Single-ended & differential type analog I/O cards

[bmsg]

I have a basic doubt:

1) What is the advantage of taking differential type analog I/O cards?

2) Does using differential type I/O cards necessarily ensure channel-to-channel isolation?

 

[djs]

To answer:

1. Differential inputs allow you to achieve a limited isolation channel to channel. On most cards this is limited to about 30 V. If you have a greater voltage then that between channels, then you must use an optical isolation on the signal.

2. Again you achieve isolation to a limited degree.

 

[sreid]

Differential inputs reject common mode signals. These are signals common to both inputs such as ground shifts or 60 cycle pick up. These kinds of problems are quite common and differential inputs essentually eliminate them.

 

[bmsg]

All said & done, is thare any real advantage in using differential type analog input cards for PLC loop powered 2 wire transmitters (pressure, flow, level, etc.)?

 

[duckless]

Yes there is. As mentioned above, any noise that is induced in-to/on-to the + and - conductors of the loop will be eliminated in differential mode. With Single Ended inputs this noise level becomes part of the signal reading thus leading to signal error. One example of this at our plant is our 480 VAC Variable Frequency Drives. The electrical power conduits (aluminum) are mixed with control signal conduits. Some of these runs are over 300 feet long. Our flow and level measurement signals (4-20mA) have a high level of mixed frequency noise from these drives. Differential mode works well to minimize this noise problem. Although it is not 100% rejection, this mode works well enough for our application.
If you can use some type of industrial IP network between your instruments and your PLC, you can eliminate 99% of your noise problems in addition to having complete remote access to the instrument for monitoring failures, recalibrations and a wealth of other data.