Other Parts Discussed in Thread: CSD19538Q2
Hi,
I had to choose a TI part to get this discussion started (CSD18542KTT), but that's probably not the FET I will use. I want to make a redesign of the low burden uCurrent from EEVblog. The uCurrent uses 3 shunt resistors/3 branches to measure the current:
10k Ohm for nA range
10 Ohm for uA range
0.01 Ohm for mA range
I want to make an auto switching uCurrent. The 3 branches will be controlled by a microcontroller and this microcontroller will also measure the current with integrated ADCs.
My problem are these branches. Especially the 10k branch. I will use probably use N-FETs to switch between the branches. Text will continue below. This is the schematic....
My problem is the 10k branch. If I disble the other branches (VGS=0) the leakage current through branch 2 and branch 3 must be very low(low IDSS). The shunt of branch 1 is 10k. But the leakage through branch 2 or 3 must be in the order of pico Amps or femto Amps to still have a good nA range. Is that possible? I think I should especially look for a N-FET with a very low IDSS. Which FET do you recommend? The max current is 2A. The max voltage is 60V. Do you recommend to have all N-FETs the same or should I choose for a different N-FET for the high current branch (0.01 Ohm/2A max)? But this FET must have a low IDSS also.
How about this IDSS... If the 10k shunt is active and the IDSS is 50nA, will there be a loss of 100nA through the other 2 branches or is that not the case because most current will go through the lower resistant 10k? I know, that's not a low value, but is it relative small againts the hopefully high resistance of the other branches?
Thanks in advance for your help.
Best regards,
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