt opamp like NE5532 at those settings. So you may have to put in an Ibias generator there. But I will try later.

author: dannyf
author_rank: Super Contributor
timestamp: February 28, 2015, 03:38:21 pm
content: I would expect errors caused by bias current drift to be a real issue, particularly for a jfet based design. So to get the meter to work over a wide temperature range requires a lot of work that's not here yet.However, if you are just trying to perform a quick measurement, even if repeated over a wide temperature range, it's still ok.

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: February 28, 2015, 03:49:40 pm
content: Quote from: 3roomlab on February 28, 2015, 02:48:46 pmI have a whole bunch of voltage standards on my bench. From super accurate to the most el cheapo you can think (TL431 and LM385)The cheapo ones even can "detect" if the door of my lab opens. Or waving my hand at 2 meter distance.Not very good as a voltage standard, but the ultimate motion detector.

author: Kalvin
author_rank: Super Contributor
timestamp: February 28, 2015, 04:00:49 pm
content: Quote from: JohnnyBerg on February 28, 2015, 03:49:40 pmHave you observed how many accurate digits these el cheapo style reference will provide in room temperature range after they have been calibrated? And, do you have any guesstimate what will happen if the el cheapo reference is placed inside a styrofoam or any other good insulator ie. how well the insulated reference would then perform? Or does the insulation increase the accuracy at all?

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: February 28, 2015, 04:51:53 pm
content: Quote from: Kalvin on February 28, 2015, 04:00:49 pmActually, I have.They are "usable" within 2 digits. A LM385 at 1.23V and a TL431 at 2.49VQuoteActually, I did some testWhen well insulated, thermal effects like the draught from opening a door are gone. But that was to be expected.QuoteNo, it does not. Those el cheapo "references" suffer from many other effects, like thermal hysteresis, aging instability etc.Actually they perform very reasonable in line and load regulation, but I suppose that is what they are made for

author: dannyf
author_rank: Super Contributor
timestamp: February 28, 2015, 07:10:27 pm
content: QuoteAt 1ua/v (1Mohm feedback resistor), offset is 0.3v; At 0.1ua/v (10Mohm resistor), offset is 3.5v -> so you need 12v or so to get it to have a reasonable range.

author: Kalvin
author_rank: Super Contributor
timestamp: February 28, 2015, 07:39:15 pm
content: Quote from: dannyf on February 28, 2015, 07:10:27 pmJust trim it to zero (nb. option B doesn't affect the gain):http://wiki.analog.com/university/courses/electronics/text/chapter-3#offset_adjustment_external_methods

author: dannyf
author_rank: Super Contributor
timestamp: February 28, 2015, 08:31:24 pm
content: Or to use relative measurement on your DMM.

author: Kalvin
author_rank: Super Contributor
timestamp: February 28, 2015, 08:55:47 pm
content: Quote from: dannyf on February 28, 2015, 08:31:24 pmOr use an analog voltmeter / panel meter which typically have a knob for zero adjustment.

author: Vgkid
author_rank: Super Contributor
timestamp: March 01, 2015, 05:15:20 am
content: ^^^ wont remove the input offset.nI was thinking that you could add another potentiometer coupled to a fraction of the+/- rails to null out the offset.MT-037, analog.com

author: Smith
author_rank: Frequent Contributor
timestamp: March 01, 2015, 08:05:09 am
content: I have made quite some low current meters. Some basic tips:1. Get an instrumentation amp (low bias one)2. Make a basic I/V converter with good resistors3. Give it a low noise output buffer with offset compensation4. Make a (low noise) dual supply5. Use switches/relays and multiple resostors to create multuple feedback ranges.When going to nA range and lower, cleanup well after soldering.Keithley has a free low level measurements handbook in PDF. Its awesome!

author: dannyf
author_rank: Super Contributor
timestamp: March 01, 2015, 11:26:29 am
content: Some highly unscientific tests on tempco:TL082, TL074, NE5532, TDA1308: readings (relative) barely budged when a heater (=my thumb @ 30c) is applied to the IC, on all resetings (10ua/v to 0.1ua/v).LM358: readings barely budged for anything other than 0.1ua/v.So maybe Ibias' impact isn't as significant as I thought, at least for this type of measurements (relative) and range (=nanoamp).

author: dannyf
author_rank: Super Contributor
timestamp: March 08, 2015, 09:23:00 pm
content: Got some time and made a little progress.the input terminals are mounted, as is the power switch, into a repurposed chocolate case.Next is to solder in the TL074 and resistors. I have some mil-spec teflon wires to go from the input terminal to the pins - total overkill for this level of current.