4-20mA Converter Question

Thread Starter

scubasteve_911

Joined Dec 27, 2007
1,203
Hello everyone!,

I have a pressure transducer that I need to interface with a 0-3.3V ADC. The output of the sensor is the typical 4-20mA.

I have seen Maxim's appnote http://pdfserv.maxim-ic.com/en/an/AN823.pdf , which converts 4-20mA to 0-5V. This is done by a current shunt, with an appropriate gain, then a subtraction for the 4mA offset via a reference. This is a 'high-side' scheme, which requires the opamps common-mode input voltage to be high, thus necessitating high common-mode rejection.

My question is, why not use low-side current sensing? You can use much better opamps (lower offset, offset tempco, noise, cmrr, etc.). Is there some industrial reason for this?, like detecting faults or something?

I want to get as much precision as possible with the circuit, since it is a one-off prototype and money isn't too much of an issue.

Thanks!

Steve
 

Thread Starter

scubasteve_911

Joined Dec 27, 2007
1,203
Okay, but I am not looking for a pre-packaged solution. I'm looking to see what the reasons were why people typically do not employ low-side current sensing. Thanks for the link though, I was unaware that there are completely integrated solutions.

Steve
 

SgtWookie

Joined Jul 17, 2007
22,210
Hey Steve,
I thought 4-20 was used for digital signalling. I'm unaware of 4-20 devices that may respond in an analog way. I don't claim to be experienced in 4-20 circuits, but have done a fair amount of reading up on them.

Your initial post suggested to me that you believe the output of the sensor is linear in response. I just don't see that happening on a 4-20 device; it's a logic 1, a logic 0, or an error.

But I really don't have enough information. It would help a great deal to know what device you are considering.
 

Thread Starter

scubasteve_911

Joined Dec 27, 2007
1,203
Wookie,

4-20mA can actually be used for digital communications and Analog circuitry. Good old Wikipedia says:

Analog
Analog current loops are used where a device must be monitored or controlled remotely over a pair of conductors. Only one current level can be present at any time.

4-20 mA ("four to twenty milliamp current loop"), also referred to as "process current", is an analog electrical transmission standard for industrial instrumentation and communication. The signal is a current loop where 4 mA represents zero percent signal and 20 mA represents the one hundred percent signal.[1] A "mA" is a milliampere, or 1/1000 of an ampere.

The "live zero" at 4 mA allows the receiving instrumentation to distinguish between a zero signal and a broken wire or a dead instrument.[1] The feature also allows low-power instruments to be directly powered from the loop, saving the cost of extra wires. The 4-20 mA standard was developed in the 1950s and is still widely used in industry today. Benefits of the 4-20 mA convention are that it is widely used by many manufacturers, relatively low-cost to implement, and it can reject many forms of electrical noise. The inherent noise rejection of 4-20 mA allows it to be used where the transducer is located far from the measuring instrument. Cable lengths of 50 meters or more are common.

Given its analog nature, current loops are easier to understand and debug than more complicated digital fieldbuses, requiring only a handheld digital voltmeter in most situations. Using fieldbuses and solving related problems usually requires much more education and understanding than required by simple current loop systems.

Additional digital communication to the device can be added to current loop using HART Protocol. Digital process buses such as FOUNDATION Fieldbus and Profibus may replace analog current loops.
The device has a non-linear sensor, which gets digitized, then multi-point compensated, then converted to a linear 4-20mA signal.

Steve
 

Gina Hann

Joined Sep 19, 2008
1
I support Texas Instruments Signal Chain Analog, and some of the challenges of low side current sensing typically include sensitivity to additional resistance in the ground path, noise coupling from ground, or the difficulty finding a rail-to-rail input and output amplifier that meet the requirements of the full current range (4mA and 20mA). You can use an amplifier, but will probably end up needing to put it in a difference amp configuration and adding some common mode offset. The OPA333 has a low and high side monitor example in the datasheet http://focus.ti.com/lit/ds/symlink/opa333.pdf. An instrumentation amplifier might be a better choice to help minimize your efforts in resistor matching, and there is a good single supply rail to rail in/out INA called the INA326. The datasheet has a low side current monitor application that may be of help. http://focus.ti.com/lit/ds/symlink/ina326.pdf

Best of luck,
Gina Hann
 

Thread Starter

scubasteve_911

Joined Dec 27, 2007
1,203
Gina,

Thanks for the suggestion, I really liked the INA326, the specs are actually pretty good. I have had trouble finding a good instrumentation amplifier with high common mode input voltage capability, too bad you weren't suggesting that particular part when I was designing a high volume circuit two months ago :( We ended up going with an Analog Devices INA. Maybe next time !

I don't see the stray ground resistance an issue because the differential nature of the opamp, which should take care of that. I will checkout that part though, thanks!

Steve
 

cwilson

Joined Oct 21, 2008
1
Hi Steve,
I'm looking into doing exactly the same thing (4-20mA -> 0-2.5v ADC) and I'm running into the same question of whether to do high vs. low side current sensing. I can't find a good answer on which is the preferred method of sensing. Did you have any resolution on this question?

I'm looking at using something from the MAX407x series (the MAX replacement for the MAX471). Have you decided upon a current sense amp? I'm brand new to 4-20mA in general, so any pointers you can give would be much appreciated.
--Chris
 

Thread Starter

scubasteve_911

Joined Dec 27, 2007
1,203
Hey Chris,

Sorry, I actually didn't finish this circuit since I didn't get the sensor that I wanted. I was bidding on it on ebay and planned on buying it, but it wasn't exactly what I needed, so I let someone else win it.

I personally haven't done this myself either, only researched it.

Steve
 

vetterick

Joined Aug 11, 2008
35
Hey Steve,
I thought 4-20 was used for digital signalling. I'm unaware of 4-20 devices that may respond in an analog way. I don't claim to be experienced in 4-20 circuits, but have done a fair amount of reading up on them.

Really digital? I've been involved with many 4-20 systems I've never seen a digital one, the features to the 4-20ma setup is, very good noise imunity, cable length dosn't matter, you can put multiple recievers on one cable (in series current stays the same), and if the cable breaks the controller knows it and can alarm.

This is why the 4-20 is still widely used and interfaced to you guesed it digital systems.

We used to buy devices known as "AGM modules", they plugged into 8 pin relay bases and would provide conversion from 4-20ma to 0-10v, 0-20ma,
0-15V, I'm sure they would have you covered, don't use them anymore since most analog inputs now cover all common schemes.

Rick
 
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