LMP91000EVM: How to select the FET to short RE and WE

Dear Liu - 

On this PCB, Q1 (the P-Channel FET) has SOT-23-3 footprint and is used as switch, with the gate pulled high by R12, neither are populated.  

(page 6 of the same user guide you point to LMP91000 EVM User Guide )

The important parameter to start with would be the package, which is SOT23-3, as this is the footprint on the PCB. The devices in Table 2 on page 2 are 30V 1/4W P-Channel JFETs and as they are used as a switch, any general purpose device with those characteristics and was in stock would suffice. 

Here are some I found, that were also available and should be direct crosses: 

https://www.digikey.com/en/products/detail/linear-integrated-systems-inc/SST177-SOT-23-3L-ROHS/14312313 

https://www.digikey.com/en/products/detail/onsemi/MMBFJ176/1050155 

https://www.onsemi.com/pdf/datasheet/mmbfj175lt1-d.pdf 

Dear Tom - 

This is a P-Channel JFET MOSFET switch circuit, which will conduct from source to drain, with no voltage applied to the gate. (i.e. if Q1 was installed by itself, the terminals would be connected, likewise they would be connected when no power is applied to the board) When a voltage is applied to the gate of Q1 through R12 (when it is installed) - this would stop current flow from source to drain and allow for normal sensor operation.

Here is an application note from a sensor vendor which discusses the need for this. 

https://www.alphasense.com/wp-content/uploads/2022/10/AAN_105-03_App-Note_V0.pdf 

Here is screen shot of that section, found on page 4. 

I provided links to direct crosses above for the JFET. I think it is not any more complicated than that.  

Dear Tom - 

in this use case (as a on/off switch for very low voltage), something damaging the part or PCB physically would be probably the only way the JFET would break, as it is not being stressed out like it would be in an amplifier circuit being operated at a frequency. Remember, in this circuit, the JFET is on when there is no power applied to the board. 

Dear Tom - 

A simple test would be to arrange a prototype circuit having a pulsed output voltage source driving the gate bias resistor. The duration of/off times would be probably be same as the amount of time you intend to have sensor powered / not powered, and I would recommend you consult your sensor vendor to see if they have additional guidance, similar to the application note from previous post. 

Something like this (image below) should work for you to test this JFET switch.  

or this

Dear Tom - 

The FET you mention is N-Channel FET. This circuit requires a P-Channel JFET, so I would think here your failures were due to using the incorrect part.