Resistance measurement through multiplexer

Thread Starter

nirbec89

Joined Feb 25, 2021
36
Hi,
I made a schematic for testing resistance on LCC package pins.
I need to test resistance between 1 fixed pin from the LCC package (Pin #8) against 7 different pins with automatic switching and measuring.

For that i chose ISL84781IVZ-T 8:1 multiplexer ic controlled by MCU,
The fixed pin (Pin #8) is connected to one side of the DMM through U5 testpoint ,
while the COM pin from the multiplexer is connected to the other DMM input through U3 testpoint.

The issue is that when i connect all the wires from the multiplexer to the LCC package (SRC1_1, SRC1_2, etc..) , i get wrong resistance measurements (much lower values then expected).

Only when i disconnect all wires and leave only one connection (SRC1_1 for example), i receive correct measurements as expected on the single line.

Can you find any issues with the multiplexer concept ? is this behaviour expected? (any kind of crosstalk inside the multiplexer?)

Please advise,
Thank you
Nir.schematic.png
 

dendad

Joined Feb 20, 2016
4,142
First off, try to measure voltage to see if there is any fed through that would upset the reading.
And, what is the supply volts to the ISL84781IVZ-T?
It could be your multi meter volts on the ohms range is too high.
I have not used the ISL84781IVZ-T so am not familiar with it.
It could be a good idea to have one switch wired up to "short" the multi meter as a calibration.
 

Thread Starter

nirbec89

Joined Feb 25, 2021
36
Hi dendad,
Thank you for your fast response.

I actually also do not have experience with this mux, I just chose it since it have very low ON resistance so it will achieve higher accuracy measurements.
I power it with 3.3V from the MCU.

the DMM is Keysight 34410A so according to its datasheet on 10KOhm range that i need it sources 100 µA .

I will make the tests according to your suggestions and update here with the results..
Thank you
 

Thread Starter

nirbec89

Joined Feb 25, 2021
36
First off, try to measure voltage to see if there is any fed through that would upset the reading.
And, what is the supply volts to the ISL84781IVZ-T?
It could be your multi meter volts on the ohms range is too high.
I have not used the ISL84781IVZ-T so am not familiar with it.
It could be a good idea to have one switch wired up to "short" the multi meter as a calibration.
Sorry for my late response,
I'm adding the desired test results to my last reply.
the switch was wired up to "short" the multimeter with 0.8Ohm resistance as a result,
and during resistance test it shows 0.0mV (both results were taken with fluke 179)
 

MrSalts

Joined Apr 2, 2020
1,762
the switch was wired up to "short" the multimeter with 0.8Ohm resistance as a result,
and during resistance test it shows 0.0mV (both results were taken with fluke 179)
Why does the resistance test show an output in milliVolts? Is your meter set to the right mode? Zero ohms should be the desired reading of a closed (connected) switch. Infinite ohms ("OL" on a Fluke .meter) should be the desired reading when the switch is open.
 

Thread Starter

nirbec89

Joined Feb 25, 2021
36
Why does the resistance test show an output in milliVolts? Is your meter set to the right mode? Zero ohms should be the desired reading of a closed (connected) switch. Infinite ohms ("OL" on a Fluke .meter) should be the desired reading when the switch is open.
Sorry, i wasn't clear.
when i wire up the switch to "short" the multimeter (according to "dendad" suggestion) , DMM gives 0.8Ohm,
I then move the DMM to voltage mode (again according to "dendad" suggestion - if i understood it correctly) and get 0.0mV.

when the switch is open i get around 250KOhm.
 

crutschow

Joined Mar 14, 2008
29,828
For the most accurate resistance measurement you would use a 4-wire (kelvin) measurement.
An accurate constant-current is put through one set of mux contacts, and a voltage measurement is done through another set.
That way the mux contact resistance has no significant effect on the resistance measurement.
The resistance is, of course, Vmes / Iapp.
 

Thread Starter

nirbec89

Joined Feb 25, 2021
36
For the most accurate resistance measurement you would use a 4-wire (kelvin) measurement.
An accurate constant-current is put through one set of mux contacts, and a voltage measurement is done through another set.
That way the mux contact resistance has no significant effect on the resistance measurement.
The resistance is, of course, Vmes / Iapp.
crutschow, thank you for your answer,
I actually chose the ISL84781IVZ-T since it features 0.4Ohm ON resistance according to its datasheet (which is much more precise than i need) in order to avoid complicated wiring.

Anyway my DMM is Keysight 34410A so its capable of measuring resistance using 4-wires, but i didn't catch the concept of using two mux, could you please prepare a simple sketch? hopefully it will yield better results
 

crutschow

Joined Mar 14, 2008
29,828
Anyway my DMM is Keysight 34410A so its capable of measuring resistance using 4-wires, but i didn't catch the concept of using two mux
It's quite simple.
Since you have one fixed pin on the package you are measuring, you connect one of the high side and the corresponding low side DMM inputs to that pin using two separate wires.
Then you run the other high side DMM input through one MUX and the other low side DMM input through the other mux using two separate wires.
The muxes both are controlled by the same address command, and both corresponding outputs go to the pin to be measured, again using two separate wires.

Make sense?
 
Last edited:

Thread Starter

nirbec89

Joined Feb 25, 2021
36
It's quite simple.
Since you have one fixed pin on the package you are measuring, you connect one of the high side and the corresponding low side DMM inputs to that pin using two separate wires.
Then you run the other high side DMM input through one MUX and the other low side DMM input through the other mux using two separate wires.
The muxes both are controlled by the same address command, and both corresponding outputs go to the pin to be measured, again using two separate wires.

Make sense?
Thank you for the detailed explanation , it does make a lot of sense.

I will try it out.
 

crutschow

Joined Mar 14, 2008
29,828
Some further thoughts:
You need to determine the output voltages of the DMM since you don't want them to go above the Mux supply voltage, or below ground.
You should probably connect the common of the Mux supply to the DMM common.
 
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