Internet PLC Forum

General => Technical support => Topic started by: CMS on July 30, 2006, 11:45:27 PM

Title: ANALOG OUTPUTS RISING TO 4096 N/C
Post by: CMS on July 30, 2006, 11:45:27 PM

When I activate the ADC command and the 0-5v sensors are not connected the value slowly rises to 4096, i e 5 volts.  If a wire to a sensor should break the same will occur? As these sensors can trigger a shutdown alarm when a preset value is met, if a wire breaks then the value will rise and meet the setpoint and cause a shutdown.

How can I avoid this??

Title: Re:ANALOG OUTPUTS RISING TO 4096 N/C
Post by: evanh on July 31, 2006, 04:12:13 AM

If such a situation is a concern then using a biasing pulldown resistor is recomended.  Assuming your transducer is actively power it will have an output impedence of no more than 1000 ohms, more likely 10 - 100 ohms.  If you use a biasing resistor of atleast 100 times (Preferably more) that figure then you'll be introducing very little distortion, especially if the bias supply is say -5 volts.


Evan