I'm thinking of doing a power meter for an e-bike.
The whole thing will be power from the battery which is 54V to 39V depending on the charge level so I'll need a voltage regulator for everything but the current sense amp which can work up to 60V.
This is most probably an overkill and not needed but I wonder if I can make it high dynamic range, e.g. from 5 mA up to 20A.
There's this high dynamic range current sense amp LTC6102 that converts a sense voltage to a sense current, it is recommended that the sense current is not greater than 1 mA full scale (which translates to just 60 uA of current for 1 LSB of the ADC).
The current waveform is 100 KHz at most accroding to my measurements.
So I'm thinking of oversampling it and the voltage and integrating (just summing) the product.
I'm looking at LTC1407A for the ADC which is 14 bit 1.5 with 2 channels and simultaneous sampling.
Thought I could buffer the current sense and voltage (from a resistive divider) signals with OPA2365 which has a low offset voltage which would allow to measure the lower end into the milliamps (1LSB would be about 100 uV for the ADC).
There are probably a ton of other thins that would prevent me from measuring the low end and getting this high of a dynamic range but one issue that I'm considering now is providing a negative rail for the opamp for the output to get near ground (1LSB is about 100uV).
The datasheet says that the negative rail need not be lower than -0.3V but still how do I do this with low noise.
I've found that one can do a virtual ground with a resistive divider and an opamp.
In my understanding this virtual ground will have to be shared by all the chips: the current sense amp, the opamp, the ADC and the FPGA so I'm worried that it would spread create noise and I won't get the dynamic range.
On the user hand at the low currents the motor is not working so I think the current will be pretty much DC, but there will be the ADC and the FPGA with their switching still.
Is this dynamic range that I'm aiming at not achievable this easily and I should just give up the idea?
If it is how should I go about the negative -0.3V for the negative rail for the opamp?