asdf44
Advanced Member level 4
I'm concepting a current sense application covering about 5 decades from 500u to 5A where high precision <0.1% AC/DC and high bandwidth ~1Mhz are required (high precision applies to <<1mhz). Ultimately the signal is fed to an ADC (>5MSPS)
So first I think it's obvious this requires multiple shunts and my thought is to space them every decade -> 0.01/0.1/1/10/100. These have to be low PPM 4 terminal type shunts with kelvin sensing.
Next I'm wondering about a few options for muxing these shunts and given the bandwidth and precision requirements I'm worried about the challenge of switching such low level signals. The shunts themselves have to be switched with fets or relays.
1) Use analog muxes to switch the shunt sense leads into a single differential amplifier stage
Pros: Shares one high quality diff-amp stage.
Cons: Has to switch 50mV type signals and lengthens their traces
2) Pair a differential amplifier with each shunt and mux their outputs
Pros: switching happens on higher amplitude signals. Can optimize layout around the shunts
Cons: Differential stage (with matched resistors) must be repeated
2A) Use opamps with enable/disable and wire OR their outputs to implement the mux. For some reason I've been studying this lately and like it
Pros: Eliminates analog switches with their associated parasitics/cost/size
Cons: Limits opamp choice (needs enable feature and 'mux friendly' input).
Thoughts about these options or the problem in general?
So first I think it's obvious this requires multiple shunts and my thought is to space them every decade -> 0.01/0.1/1/10/100. These have to be low PPM 4 terminal type shunts with kelvin sensing.
Next I'm wondering about a few options for muxing these shunts and given the bandwidth and precision requirements I'm worried about the challenge of switching such low level signals. The shunts themselves have to be switched with fets or relays.
1) Use analog muxes to switch the shunt sense leads into a single differential amplifier stage
Pros: Shares one high quality diff-amp stage.
Cons: Has to switch 50mV type signals and lengthens their traces
2) Pair a differential amplifier with each shunt and mux their outputs
Pros: switching happens on higher amplitude signals. Can optimize layout around the shunts
Cons: Differential stage (with matched resistors) must be repeated
2A) Use opamps with enable/disable and wire OR their outputs to implement the mux. For some reason I've been studying this lately and like it
Pros: Eliminates analog switches with their associated parasitics/cost/size
Cons: Limits opamp choice (needs enable feature and 'mux friendly' input).
Thoughts about these options or the problem in general?