Recommendations on techniques to measure 100nA to 500 mA automatically - Page 1
Forum section: electronics / general

author: PICmonster
author_rank: Regular Contributor
timestamp: January 20, 2014, 09:42:39 am
content: Hello EveryoneI am looking for ideas and advice on the best way to design an energy datalogger, the front end poses a technical problem as I am not sure what the best approach might be, no double some of you will have experience in this area.The problem is I want to measure to with 1% or so a current from 100nA to 500mA automatically using an ADC, also voltage, but the voltage range does not pose a problem, ideally I would want a sampling rate of a least 1mS, but this is secondary to the current measurement required.I have been looking at multi-channel 24 bit sigma-delta ADC that can measure using a 2.5V reference 2.5/ 2^24 = 149nV/Bit, but there is too much noise as expected on this, I have been experimenting using the PmodAD5 ADC module that uses an AD7193 by Digilent Inc, I have screened this module in a die-cast grounded can an it is running from battery to eliminate power supply noise as much as possible, I am aware of noise being dominant as we are measuring very small currents indeed approaching what is technically possible. Also the number of bit effective bits reduces with increasing sampling rates. I have looked into this quite a bit but I am not experienced at measuring these very low signals so I am a looking for advice and guidance from you.The 24 Bit ADC was thought to be a good idea because it can resolve without instrumentation amp front end the low currents, but I am now thinking about using two 10 bit ADC channels in a PIC for two current ranges, low and high, using two instrumentation amps for each channel set with different gains.I would be grateful of your help and interested in hearing your thoughts on this problem.

author: tszaboo
author_rank: Super Contributor
timestamp: January 20, 2014, 11:24:23 am
content: ADS1246 or similar has a built in 128 gain amplifier, and it gives you 15 effective bit with 2KSPS and 128 gain. You might be able to use it without any amplifier, if the circuit is designed well.

author: EEVblog
author_rank: Unknown
timestamp: January 20, 2014, 11:53:33 am
content: As you have found, to get 100nA resolution on a 500mA range is pushing the limits of a 24bit ADC.For a single fixed shunt resistor solution you'd have to size the shunt value big enough to be just tolerable on he 500mA range.e.g. 0.1ohm with a 50mV drop would be reasonable, and an amp gain of 100 to give a 5V ADC range. Then if you had say a 1mA FS range (good enough for 100nA) with the same shunt, and use a nice 16bit converter, that's a 100uV shunt drop FS with a required amp gain of 50,000 - ouch. Then your DC offset errors become pretty unworkable.So that pretty much forces you into a switched autoranging shunt arrangement. Doable, but you potentially lose data on the switchover of course.

author: EEVblog
author_rank: Unknown
timestamp: January 20, 2014, 11:54:58 am
content: The other thing is, is your required resolution 100nA, or do you want meaningful measurements at the 100nA scale?

author: PICmonster
author_rank: Regular Contributor
timestamp: January 20, 2014, 01:25:27 pm
content: Thank you for your help with this, I will attempt to answer the questions below, the minimum I would like to measure is 1 uA, so to resolve this we have to use something lower, I know 100nA is only 10 counts to 1uA but I think this would be ok maybe, yep the switched auto range is where I was going mayself.I was considering datalogging the two ranges separately through two ADC channels, so I might have a range of say 100nA to 10mA and another range of 10mA to 500mA, there would be two data log graphical outputs generated by the data logger scaled appropriately so I can see activity below 10mA and activity above.The application is to monitor the energy taken from a 3V 5Ah lithium Manganese Dioxide battery when the micro is in various modes of operation, the application is a wireless alarm sounder with strobe that uses 868MHz tranceiver to commumicate with the main alarm panel, the microcontroller sleep modes with various peripherals enabled take differing amounts of power from the battery, the high scale is for a piezio sounder that pulls approx. 300mA or so, this tool would allow me to gather data on the energy use of the device and thus design and make changes to optimise the battery life of the application.Your comments are most welcome, do you think this approach using two indepenent ranges would work?

author: KJDS
author_rank: Super Contributor
timestamp: January 20, 2014, 01:52:00 pm
content: If it's just for a one off to help with a product design, use a couple of decent multimeters. You only need worry about sorting out which range they are on rather than a whole raft of design issues.

author: PICmonster
author_rank: Regular Contributor
timestamp: January 20, 2014, 02:14:43 pm
content: Quote from: KJDS on January 20, 2014, 01:52:00 pmThanks KJDS, I have been around this loop initially, but it was deemed too expensive by my boss

author: KJDS
author_rank: Super Contributor
timestamp: January 20, 2014, 02:41:21 pm
content: Quote from: PICmonster on January 20, 2014, 02:14:43 pmAt some stage, when you've spent four weeks of effort and £500 on parts, he'll realize he's made the wrong decision, but I suspect you know that already.

author: tszaboo
author_rank: Super Contributor
timestamp: January 20, 2014, 03:41:29 pm
content: Quote from: KJDS on January 20, 2014, 02:41:21 pmNot to mention, a good multimeter will easily pay for itself in a few months, and absolutely essential for any serious work. Tell your boss, he is the pointy haired boss from Dilbert. Or dont, and keep your job. Anyway, good luck with him.

author: don.r
author_rank: Frequent Contributor
timestamp: January 20, 2014, 03:57:13 pm
content: Quote from: NANDBlog on January 20, 2014, 03:41:29 pmLOL! I'm so glad I no longer work for anyone.