0 - 20mA and 4-20mA current loop

Brian,

I'm not really the "ADC guy" but I am familiar with a majority of these "loop" designs, so thought I could at least try to get the thread going in the right direction and clarify what your goals are and which devices you need help selecting.

0-20mA and 4-20mA current "loops" each come in a variety of configurations, primarily impacting the structure and design of transmitter. I've shown the high-level configurations below:

The transmitter in the 0-20mA loop structure could come in either 3-wire or 4-wire configurations but not in 2-wire. This is because in the 2-wire configuration the lowest loop current value must correspond to a current that is greater than or equal to the transmitter's quiescent current, and 0mA does not allow any current consumption for the transmitter at all.

The transmitter in the 4-20mA loop structure could come in any of the three varieties since the lowest loop current value is non-zero, in fact 4mA of budget for quiescent current in the transmitter is very common and quite reasonable to achieve. 

The analog input or receiver in all of these cases would  look similar and should be selected based on design requirements for the receiver such as bandwidth, resolution, sampling rate, etc. In all cases the receiver is not powered by the loop and therefore power is usually not a major concern.

What is not shown in these illustrations sensor placement for sensor transmitter designs. A sensor and ADC to digitize the analog data from the sensor is also included with the transmitter in sensor transmitter designs. A sensor transmitter may be designed in 2-wire, 3-wire, or 4-wire configurations. The ADC in the sensor transmitter would be selected with similar criteria as the ADC in the receiver, except in the 2-wire sensor transmitter design the ADC will also need to be power-conscious.

In order to help you out...

1. What are you designing? A sensor transmitter? A transmitter? A receiver? Some of all?
2. Which configuration will the 0-20mA loop be implemented with?
3. Which configuration will the 4-20mA loop be implemented with?

There will be more questions to follow, probably from some of my colleagues, but I hope this helps.

Hi Kevin, thanks for the response. I'm looking at receiving in the signals 0-20 and 4-20ma. As far as the configuration, since I'm receiving in the signals, does that pertain to this? Also, I realize I'm currently using an ADS1232 that has 5V on the Analog side and 3v3 on the digital side. I was thinking of switching to the ADS1242 to address the two current loops that's been added to the project. What do you think?

To clarify, I'm not making a transmitter, just a receiver.

Brian,


There are a lot of different options for the ADC. What sort of noise floor are you looking for? What data rate do you need? Do you need the integrated PGA?

You could move to the ADS1242, but there are a few other options that you could go with. If you're familiar with the ADS1232 already, you could use the ADS1234, which is a similar device but with a higher channel count.

Another device that has multiple channels would be the ADS1224, but it doesn't have the PGA.

Yet another device would be the ADS1248, with multiple channels, the PGA, and a few other integrated features that might be useful (excitation currents, internal reference).

If you need to make 3 measurements you could use the ADS1220, as long as the three measurements have one common voltage from which to measure.


Joseph Wu