INA321 multiplexing

Hi Chris,

Welcome to the e2e forum! The main issue I see with multiplexing the power for each INA321 is that the output pin of the device has an ESD diode to each of the power supply rails, here is a simplified schematic:

Therefore, if the VDD line is pulled to ground on the "disabled" device, then the voltage at the output will be clamped to within 1 diode drop in either direction (positive or negative) from ground. This is not the case with the device is simply in shutdown mode, because both diodes remain reverse biased as long as the signal range is within the supply voltages. 

Thanks John,

I just don't understand enough about the internals of these devices to work out what ifs without actually building the circuit

and I'm a bit too long in the tooth to use a simulator

from your explanation I can see that is the case if VDD is pulled to ground..  

and I can see that if both VDD and VSS are left to float, the output can only be fully floating and therefore commoning the outputs would work

but what happens when VDD is just left to float ? or if VSS is left to float ?

using a demultiplexer I can arrange for either VDD or VSS to float, but it would take two demultiplexers to make both float..

an alternative would be using the demultiplexer to cut the power to VDD (to reduce power consumption) and also operate some analogue switches to select the output.

a pair of 74HC4066s I guess

even that is easier than arranging for 4 way cabling to the devices..

Chris

Thanks for that John,

I can see that tying VDD to ground or VSS to +V would have that effect..

If both VDD and VSS were left to float (by using a pair of demultiplexers to only connect the VDD and VSS of the device that I want to read, then the outputs of the others would be truly floating and it should work and save power by only powering up the device required.. 

but what happens when I allow to float only VDD  OR  VSS of the devices I don't want to read ?

Thats where my limited knowledge lets me down

I know I can use the multiplexer to only power up the chip I want, and an analogue switch (operated also by the multiplexer ) to select the output, but its all a bit clumsy

Chris

Floating the supplies of the devices that you don't want to read should be fine because then there is no way for their ESD diodes to clamp the signal. 

Thanks John,

I will make up a test pcb just to confirm that it is stable before I incorporate it into my project, but its good to know that I'm not barking up the wrong tree.

I'm OK with a hammer and a screwdriver, but I quite often let the magic smoke out !

Chris

After a little bit of experimentation.. I am a little further down the road

Floating the Vcc of the unused amplifiers does indeed allow commoning of the outputs

Floating the ground does not work

To put this into practice is not quite straightforward, mainly because I am limited to 3.3V 

but it looks like I may be able to use a 74HC238 to supply Vcc to the INA321(and what it is reading) that I want to read

I will have to use schottky diodes to prevent the supply of the unused INA321s being pulled to ground by the 74HC238 when not selected.

parts on order, I will post if it is successful as it may be useful to others