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MCC118 / analog front-end schematic


didierjehanno
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[2022.08.04 - 16:34]

Hello,

When the analog inputs of the MCC118 are not connected, we measure 1.75V. The electrical specifications read that the input front-end is a 1MOhm connected to the GND. Then, why do we measure 1.75V ? Is the front-end schematic available ? Where ?

Many thanks for your help.

Didier, France.

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The reason you see a voltage on unterminated inputs is due to the design requiring an active front end circuit.

A/D converters can only make measurements within the limits of their power supplies.  The MCC 118 is powered by 5VDC and ground.  To make it operate and make measurements within the range of +/-10V, an additional circuit is added to the front end.

The only side effect is when the inputs are left to float, they read 1.75ish VDC.  if you short the input to ground it will read 0.0 VDC.

This is similar to the design of our USB-1208FS-PLUS and USB-1408FS-PLUS, in that when the inputs are left unterminated, they float to ~ 2.4 VDC.

Sorry, schematics even partials are not available.

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