[SOLVED] Multiplexer garbage values

[venkates2218]

This is the circuit used to switch the analog value from pressure sensor's output to microcontroller.Output from the pressure will be 0.10mV to 5VDC.
These output are connected to Y0 to Y7 pins.Channels are selected by using S0,S1,S2 and EN pins.If any output is not required means,EN pin is enabled and output from the multiplexer is disabled.

Problem is even if the multiplexer is disabled by enabling the EN pin,some of the output voltage are measured in both microcontroller and multimeter(For checking purpose).The volatge will be 0.25mV to 1VDC.

How to eliminate this error voltage..?

 

[wwfeldman]

if the multiplexer is disabled, you apparently do not care about the sample voltage,
so why does it matter if there is some output voltage? don't look at it

presumably the microcontroller controls the enable and the address, so you know (can know)
when to not look at the sample

i saw nothing in the data sheet, except maybe the off state leakage current, +/- 5 uA,
that may be the cause of the observed voltage

 

[betwixt]

Two potential causes are:
1. if there is any difference in ground potential between the MUX and DAC it will show as an error.
2. If disabled (~E is high) the output will float rather than go to zero. You may need to put a pull down resistor from the DAC input to ground to discharge residual leakage and charges.

Brian.

 

[KlausST]

Hi,

Disabled outputs are floating = high impedance.
This means the voltage is not defined...and thus is what you see.
Additionally high impedance nodes are prone to pick up any noise and induced voltage.

This behaviour is quite expectable.

So why don't you connect Y7 to GND and switch to this node ... then you have a valid, low impedance zero signal.

Klaus