MD2424 ADC conversion unstable

[Alan]

I have wired an LM34 temp sensor to ADC(1).  The sensor outputs 10mV per degree F.  I scan the port once a second.

The actual bits monitored at the PLC can vary from 600mV to 640mV, which gives a temperature variation of 60 to 64 degrees.  Very unstable.

Vs is wired to +5V ref.
Grn is wired to 0V
Vout is wired to ADC(1)

I have experimented with the sensor wired to a 16bit DAC card and found it to be stable when read at 16bits.

Do I have something misconfigured?

Thanks.

Alan

[support]

The ADC input on the PLC can be affected by digital noise in the digital section of the PLC. This include the switching noise in the power supply and the digital noise in the microprocessor circuit.
 
You may want to use a low noise linear power supply to reduce the noise that could affect the reading.

The ADC input built-in to the microprocessor can vary about +/- 3 LSB. Which means for 0-5V range, the variation is about:

5000*3/1024 = +/-15mV.

So it is not uncommon that the reading will jump around a median value. Such variation are quite typical of ADC circuit that are built into microprocessor.

The external analog module such as the I-7017 has its own isolated DC-DC converter in order to give very stable reading (16-bit resolution). This also partly explain its higher cost ($230) compared to the  built-in ADC on the PLC.