Measuring average current to estimate battery capacity

[msr]

Hi everyone,

I designed a circuit with a PIC24F and a couple more components and now I want to measure it's average current in different modes (run, idle, sleep) to later estimate the battery capacity I'll need. What sould be the best procedure to accomplish that? Im expecting to get very small currents, in the range of ~100mA (run/idle mode) and ~10's uA (sleep) or even ~100's nA (deep sleep).
I have a Rigol DS1052E Osciloscope and a Batronix BXM98P multimeter (true rms, 4 1/2 digit). I can also borrow an current probe if needed.

I considered to use a 1ohm resistor in series with circuit's VDD, two osciloscope probes and use ADD function of the osciloscope to get a differential voltage across the sensing resistor. I don't know if this method is reliable to measure such small currents due to osciloscope's vertical noise and CMRR between the two probes.

What are your suggestions?

Thanks

[SeanB]

Buy one of Dave's microcurrents...........

[T4P]

100mA ... Didn't know the 24E was so power hungry, that puts in on par with the STM32F407VGT6 ( Not that it matters ... )

[msr]

100mA ... Didn't know the 24E was so power hungry, that puts in on par with the STM32F407VGT6 ( Not that it matters ... )

PIC24F are not that power hungry. If you see the datasheet you'll notice it consumes (typical values) ~15mA at 16MIPS ("normal" mode), ~3.5mA in Idle mode. But in the board I designed I can have additional modules like WiFi or SD card which will certainly consume more current (~10-100's of mA).

As far as I can understand Dave's uCurrent is good to use with multimeters due to Burden Voltage. However I don't know if that voltage also affects osciloscopes. If don't, it seems it doesn't make sense to buy uCurrent to use with an o'scope. Please let me know if Im wrong.

[MikeK]

If the max current is going to be 100mA through a 1 ohm resistor, why not feed that voltage to an op-amp with a gain of at least 50?

[SeanB]

Microcurrent is a low drop current to voltage converter, and has a pretty good bandwidth, so will work wel as a scope probe add on. Just make sure to consider the grounded lead of the scope does not cause any problems, either by using a differential probe or an isolated scope.

[T4P]

100mA ... Didn't know the 24E was so power hungry, that puts in on par with the STM32F407VGT6 ( Not that it matters ... )

PIC24F are not that power hungry. If you see the datasheet you'll notice it consumes (typical values) ~15mA at 16MIPS ("normal" mode), ~3.5mA in Idle mode. But in the board I designed I can have additional modules like WiFi or SD card which will certainly consume more current (~10-100's of mA).

As far as I can understand Dave's uCurrent is good to use with multimeters due to Burden Voltage. However I don't know if that voltage also affects osciloscopes. If don't, it seems it doesn't make sense to buy uCurrent to use with an o'scope. Please let me know if Im wrong.

Oops. F it is then, but if the F (which is a slower part) is that much power already, the E will definitely chew quite a lot, i just check'd the schem and it is indeed 100mA ... for the E