MUX ADG1408

Another question: all of my 64 MUX receive the same 6 inputs, which means they are placed in parallel to one another. Is this design correct? Might this be problematic?

Hello Ben,

Please could you provide me with a schematic of your circuit. Just the circuit with 3 MUXes will suffice at the moment.

Is the -12V seen at both the input and the output? Is it always there or can you switch away from the signal?

Regards,

Sean

Hi Sean,

I haven't design the circuit myself, I'm a summer intern working on top of someone else's previous work. I don't have access to a schematics editor for the moment, so all I can do is sed you the whole thing / circuit, which is a bit complex as it makes use of 64 MUX.

The -12V is not seen at the output unless I turn on the switch corresponding to the input showing -12V. I can basically control every other switch with no problem at all.

I was wondering whether this might simply be because some of my MUX are not good anymore.

I appreciate your help,

-Ben

here's the schematics with only 3 MUX.

Regards,

Benoit

Hello Benoit,

Which inputs do you see the -12V on?

Regards,

Sean

Hi Sean,

On this PCB, it's MUX's pins # 3,4 that are at -12V.

But I have this other PCB with all 64 MUXes assembled on it, which display -12V on every of the 6 inputs (MUX pins 3-5 and 7-9) we're making use of.

I know pins 2 and 10 should be grounded as we're not using these switches; I've put that as recommendation for future update on the PCB schematics.

Thanks for your help,

Benoit

Hello Benoit,

Your schematic and PCB layout do not indicate any obvious issues to me. The parallel design you are using is fine and should not be causing the issues you are seeing. Grounding unused pins is good practice if it isn't explicitly stated in the datasheets, but this would not be causing the issue you are seeing here.

Because there is a difference between the 3-MUX and 64-MUX boards, I feel that it may a physical connection problem and not just damaged MUXes. This can be tricky to isolate and my first suggestion is to measure the resistance using an Ohm-meter or handheld multimeter between the VSS (-12V) rail and the input pads on an unpopulated board. This will indicate if there is an electrical connection between these pins without the multiplexers present.

For the 3-MUX board I would check if there is a diode between pin 3 or pin 4 and VSS. This will indicate if there is a short internally. A handheld multimeter with a diode setting should be able to identify this.

Regards,

Sean

Hi Sean,

It appears the problem was related to the assembly of the MUX on the PCB. As far as I could tell, there was a short-circuit between power supply and an input gate on one of the 64 MUX. 3 other MUXes were not properly connected to the power supply (either +12V or -12V was not connected).

Troubleshooting is dfficult because of the component package, which prevents direct observation of soldered pins. Since all MUXes are in parallel to one another, it's impossible to tell which hold the short-circuit with power supply. PCB design didn't allow to shut down MUXes one by one.

At some point I decided to verify what tension there was at the MUX output pin. One of the MUXes showed power supply tension value. I guess the short-circuit between the input gate and the power supply also included the output pin.

After I removed this MUX and the 3 others I could easily control the others.

Thanks for your help. -Benoit