INA826 (instr. amp) - can I use mosfet to switch different gain resistors?

[dusan]

I have INA826 and I'm trying to measure current from 1uA to 100mA using 1R shunt and I want to switch between 2 ranges (100x Rg=470, 1000x Rg=47). Can I use mosfet across that 470R gain resistor and when I want to "remove" it I simply sort it with mosfet?

The image is from datasheet https://www.ti.com/lit/ds/symlink/ina826.pdf example and shows 3.5mOhm I'll use 1ohm shunt, I don't have actual schematic yet.

2 Gain resistors and 470R shortable by mosfet:

Code: [Select]

     |
  47R
     |-------drain
   470R
     |--------source
     |


[Zero999]

It's generally a bad idea, because the MOSFET needs to be biased on which requires the gate voltage to be taken above the source. An analogue switch would avoid this, but it will have a much higher on resistance. A reed relay would be better.

Why not use a programmable gain amplifier?

[harerod]

This can be done. However, you need to know a bit about the internals of your INA to get the bias of the MOSFET right:
https://www.eevblog.com/forum/projects/ad627-rg-connection/msg5221824/#msg5221824
ADI support was a bit slow to understand and react to my request. I only got an answer after I had built the prototype. Works like a charm, btw.

[Doctorandus_P]

As an alternative, you can also use mulltiple shunt resistors either in series or parallel, and then switch some of those with MOSfets. Even SOT-23 Fets can switch multiple amps these days and can have a low Rds-on.

But whatever method you choose, you will have to deal with this resistance of the FET itself (which is both temperature and voltage dependent) and with current leakage.