Searching low cost µA measuging device - Page 1
Forum section: electronics / testgear

author: PeterFW
author_rank: Frequent Contributor
timestamp: April 27, 2015, 01:24:57 pm
content: Hello and yes, i know!Stupid question since there is plenty or information out there allready.Yesterday i decidet to give up on the idea to score a decent bench multimeter on ebay, for some reason people here pay ludicrous amounts of money for 20 year old test gear.I got two multimeters wich seem to be able to measure mV with reasonable accuracy.The µCurrent is out of my budget and dave understandably ships no longer to this customs hell hole.My best option would be to route my own board from the µCurrent files and hope it will be accurate enough without calibration/function check.I would like to measure the current consumtion of a few low power components and the sleep mode of a microcontroller.Greetings,Peter

author: EEVblog
author_rank: Unknown
timestamp: April 27, 2015, 01:44:17 pm
content: Quote from: PeterFW on April 27, 2015, 01:24:57 pmIf you use the same precision parts, it will be.

author: TiN
author_rank: Super Contributor
timestamp: April 27, 2015, 03:45:24 pm
content: Age is no indication of quality or performance for most test equipment. There are many examples when 20yr old gear is better than new fancy meter with some touchy touchscreen TFT display and Wifi integratedThere are some34401A'sfor a hundred or so..

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: April 27, 2015, 03:52:35 pm
content: Quote from: PeterFW on April 27, 2015, 01:24:57 pmThere's no old school stores in your area? A simple 50uA meter movement does the trick for me.

author: XFDDesign
author_rank: Frequent Contributor
timestamp: April 27, 2015, 03:52:56 pm
content: How much current are you wanting to measure? To what accuracy?

author: PeterFW
author_rank: Frequent Contributor
timestamp: April 27, 2015, 04:13:09 pm
content: Quote from: EEVblog on April 27, 2015, 01:44:17 pmThanks for the reply!The best i can get without jumping through hoops are 1% and 50ppm resistors, looks like i have to put on my clown outfitQuote from: TiN on April 27, 2015, 03:45:24 pmI know, i know, i was talking about something else.I can buy a old keithley meter for ~70 bucks including shipping from the USA to europe.The price of the same equipment within germany is above that without shipping.Or around that, i have not kept a log about specific numbers. The real numbers may varyQuote from: XFDDesign on April 27, 2015, 03:52:56 pmThe datasheets of the various ICs range from 1µA to 500µA, so around that, maybe 0 to 1mA with 1µA accuracy and resolution.

author: XFDDesign
author_rank: Frequent Contributor
timestamp: April 27, 2015, 04:46:26 pm
content: For your solution, do you have access to split supplies?

author: PeterFW
author_rank: Frequent Contributor
timestamp: April 27, 2015, 04:56:40 pm
content: Quote from: XFDDesign on April 27, 2015, 04:46:26 pmI would be willing and capeable to build one with two linear regulators powered from batteries.That should be fairly stable and low noise if i use decent regulators.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 27, 2015, 06:19:12 pm
content: If it is just for measuring the µC supply, there is no need for a really low noise supply - the µC will likely cause enough noise. So the circuit will have too be able to measure in a somewhat noisy environment. Whether you need a dual supply depends on the circuit. There are several option to build such a circuit:One would be a circuit with a shunt and zero Offset amplifier, just like the µ-current. To work with highly variable load, use plenty of decoupling caps (may be well in the 100 µF range), and allow for current peaks. So something like a 10-50 Ohms shunt may be the right choice. Having the shunt low side would profit from a dual supply. Something like a 0.6 V from a diode drop may be enough - so a single external supply (e.g. 12 V wall-wart) may also work. Modern Zero Offset OP's are often Rail to rail types, so no large negative voltage is needed.The second option would be using a transimpedance amplifier with a relatively large capacitor parallel to the feedback resistor. This is especially an option if the peak current does not go above 10-20 mA, so a more normal OP-Amp can deliver it. This circuit may want to have a negative supply, especially if the current is measured on the high side.

author: dom0
author_rank: Super Contributor
timestamp: April 27, 2015, 06:34:53 pm
content: Quote from: PeterFW on April 27, 2015, 04:56:40 pmThere is little point in regulating batteries ; they exhibit lower noise than most integrated regulators.By the way, the LTC1050 (R2R, chopper) has better availability here in Germany (reichelt and conrad stock them) than the MAX4239 (or so) used in Daves' design./e: 1050, not 2050. The 1050 is a little worse in most aspects, but nothing dramatic.