My customer has a problem when selecting one channel
and running current in the other it effects the selected one.
Please advise how to solve it.
I'm Freddy's customer and to answer your questions:
1) Supply voltage is 3.3V
2) Signal Voltage is 4-20mA from standard sensors.
3) The issue is seen on any 2 channels combination.
4) We have many products( the same model) and it appear on all of them.
The voltage on the "OFF" channel is around 4V. I've even disables the "OFF" channel and measured the voltage on the only "ON" channel and it's 3.6V. in both cases it's above the supply rail.
My circuit include two 4-20mA inputs. Pins 4 and 14 are connected to the 4-20mA supply A (sensor A) through 80.6R resistors on each line and pins 5 and 12 are connected to sensor B. There is a 21R resistor connected between pins 8 and 9. I would like to know how that current loop can effect the chip ( I don't understand how the relation to the chip ground).
The current loop itself should not affect the multiplexer. However, you are generating voltages that are above the supply voltage of the part. This is operating the part at or above the Absolute Maximum Rating. Using the part in this manner is not recommended or specified and can cause reliability issues and affect the lifetime of the part.
I would recommend increasing your voltage supply to the part to encompass the signal voltages.