Converting Analog Inputs


Thread Starter


I would like to use a momentum ANR-120-91, which has eight 0-10v analog inputs, but all of my devices are 4-20ma. I was told to put a 500ohm resistor in parallel and it would work. Has anyone done this with the momentum I/O base or any other similar module.

Thanks for the help.

Steve Myres, PE

This is what most ma input I/O's are doing internally anyway. There will be two things you need to keep in mind: First, a normal resistor has a resistance tolerance of +/-5%, so you could get 5% error introduced by the resistor. This may not be a problem, anyway, as normal non-precision resistors do not come in 500 ohm anyway, so if you find a 500 ohm resistor, it is likely to be a higher precision resistor sold specifically for this purpose. Second, because of the 20% offset, your input signal will actually be 2-10vdc rather than 0-10vdc, so you'll need to accomodate this in software. The good part about this, though, is you can monitor the signal for sensor/wiring errors (the signal normally can never go below 2v, so if you see 0 or 1 volt, you can alarm a sensor or wiring problem)
This is a standard practice. However, your input range will be changed to 2-10V, since there will be a minimum 2 volts drop across the 500 ohm resistor when your device sends 4 mA.

Since it is an analog input card, you may as well considering reading in the raw count present at the input address, and scale it to your requirements, using ladder logic or any other language being used by you.

Hakan Ozevin

Check if your devices support 500 ohm load (some do not). If yes, you can use, but buy 0,1% tolerance resistors.
It is very easy and common practice to use a precision resistor to convert the mA signal to a volt signal. However, while using the resistor care need to be taken to ensure that the resistance is within the load capacity of 4-20mA signal source. Of course, all standard 4-20mA signal sources support 500 Ohms resistor as load..
Please go ahead and use the resistor for volt signals...
Yes, this works OK. We've been doing something like this with KMC controllers having 0-5 VDC inputs but we use a 250 ohm resistor instead.
Some points to remember:
1. Connect the resistor close to the controller. This will preserve the 4-20ma advantage of ignoring wire/connection resistances.
2. If your controller input has one side grounded, you won't be able to connect other devices in series with it unless it's the last one.

The 500 ohm resistor idea will work. It needs to be a precision resistor, since the voltage it develops at the terminals will only be as accurate as the resistor itself. Also you need to make sure it can dissipate enough power to avoid self heating errors. At 20ma, a 500 ohm resistor will need to be able to dissipate 0.2 watts without a significant rise in temperature.

Rick Daniel
Intelligent Instrumentation

Jon Schacher

There are 4 other Momentum bases that support 4-20ma input directly. So unless you absolutely need 6ai, 4ao, 8di, & 8do that the -91 supplies, I suggest using on one of the others.

For instance the 170 AMM 090 00 has 4 di, 2 do, 4ai, & 2 ao.

Jon Schacher
Sr. Automation Engineer
Graybar Electric