I heard a professor saying "it's a null method, so it is very precise".
He was talking about the force balance pressure measuring method.
I think in this cathegory of methods one can include the Wheatstone bridge (with balanced arms) or other methods where a galvanometer is used.
But why is this more accurate than other methods? My intuition tells me that the accuracy of a non-zero reading can be "more affected" by many factors…, but i'm not sure…
Electronic – Why are null measuring methods considered to be very precise
current measurementmeasurementvoltage measurementwheatstone-bridge
Related Solutions
So you need to measure supply current at 10 ksamp/s from 100µA to 30mA, which is 300:1 range.
That by itself sounds doable enough. Even a 10 bit A/D built into a microcontroller is enough resolution if the signal is amplified properly. 10 kHz sample rate is also quite doable. In fact, I'd want to sample faster than that and do a little low pass filtering and decimation in the micro. 100 kHz sample rate isn't even pushing it for something like a PIC 24H. At 40 MIPS that would leave 400 instructions/sample. That is much more than needed for a little low pass filtering and background bookeeping, so that checks out fine too.
The real question is what does the power feed look like and to what extent can you break into it. Are the units under test powered with LDOs? That would be useful, since a small current sense resitor before the LDO wouldn't effect the unit under test power voltage at all. You'd have to subtract off the LDO current, but that is doable. By putting the current sense before the LDO, you can afford to have it drop a little more voltage since the LDO will make sure the UUT still sees the same supply voltage. This of course assumes there is enough input voltage headroom to play with.
If you have to put the current sense directly in line with the UUT, then you have to carefully consider voltage drop versus sensitivity and therefore ultimately signal to noise ratio. Maybe 1Ω is reasonable. That would only drop 30mV max, which wouldn't effect most devices much at all. You'd need a differential amplifier and a overall gain of 100 so that 0-30mA results in 0-3.0V, which is just about the right target for a processor running at 3.3V. Various folks make such diff amps or specifically high side current sense amps. If this is a one off, I'd start with Analog Devices. A 10x diff amp with 1 MHz gain-bandwidth shouldn't be hard to find. That would need to be follwed by a ordinary 10x amp before the micro, again with 1 MHz gain-bandwidth being adequate. You could try doing the whole thing with a single 100x diff amp, but the gain-bandwidth product should be at least 10 MHz so the choices will be more limited.
Try this idea: -
5mA is generated by X1 and X5 - you'll need a stable voltage source where X6 is - it's referenced to the +5V to make life easier. If you are happy with your 5V rail stability it can just be a potential divider instead of the precision zener and R6 shown. X5 (AD8628) I used on a recent job but any 5V op-amp with decent offset voltage and inputs/outputs capable of getting near either rail will work. There are Plenty of P channel FETs that will do the job too - i picked this one because it has low Vgs(threshold) and there would be no-doubt it would work across the loads and power rails.
The AD627 is fairly standard and, for every ohm that R4 changes, the output (V out) alters 394 mV. The output offset is 2.5V (with a perfectly balanced bridge) but it could be set to 1V or if you had split supplies, 0V. Good luck.
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Best Answer
There are two key benefits of null-balance measuring methods.
The first is that the quantity being measured is not affected in any way by the measurement when the system is in balance; i.e., there are no "loading" effects.
The second is that since the measurement is always made at the same point on the indicating device, any nonlinearities in the indicating device itself do not affect the measurement at all. The only requirement is that the indicating device must have good "repeatability", which means that it always comes back to the same point for the same quantity being measured, regardless of its previous history.