Switching a constant current source through a MUX

[jg001]

I’m trying to build a circuit that I thought was going to be simple. So far, no luck. Here’s my problem:

I have a 1mA constant current source that I need to input into multiple strain gages to measure the resistance in 4-wire setup. I’ve tried using the ADG406 multiplexer. It doesn’t work. For example, I connected a gage to one port and tried to switch this port ON and OFF using the MUX control lines. I had a DMM connected in series. Theoretically I should have seen my current through this strain gage to be 0 when this particular port was OFF and 1mA when it is ON. Instead, when the port is OFF, I see 0.6 mA and when it is ON I see 1.0 mA. The rest of the MUX input pins are floating (nothing connected to them).

I’ve also tried this with a 4053 MUX chip. Same thing! Also, my Vdd keeps changing between 8 V and 0.1V as I turn a port ON and OFF??!!

Can someone please give me some input into this situation? My circuit diagram is attached.

 

[JoannesPaulus]

Why do you have 47K in series with the power supply? Removing it might help...

 

[jg001]

Why do you have 47K in series with the power supply? Removing it might help...

And it did ... with the Vdd problem. My line current still is between 0.6 and 1 mA, not between 0 and 1mA

 

[JoannesPaulus]

Where is the input current supposed to go when it is not going thru the strain gage? I believe you are not showing the whole schematic. Try to add a resistor between, say, S1 and ground and switch the current into S1. If the current in S12 goes to 0... there is your answer!

 

[jg001]

Thanks! It worked.

So, for any given control input, a current path to ground must exist through that switch. Thanks again.