Nano ammeter project - Page 1
Forum section: electronics / projects

author: craftycoder
author_rank: Contributor
timestamp: September 08, 2012, 04:07:06 pm
content: Update: Look below for the surface mount custom PCB version video.Video of a transimpedance amplifier I put together for measuring small currents. Operates similarly to the uCurrent.

author: craftycoder
author_rank: Contributor
timestamp: September 09, 2012, 05:36:19 pm
content: Sorry, I had the video linked up wrong yesterday. I got it figured out now.

author: Stephen Hill
author_rank: Regular Contributor
timestamp: September 11, 2012, 12:26:32 pm
content: Nice little circuit. I'm surprised no one has commented on this thread so far. I have two questions:How low can the battery supply voltage go?How much quiescent current does the circuit use at +/- 4.5V?Many ThanksStephen

author: ejeffrey
author_rank: Super Contributor
timestamp: September 11, 2012, 05:15:41 pm
content: OP 277 is not a great choice here, the bias current is 1 nanoamp at 25C. Try a precision JFET or an auto-zero CMOS opamp.

author: PSR B1257
author_rank: Frequent Contributor
timestamp: September 11, 2012, 07:19:49 pm
content: QuoteRegarding to the datasheed: +-2VSo for an single supply minimum of 4V.@craftycoder: Two complains to your circuit:1. Why did you drew it thatcomplicated? Especially R1 and R22. About your description of how the circuit works. You said, that the current will flow out of the input of the op amp.Ant I bet you will confuse some electronics-newbie with that statement. Of course there is now current neither flowing out nor into the input of the op amp (disregarding the bias current).The current will flow in the output an than trough the supply-pin and than trough the voltagediving op amp trough the virtual GND. So the measuring op amp sinks the measure-current an the other op amp source the same current.

author: sorin
author_rank: Frequent Contributor
timestamp: September 11, 2012, 09:24:04 pm
content: Measuring nA is not so difficult, especially when you use high value resistor, which are expensive and produce high noise.The problem is measuring pA or fA...

author: PA4TIM
author_rank: Super Contributor
timestamp: September 11, 2012, 10:11:29 pm
content: Bob Pease troubleshooting analig circuits page 52. A nice simple logarithmic leakage current meter.The range is from less then 1 pA to 1 mA. Elegant circuit. Plan to build it just for fun.But I agree meauring nA is not very difficult. I managed to come down to 20 pA at 1KHz using opamps. I had to clean the whole board with ipa, use guarding, shielding ect. fA and pA is not a piece of cake. I had to measure 20 to 40 pA during aligning a multimeter a few weeks ago. I used guarded cable on a HP meter that can measure from 50 fA but it took forever to stabilize, leakage current through dirt ect between connectors makes a diffence and I had to stay a meter or so away from the setup and turn of the led and halogene lights.

author: craftycoder
author_rank: Contributor
timestamp: September 30, 2012, 08:38:42 pm
content: Good point about the power supply needing to be 4V. In reality it can be a little lower (it works at 3.5V), but I am using 2 3V cells. Also, good point about the OPA227 bias current. I mention that in the video in fact, but since my interest is only in measuring nano amps it works great for my needs. I got a custom PCB and built it up today. Video posted.

author: shadowncs
author_rank: Newbie
timestamp: October 30, 2016, 04:14:00 pm
content: Hey craftycoder resurrecting this thread to share my homemade LightningBug. I just completed it - see attached pictures.PCB is home-brew, resistors are 0402 I only had 5% tolerance on hand. ICs are SMT variants.Got an SMT mini switch for power and another SMT mini switch for nA-uA-mA range.I thought I could power this off a single 3V coin-cell but I was wrong. Circuit needs more voltage. 5V from ISOLATED USB works fine though.I also noticed it is hard to measure at the beginning of the scale i.e. into a few mV output. Would be useful if the scales overlap 10% which means using resistors 10MOhm (10mV = 1nA), 1MOhm (1mV = 1nA), 10KOhm (10mV=1uA), 1K (1mV=1uA), 10ohm (10mV=1mA). There is no point in going above 10mA measured current since opamps can't source / sink it.All in all, great little project and fun to work with. Works pretty good - see attached precision graph. That graph in itself was done using 5% test resistors so part of the error is from load tolerances, part is from scale R tolerances. Even so it's pretty impressive and very useful little gadget to have around.Thanks for sharing!

author: Kleinstein
author_rank: Super Contributor
timestamp: October 30, 2016, 05:01:13 pm
content: The input has no extra protection - at least something like a 220 Ohms series resistor would be a good idea. This should also isolate the input from to much capacitance, that could otherwise make the circuit oscillate. There should also be a capacitor in feedback to reduce the tendency to oscillate. There should be no really need to have the ranges in 1:10 steps. At least 1:100 steps should be possible.The OPA277 is not the best choice for the OP, I would prefer something like the LT1012 (lower bias current, current noise and lower supply), or maybe an auto zero OP like AD8551 or max4238. A dip version in a socket might be a good idea if the input is not protected.The shown layout is not that good, as there may be quite some leakage. One should generally keep the pad connected to the inverting input of an OP small.

author: shadowncs
author_rank: Newbie
timestamp: October 30, 2016, 06:25:36 pm
content: Quote from: Kleinstein on October 30, 2016, 05:01:13 pmThanks, all good points. Good OP suggestions. I've seen no leakage on the layout but yes due to only using basically 1 layer PCB home-made + switch I had to route the feedback like that. To tell the truth I have zero experience in PCB design, I'm actually a SW eng so this is educational for me. I'm surprised that I could measure 3nA quite accurately using 1Gohm resistors on a 3V supply, to tell the truth.The proposed scales are 1:100 steps as opposed to original 1:1000 steps.