ADC Jumping Around

[Jeff Bauman]

I'm using ADC channel 1 with a 4-20ma simulator connected to it with a 1% 250ohm resister.  The values are not steady, they move around +/- 1-2% without me changing values on simulator.  Any ideas?

[support]

The built-in ADC has a repeatability of about +/- 3 LSB, which for a 10-bit ADC translate to approximately +/-0.3%.

Note that the reading you see on screen is multiplied by 4 because of the normalisation to 12-bit. i.e. for +/- 3 LSB the reading will fluctuate about +/- 4,8 or 12.

If the reading jump more than that, then ripple on the PLC power supply may play a factor if I assume that your 4-20mA simulator has ultra steady power supply.

If you use the PLC's built-in 5V to power a potentiometer and take measurement the reading will be much more stable.

[Jeff Bauman]

So thats it? Nothing can be done about 4-20mA signal jumping around?  I'm trying to use plc for remote water tower level monitoring via a 50psi pressure transmitter with 4-20mA output, so using a pot is out of the question.  I guess I might have to try one of the RS-485 analog input modules, which puts cost of your plc above traditional units.  Any thoughts? Have you considered offering two different plc models? One for high end analog users (added cost) and maintain lower quality ADC for low end analog users?  How much more could new chip cost?

[support]

Have you actually tried the sensor interfacing with the analog input instead of using a simulator? Depending on the quality of the PLC power supply as well as sensor power supply, the presense of electrical noise etc you can get different result. So far many users seems satisfied with the slight flickering of the reading.

For water level measurement, since the reading does not change rapidly, you could take a moving average of the signal that you read in. That can be easily accomplished using the math capability of TBASIC.  You can take a moving average of 3 or 5 samples as your final result.

For absolute stability the RS485 based I-7017 will give you excellent result since it is a 16-bit unit with its own isolated power supply. Considering that it gives you 8 channels of 16-bit analog inputs the price is not too expensive.


If you want absolute stability in the reading