Analog Input 1771-IFE


Thread Starter


The manual of 1771-IFE states that the input signal must be within +/- 14.25 V referenced to the module common otherwise a crosstalk may occur and cause invalid input. Is that mean I can't use transmitters require 20 to 24 volts in the input loop to transmit 4-20 mA?

I know that this module type doesn't supply loop power.
No, it does not mean that you can't use a 24V power supply for 2 wire transmitters. Yes, you can use a 24V power supply.

That statement means that a 10V full scale signal from 0-10Vdc output on a 4 wire device that has +6Vdc common mode between the module common and the signal common will exceed the +/-14.25V limit (10V signal + 6V common mode = 16V), which will saturate the card and probably drive all signals off scale, up scale.

A 4-20mA signal develops a maximum or 5Vdc across the AI card's internal 250 ohm resistor, so a 4-20mA signal would have to have over 9.25V common mode to 'crosstalk' (5v + 9.25V = 14.25V), as A-B says, which is far less likely because 2 wire transmitter outputs are almost always isolated from ground.
I know that this a bit late, but this is the common mode rejection specification problem. One of the things that this spec means for a 24 VDC/4-20 mA transmitter loop is that the input card should not be the first thing in the loop (e.g. should be after the transmitter) where it would typically see 24 VDC to ground (250 ohm resistor) on the plus terminal and 23-19 VDC on the minus terminal, both of which exceed the +/-14.25 VDC spec. This can result in channel-to-channel crosstalk and can cause invalid input readings and potential invalid underrange or overrange bits. This essentially a ground loop problem where part of the signal returns through a different path than normal through an internal return path in the I/O card back to the power supply where it came, which will result in an erroneous signal value.

A real life example of this type of problem occurred in a laboratory where a current signal went to a recorder right off the power supply before the transmitter in the current loop . It was noticed that the of the recorder output did not agree with the current indicator on the transmitter in a laboratory hood. The actual transmitter current output was checked and agreed with the current indicator. The voltage drop across dropping resistor on the back of the recorder was checked and it was also agreed with the transmitter output. The loop was checked without the recorder; transmitter was checked; and power supply voltage were checked, all of which checked out. The recorder was replaced and the problem persisted. By this time the instrument technicians were pulling their hair out, which is where I got involved. It seems that if you have something really strange occurring in your instrument loop, the ground is almost always involved. I checked the CMR spec and found that it was exceeded. The recorder was relocated down stream of the transmitter and the problem was solved and the recorder loop worked correctly. I cannot say if all such loops will react that way but that one certainly did.

William (Bill) L. Mostia, Jr. PE
ISA Fellow, SIS-TECH Fellow,
SIS-TECH Solutions, LP

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