Here's my requirement:
I need an analogue switch that can do SPST/DPST/DPDT with very low On-Resistance (<10mOhms). There would be current flowing through this switch, albeit in the range of a <10mA. This current DOES need to flow across, and hence my understanding is that I can't use any buffered(?) switches. I want the voltage drop across the switch to be absolutely minimal - very critical. Effectively I want the same voltage seen on the input to be present in the output, while allowing a small current to flow through.
The requirement for the very low On-resistance is an effort to keep the voltage errors below 0.1 ppm seen at the output. This is for a precision instrumentation circuit where the switches select various range resistor networks. Currently, these switches are mechanical and the contacts are gold plated. But I wanted to see if I can use solid-state switches in an effort to allow for the instrument to be more automated.
I understand this seems a difficult thing and it might be near impossible, but I wanted to see if anyone had any thoughts.
Here's my question/problem:
A MOSFET Mux seems that ideal candidate and is - except the lowest R-ON I could find was around 250 mOhms. This introduces errors in the range of 2-3 ppm.
And hence the first thing I was looking at was making my own MOSFET Mux using Discrete MOSFETs. The issue I'm having is that I can't seem to find a MOSFET that is 4 pin with a dedicated body-pin. My understanding is that to construct a MOSFET Switch/Mux, you need the body pin of the N-MOS to be connected to VSS and of the P-MOS to be connected to VDD.
I also did have a look at using series JFETs as switches, but they exhibited significant voltage drop across the FET.
- Is there a way to construct a MOSFET Mux/Switch with conventional MOSFETs that have their body connected to the source?
- Is anyone aware of 4-pin MOSFETs (with source and body disconnected) and if there was any specific terminology I might use to find them?
- Is there any other type of FET/BJT that can be used to accomplish what I want.
- Is anyone aware of low R-On Mechanical Relays?
Thank you for all your help. Let me know if this is a hopeless endeavor :)