CD74HC4067: multiplexing current sources

Hi Andre,

From my understanding the multiplexer is actually functioning properly and that 16RCVs connected to 1 multiplexer would function properly.

Analog multiplexers like the CD74HC4067 should be able to pass 4-20 ma and voltage signals through the io pins.

In addition a precision multiplexer like the MUX506 can also work in this application 

I am not a expert in RVC devices but you could talk to the team responsible for those devices and see if they have a more compact solution for the RVC you are using.

Regards,

Kameron

kameron

Thanks for your answer. Let me rephrase  the question. Instead of using 16 RCVs 420 I´d like to use CD74HC4067 as a current source multiplexer. In order to do a small test, I created a PCB with 8 Howland improved current sources (“Improved” Howland current pump circuit) . As soon as connect more than one source current and skeep the same selected channel , the voltage output from RCV 420 changes to a crazy value as soon as I connect a second, third, etc source. My question now is : Regarding a Howland current source, maybe the output impedance could be affecting the whole circuit ?

Regards

Andre

Hi andre,

Thank you for the extra information and now I understand this a bit more

So because you are placing current sources on the unused inputs of the device this is something our team calls injected current.( Forcing current on unselected pins of the mux)

This can cause improper operation of the mux and cause the reading errors you are seeing.

Some good devices you could try for this application are the TMUX1308 and TMUX1308A which have injection current control. and can help reduce the error you are seeing because of their internal design

Looking at the op amp app note I’m not sure what voltages would be present at the input of the 1308 devices but if they fall in the recommend operation section of the data sheet external components might not be needed 

Finally we could look into using multiplexers with a feature called over voltage protection like the TMUX7308F. These devices can withstand a increase in voltage on unused pins from something like a current source as long as that current source clamps out at some point to a voltage with the ovp range of the device.

For the TMUX7308F that is +/- 60V.

The voltage clamp of the op amp would be important here.

I personally am not a expert in the other parts of your design but the devices I mentioned should be able to help here.

Please let me know what you think about this and we can continue to look at this.

Kameron

Kameron. I appreciate your answer but I still have one last question: Just for my understanding, how precise is this circuit ? I mean, how much will I loose of information in case of 8 current sources being connected to it ? My concern is related to potential errors converting the current source value which maybe affected by other current sources. Also I'd like to ask if there is a similar model with 16 inputs ?

Warmest regards

Andre 

Hi Andre,

So I actually added something to my answer when you responded and maybe you could also look at  Multiplexers with OVP like the TMUX7308F.

But for your question, I did talk to the design team for our parts and their thought was there is a possibility the  mux would cause to much leakage in this kind of application.

Is there a chance you could just convert the current reading into voltage readings before the mux.

These muxes are much better in voltage signal applications.

The MUX506 would probably be our best 16:1 option here as we dont have a fault protected or injection current control device in this large configuration

Please let me know what you think about this  and happy to help further.

Regards,

Kameron