Accurate nano- and microamp current sources - Page 1
Forum section: electronics / projects

author: philpem
author_rank: Frequent Contributor
timestamp: December 16, 2013, 10:45:58 am
content: Here's a "fun" problem...I'm designing a photodiode amplifier, with input ranging from 2nA to 2mA, gain set via series resistors and a multiplexer. My problem is, how do you calibrate something like this, especially at the nanoamps range? A Keithley 225 would seem to be the "standard" or "best" option, but they seem to be selling for rather a lot of money these days.Generating a 200uA reference is easy (the good old REF200 or a VRef and opamp current source) and higher currents can be generated with an accurate voltage reference and an opamp, but what about sub-microamp currents? Surely the inherent error (offset et al) of the drive opamp and shunt resistor would frustrate any attempt to make an accurate source?Cheers,Phil.

author: TopLoser
author_rank: Unknown
timestamp: December 16, 2013, 10:50:12 am
content: Still got a surplus to requirements Keithley 2636A on my desk, can source and sink down to 1fA if you want to borrow it for a while!

author: george graves
author_rank: Super Contributor
timestamp: December 16, 2013, 11:16:51 am
content: I'd assume at that low current, you've want everything shielded?

author: robrenz
author_rank: Super Contributor
timestamp: December 16, 2013, 02:14:41 pm
content: At single digit nA levels and below you will want to use triaxial cables. The SMU drives the inner shield to the same potential as the source on both the source high and sense high leads to minimize leakage. The outer shield of the triaxial's are connected to the SMU chassis.

author: minime72706
author_rank: Frequent Contributor
timestamp: December 16, 2013, 03:14:10 pm
content: Just a thought: do the photodiodes get tested separately elsewhere? If you know the dark current experimentally, you might be able to get halfway there. (at the same temperature!)

author: tszaboo
author_rank: Super Contributor
timestamp: December 16, 2013, 06:30:57 pm
content: How accurately do you want it? As far as I remember, there is subtreshold conduction for MOSFETs you can use it to generate nanoamps, and there are opamps which have input bias in the femtoamp range, to measure it back.Let us know how the project proceeds.mod: Before I forgot, link:www.aldinc.com/pdf/SubthresholdDesign.pdf

author: Andreas
author_rank: Super Contributor
timestamp: December 16, 2013, 06:36:33 pm
content: Quote from: philpem on December 16, 2013, 10:45:58 amGood photodiodes have tolerances of e.g. +100/-50%.Plus a large dependancy on temperature.Why in the hell do you want to calibrate the amplifier?I would try to calibrate the whole system.With best regardsAndreas

author: dannyf
author_rank: Super Contributor
timestamp: December 16, 2013, 10:22:11 pm
content: Take a look at service manuals of old hp ohm meters. Some of them utilize ccs.

author: philpem
author_rank: Frequent Contributor
timestamp: December 17, 2013, 12:02:08 am
content: First it looks like I got the measurement range wrong. On the meter design I'm looking at using as a starting point, the range is from 20nA to 20mA in the usual x10 (1-10-100) steps, plus 50mA. So 2nA is unnecessary. Specs are:Range - Cal point - Accuracy20nA - 19nA - 106pA200nA - 190nA - 436pA2uA - 1.9uA - 1.42nA20uA - 19uA - 12.3nA200uA - 190uA - 104nA2mA - 1.9mA - 1.04uA20mA - 19mA - 10.4uA50mA - 50mA - 18.1uAQuote from: robrenz on December 16, 2013, 02:14:41 pmYou're right - there's a footnote in the calibration instructions about that... "Triaxial cable should be used between current source and sensor adapter". Oddly enough, the LPM I have on my bench doesn't use triaxial cable - it's just bog-standard 9-conductor insulated stranded cable.Quote from: Andreas on December 16, 2013, 06:36:33 pmI can get calibrated photodiodes from (to name but a few) Thorlabs, or I can (at least in theory!) have a local cal lab provide calibration data for them. That data takes the form of A/W (amps of output per optical watt) values.The problem is that I don't have an optical power reference. So I can't do an end-to-end calibration - I'd have to send it out to the cal lab. If the circuit is wrong, the cost could mount up pretty quickly. Also, there's roughly a two week turnaround.So my thought was, given that I have accurate voltage and current measurement hardware here (a Solartron 7150Plus bench DMM) I figured I could calibrate for current, then apply the A/W calibration data to obtain optical power.This is all an academic exercise now, as I just snapped up an Advantest Q8230 power meter and 200mW visible/NIR sensor head on eBay for the quite reasonable sum of £125 (less than a new calibrated photodiode sensor). I will be keeping an eye on this thread, because I'll need to design a PD amplifier for my laser diode driver (this year's Christmas project).Cheers,Phil.

author: Rufus
author_rank: Super Contributor
timestamp: December 17, 2013, 12:53:37 am
content: Quote from: philpem on December 16, 2013, 10:45:58 amIf you have a ground reference or your circuit doesn't mind having stuff connected to whatever it considers ground then what is so hard about a resistor and a voltage to generate a known small current.You need high value stable resistors that you can keep clean. The higher you go with the voltage the less difference any ground offset/error makes.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: December 17, 2013, 01:13:17 am
content: Hi,Here is a circuit from the LTC6082 datasheet. The circuit is for a +/- 1nA current source. The 10m Ohm resistor should be 10M (Megohm).If you use a 1M resistor the range becomes +/- 10nA etc.Jay_Diddy_B