4-20ma reciver

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Dear all
I have a sensor which can transmit a 4-20ma current. I want to read it using a 16bit ADC. Should I go with a shunt resistor path or do we have Receiver Chips available?

Any suggestions would be welcome!

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Most go with a precision shunt resistor, but ti/ burr brown have a specific device X something.

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Do you happen to know the part number?

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Gp tp www.ti.com and type in 4-20ma in their search field. They have several.

Jim

 

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I have found RCV420 but the Achievable accuracy is .1%, I need 16bit's of data, do we have higher accuracy options?

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4-20ma systems are not, as far as I know, really designed for that sort of accuracy.

If the transmitter is REALLY that accurate, what receiver is recommended by the maker of the transmitter?

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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For 16 bits..

What resolution do You really need on the old fashioned 4-20mA loop?

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Quote:
What resolution do You really need on the old fashioned 4-20mA loop?

As I said 16Bit,I have found 16bit DAC transmitters available, but there is nothing for 16bit receiver!? What should I do? Should I choose a 0.01% resistor? Is that my only option?

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I guess the question is do You really need 16 bit resolution?
But if You do do need 16 bits.. well no good trying to sense the current with 0.1% components and not include some kind of calibration system in the receiver.

Probably a software cal function ( a look up table).

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The old chesnut of resolution and precision. Does the transmitter have 0.01% resistors? Probably not. So your sensor might be able to give 16 bits of resolution but not 0.01% accuracy. 12 bits is the accepted norm for 4-20mA, so I'd be looking carefully at the sensor specs to see that you won't be gold plating your design.

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There is another issue here as well. Does this transmitter draw its power from the 4-20 mA current loop. This topology is very common in the Process Control Instrumentation field. It is usually referred to as a "2-wire hook-up" or "2-wire transmitter".

A common alternative is a "4-wire" type. In this topology the transmitter's power is supplied from a power source which is independent of the 4-20 measurement loop. Often 24 VDC (nominal), but sometimes 120 VAC.

16 bit resolution implies you are measuring to a resolution of 305 nano-amps in the current loop. Insulation leakage in the loop wiring can contribute to the error picture at such nano-amp levels, not to mention all of the other electrical noise culprits.

In a 2-wire system the power source plays a role in the accuracy/resolution issues as well.

So, Ali, 2-wire or 4-wire?

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Precisely ( all puns intended), what is it that's required by the process?

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The sensor is 4 wires. And the sensor is a pressure sensor.
Let me explain more, we have 16bit transmitters like AD421 from analog and dac161p997 from Ti .So what should we use for receiving part? Dose a 0.01% 250 ohm resistor and a 16bit ADC sufficient (it will translate to 13-14Bits)? Do we have other options? Do we have 4-20ma ADC’s? Or any other option? How can we get 16bit resolution in receiver?

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Is 13 bit resolution sufficient for YOUR PROCESS... never mind what transmitters You have.

It is not about transmitters ... it is ALWAYS about process being supervised, controlled, logged etc .

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Ali,

1. Most ADCs work in voltage mode - the VOLTAGE on the input is converted to a digital code on the output. I am not sure if there are ANY current mode ADCs available in the IC marketplace. Perhaps another Freak can enlighten us. I can think of a few ways to make a current mode ADC, but they would probably be too complicated for your needs.

2. In most instances when we want to measure a current, we first convert it to a voltage using a resistor (or capacitor in some more complex schemes), then measure the resulting voltage with an ADC, DVM etc. This is the scheme you gravitated to already. As you can see it does have its limitations - witness the mythical 0.02% resistor you seek. Is that 0.02% over some operating temperature range? I can conjure up a few ways to make such a topology self-calibrating, but again we get into comlexities which may not be appropriate for your application. Or am I wrong in that assumption?

3. Many 4-20 loops have been replaced or supplemented with digitial communications schemes such as HART & Field Bus. Have you looked into any of these? The number of bits of precision or resolution are considerably less troublesome when they can be digital transmitted. If in fact you need 16-bit resolution,such a digital solution is probably your best approach.

4. What is the specified resolution of you pressure transmitter?

5. The fact that this is a 4-wire transmitter gives you a bit of an accuracy advantage over a 2-wire type, but I am still not convinced you can get 16-bit precision over a 4-20 ma loop in a practical application.

6. A big factor in the performance of a 4-20 loop is the noise and leakage environment in which it must operate. How long is the loop? What type of wire? Are the wires bundled with other wiring? What type? Is there a shield or conduit involved in the wiring? What is the temperature range that the loop wiring is exposed to over the course of its operation?

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The AD421 >>IS<< a DAC with 4-20ma output.

It still requires an ADC to read the sensor and provide the digital data to the DAC. By the time you do that conversion, your ACCURACY will already be down (though 16 bits is not that bad). But, the combination of a 16 bit ADC and a 16 bit DAC does NOT necessarily give you 16 bit accuracy. That depends on the sensor and signal conditioning between the sensor and the ADC. All of this is before you even get to the 4-20ma system!

In the 4-20ma system, you then have all of the issues that Chuck describes.

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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Thanks guys for your reply. So you say that it's impossible to implement 16 bit accuracy!

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Not impossible. Very difficult.

And, even if you have a 16 bit DAC, your SYSTEM (sensor to data user) won't be 16 bit.

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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Quote:
And, even if you have a 16 bit DAC, your SYSTEM (sensor to data user) won't be 16 bit.

Suppose it's a 16bit counter, Then my system would be 16bit!

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