Impedance of analog input to SyampPLC

[Plitch]

noobie question:

I am interested in using an industrial ultrasonic sensor with a 0-5 VDC sensor ouput range with a StampPLC equipped with a MAX1270 AD converter.

The only reference to the sensor circuit from the sensor manufacturer says "The input impedance of the measuring element should be greater than 50K ohms." I assumes this means that the StampPLC should provide resistance of 100ohms on the "analog in" pin.

In searching for information on the inputs to the Max1270/StampPLC, I found a Nuts & Volts article that mentions a 100K resistor betwen the analog input terminals on the Stamp PLC and the MAX1270 analog input pins.

Am I therefor correct in assuming that all I have to do is connect my sensor input wires to the appropriate "Ain channel" and "Ain GND" pins on the StampPLC without any further resistors in the circuit? If I have read this wrong, please direct me to a rough diagram of a correct circuit.

Many Thanks

[Phil Pilgrim (PhiPi)]

Plitch said...
...should provide resistance of 100ohms on the "analog in" pin.


I assume you meant 100K here. So, yes, this should work. However, I'd be very concerned about noise pickup if you're mounting the sensor remotely. That's much too high an impedance to be working with over even a short cable. If there's an equivalent sensor with a 4-20mA output, that would be the better choice. At the StampPLC end, you would install the shunt which converts the 4-20mA to a 1-5V input voltage.

-Phil

[Plitch]

Phil -

My apologies - I did indeed mean 100K, not 100 ohms.

I plan to interface the sensor to a StampPLC with about 5 feet of cable - actually reducing the cable length provided by the manufacturer.

I have used it "as is" with a Stamp Board of Education and ADC0831 and it seemed to work fine. I'm not sure what resistance (if any) was provided by that circuit - I didn't add any in that little experiment. I just wrote a quick-and-dirty digital voltmeter program, and the sensor voltages seemed consistent with expected outcomes and fairly steady. Not at all scientific, just a curiosity based exercise.

Where would you expect "noise" to arrise?

Thanks for your help.

Pieter

[Phil Pilgrim (PhiPi)]

Electrical noise can be induced in a cable from many sources, especially in an industrial environment. Motor relays are a common source, but I've even seen static electricity buildup on rubber conveyor belts cause problems when the inevitible discharge occurs. In my own fairly benign shop environment, I have an electric heater which triggers my 'scope every time it comes on. If these issues aren't planned for in advance, they can be maddening to fix later.

In your case, if you're committed to using a voltage output, I'd be tempted at least to buffer it at the source with an op amp that can drive a cable terminated at a lower impedance. And be sure to use shielded cable, with the shield connected to ground at one end only. You don't want any current running through the shield, as that will compromise its effectiveness. (This is a lesson I learned the hard way in the same plant that had the staticky conveyors.)

But, even with only a five-foot cable, 4-20mA would be the better choice in an industrial environment.

-Phil