analog multiplexer

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Hi guys !

 

I'm using serial analog multiplexer 8:1 LTC1391 and adc LTC1285 like in picture.

 

I've have noticed that if more analog signals are connected to analog multiplexer LTC1391 I read different values e.g.

 

I measure 1 analog signal and I read 1,245 V, but when I connect another analog signal to multiplexer I read on same channel 1,352 V instead of 1,245 V.

 

 

Anyone had similar issue?

 

Design is taken form datasheet.

 

Could you suggest me something similar (schematics)?

 

 

 

Thank you !

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Have you included the "optional filter"? Could it be that its cap is holding some of the previous signal?

 

Cheers,

 

Ross

 

Ross McKenzie ValuSoft Melbourne Australia

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valusoft wrote:

Have you included the "optional filter"? Could it be that its cap is holding some of the previous signal?

 

Yes,  R=120 Ohm  C=1nF  

 

 

Measure procedure is like this:

 

1.Close(deselect) all channels and wait for 1 second

2.open desired channel

3.measure desired signal

 

 

 

 

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This catches just about everyone.
Change mux
Wait x time
Do adc conversion.

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Kartman wrote:

This catches just about everyone.
Change mux
Wait x time
Do adc conversion.

 

OK !

 

From your experience, how much is this x time?

 

Thank you !

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T = RC
You need to factor in the impedance of the drive circuit. Or you can figure it out experimentally by putting 0V on one channel and full scale on the other. Convert both and observe the adc readings. Increase the delay until you read the correct values.
Your method of disconnecting the input was useless as the only thing changing the value on the capacitor was leakage.