Hi, I'm currently trying to do some power budgeting on yet another battery powered PIC project (once again it's a CR2032). Problem is, none of my DMMs can measure down far enough - they all start having trouble getting a decent reading once you start going down below 1mA. I was going to design a low-current ammeter (I've already done a voltmeter, just need to design a new front-end circuit to do current), but I'm not sure how to go about deciding on the sense resistor value. I thought of limiting the forward drop to about 100mV, which means a 100-ohm resistor will allow me to measure a 1mA pk-pk current. Amplifying the 100mV drop by 40x and adding a 2V offset gives a voltage span of 0V to 4V, corresponding to a span of -500uA to +500uA with zero centred around 2V. Couple that to a 10-bit A/D converter (like, I dunno.. the one in the 12F675?) and a 4.096V reference gives 1uA per measurement step. In real life, it's unlikely you'd be able to measure down quite that far due to noise issues; I suspect 5uA would be as low as you could realistically go. Does this sound vaguely reasonable? Surely I'm not the only one having trouble measuring PIC sleep currents... How are you guys doing it? I've also thrown together a quick spreadsheet for working out the parameters of low-side (and I suppose high-side too) current sensing circuits, and A/D interfacing. I'd appreciate comments on it - you can grab a copy from: (OpenOffice 2.0 format) (Excel 97 format) Like I said - I have no idea if the maths behind it is correct. White fields are user-editable values, blue ones are calculated by the spreadsheet. The voltmeter is kinda neat - fully isolated input circuitry, isolated control bus (read: GPIB over RS232), only one fixed range though. I'm still trying to figure out the best way to handle autoranging - at the moment I'm leaning towards a fixed attenuator and an opamp with a programmable/selectable gain. I don't want to use a digital potentiometer though - what other options are there? Thanks. -- Phil. | Kitsune: Acorn RiscPC SA202 64M+6G ViewFinder philpem@dsl.pipex.com | Cheetah: Athlon64 3200+ A8VDeluxeV2 512M+100G http://www.philpem.me.uk/ | Tiger: Toshiba SatPro4600 Celeron700 256M+40G -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist