RCV 420

Hello Jorge,

Thank you for your interest in the RCV420, and welcome to the TI E2E forums.

Let me first answer your question about the types of transmitters and loops:

• In a loop-powered system, the 4-20mA transmitter is provided power by the same lines on which it outputs current. Since there are two wires involved (output current and return current), this is known as a 2-wire transmitter. An example of this is the XTR105. 2-wire transmitters are commonly used to transmit data from a remotely-located sensor, and because of the distance it is convenient to only need 2 wires.

• In a locally-powered system, the device still has the same output and return current lines, but now the device is powered from a dedicated power supply line. This is called a 3-wire transmitter. An example of this is the XTR111. 3-wire transmitters are commonly used to transmit an analog control signal from a control room to a remotely-located element such as a valve. In this case, the transmitter is close to a power source so providing a power supply line to it is not difficult.

You are correct in that a resistance is needed to convert the 4-20mA current into a voltage. Since the RCV420 is designed as a 4-20mA receiver, it contains internal resistors sized so that the part can easily produce common output voltage ranges such as 0-5V with very few external components. See Figure 1 on p. 5 of the RCV420 data sheet for a example schematic of how to connect your input and output signals.

Do you have already have a 4-20mA transmitter selected? You mention a flow sensor - is that your application? Does the sensor generate a varying resistance depending on the flow rate it measures? If you provide more information about your application I can help make recommendations on your circuit design.

Best regards,

Ian Williams
Linear Applications Engineer
High Performance Linear

Hi Ian,

Thanks for your reply.

I am designing an instrument which measures ,flow,pressure and density. And for density I want to connect my design to a masic flow meter which measures flow and density too. There are some instruments with self powered mA output. I did my connection with one of these systems and the response was satisfactory.
My question now is more oriented to the instruments which not have power with the mA output. So I have 2 doubts.

1.What would be the connection when the sensor needs and external power. (I guess but I am not sure must be  like the self powered case) if not, How must be the design connection?

2.My system can provide  the supply? It must be the same power of the RCV 420? Is this correct? if yes  How must be the design connection?


Thanks......

Jorge,

I'm not sure I understand your question. It sounds like you did some tests with a self-powered transmitter, and now you have concerns about a loop-powered (2-wire) transmitter.

You asked:

1. What would be the connection when the sensor needs and external power. (I guess but I am not sure must be  like the self powered case) if not, How must be the design connection?

Can you give me a specific example of the sensor you're asking about? Can you provide a link to a data sheet? A common type of sensor is a RTD (resistive thermal device) which changes resistance based on temperature. This type of sensor is "powered" in the sense that our XTR transmitters drive a small current through it to generate a voltage drop which is measured and compared to a known value.

2.My system can provide  the supply? It must be the same power of the RCV 420? Is this correct? if yes  How must be the design connection?

An example of a complete loop-powered system is shown on Figure 8 of the RCV420 data sheet. The RCV420 has local +12V and -12V supplies, and the XTR105 shares the +12V line. An input sensor (RTD) is "powered" by current from pin 14 of the XTR105.

Best regards,

Ian Williams

Hi Ian,

1.  I must say yes I was doing tests with a self powered transmitter and I got good results but the manufacturer told me the new electronis of new transmitters have changes, so in that case if I use an external supply Is the connection the same or not. The transmitter is a masic flow meter created by Micro Motion. This the link for 1700 and 2700 electronics:

http://www2.emersonprocess.com/en-US/brands/micromotion/coriolis-transmitters/1700-2700-transmitters/Pages/index.aspx

2.   Ok the answer is yes but  I guess the power must have current enough   to circulate the current across the 250 ohm resistor(4-20mA)  (about 2A), Am I right?

Thanks.....

Jorge,

1. The Micro Motion transmitters are all self-powered, but they all still have 2-wire mA output so the connections are the same. However, if you have a "IS" type transmitter, an extra power supply is required for the 4-20mA output. The compliance voltage for the transmitter is 12V, so you'd need a minimum of 13.5V supply if using the RCV420 as a receiver since it only causes a 1.5V drop in the loop.
2. The RCV420 requires +15V and -15V supplies, while the Micro Motion 1700/2700 transmitters have a wide range of AC and DC power supply voltages. These supplies must be kept separate. The RCV420 has a typical quiescent current of only 3mA so a low-power bipolar supply is suitable.

You asked about circulating 4-20mA current across a 250-ohm resistor. If you use the RCV420 you do not need a 250-ohm resistor. The RCV420 has internal resistors which allow for a low voltage drop, and the internal amplifier and references are used to generate 0-5V output. If you would rather use a 250-ohm resistor across the 4-20mA output you will get a 1-5V output per Ohm's Law. However, using the RCV420 allows for more accurate voltage measurements to be made compared to a standard resistor.

Regards,

Ian Williams

Hi Ian,

1.Ok it is clear and I have no doubts, anyway I was speaking with Micro Motion people and according with your reply and their reply they have a selfpowered mA outs so there is no problem.

2.Yes Ian but the circuit is instaled in myu design which provides the correct complianced power and the GND line which is common to masic flow sensor and my instrument.

Finally if understood well I do not need the 250 ohm resistor so I just can drive directly the mA+ out put of the sensor to the pin #3 (in this case)?

Let me know any commnet please.

Thanks.....

Hi TI

I saw this post, and i have a similar trouble connecting RCV420KP, i tried this conection that i read in the datasheet (Basic Power Supply and Signal Connections) .

Im using a Flow Sensor Badger® Series 250 & 228 and a Data Industrial Model: 310 Programmable Analog Transmitter. This kind of transmitter has an output 4-20ma an a input 9-35 VDC. 

I want to use the RCV420KP to COMPLETE 4-20mA TO 0-5V CONVERSION.

I hope you can help me to conect, i dont know how to conect mA+ and mA-, my transmitter output has two wires 4-20ma loop(-) and 4-20ma loop(+)  i conect them too pin 1 and pin 3, but those wires need voltage too 9-32VCD. But i dont know if i  can put voltage to those pins.

I attached datasheets

6888.310_IOM.pdf

1541.228_250_i.pdf

Hello Alejandro,

Don't worry, the RCV420 has a common-mode input voltage range of +/-40V and is designed to easily interface with loop-powered current transmitters, so you can connect your transmitter output to the RCV420 input pins with no problems.

I recommend the same connection as in my previous post. Connect mA+ from the transmitter to pin 3, connect mA- from the transmitter to ground, connect pin 2 to ground, and leave pin 1 open. Also, make sure to provide a negative supply voltage of at least -1.5V to pin 4.

Best regards,

Ian Williams

Hi Ian:

Thanks for answered so fast...

I cant make RCV420 work, i dont know what im doing wrong. i connect this way:

Pin 1: open

Pin 2: ground

Pin 3: mA+

Pin 4: -12V and 1 capacitor 1μf

Pin 5: ground

Pin 6: NC

Pin 7: NC

Pin 8: NC

Pin 9: NC

Pin 10, 11, 12: connected together

Pin 13: ground

Pin 14, 15:  connected together

Pin 16: +12V 1 capacitor 1μf

i dont get de 0-5 V output, i get negative voltage without change...

Best regards

Alex

Alejandro;

Refer to Figure 4 of the Model 310 data sheet.

The "4-20mA Loop -" should connect to the RCV420 pin 3 (+In) and the "4-20mA Loop +" should connect to the 24V power supply + terminal. This powers both the Model 310 and the current loop. Connect the power supply - terminal to the RCV420 pin 2 which is the RCV420 common connection.

This should work with this transmitter.

Hi Neil:

Thanks for the information, but what about Pin: 4 and Pin:16, which voltage should i use?

Best regards

Alex

Alejandro;

Connect those RCV420 pins to the +/- 15V power supplies as shown in Figure 1 of the RCV420 data sheet. According to the Model 310 sensor data sheet, it can operate on power supplies of from +9V to +35V, so you can power the sensor from the same +15V supply that is connected to the positive supply pin of the RCV420 instead of a separate 24V power supply if you wish.

Hi Neil:

Thank you very much for the support and information, RCV420 is working perfectly using the las connection you told me.

Now my project is progressing...

Best Regards

Alex

That is good to hear; good luck with your project, Alex.

Dear gurus,

I have a doubt for using the RCV420- Firstly I think it is a great and useful chip. My doubt is related of using it as a 4-20mA passive system. In my case the design requires to measure 4-20mA current loop which will be monitoring a loop where a third party system will be energizing the loop ( lets say the DCS ). All the app notes shows the IC providing the supply to the loop. Since the measurement has to be non intrusive, I need to get one input where the current flows to my device, and one output. Besides, grouds can not be connected. Lastly, I have 24V and 5V power supply to meet 0-5 output. Could you please support me?

Thanks in advance,

Marcos

Hi,

I have a 2 wire differential pressure transmitter wich is supplied with 24 V., and the RCV420 is supplied with +/- 15V, iboth power supplies have different grounds... how can i cannect the 24 V in the current loop? Is it posible join both grounds without affecting the measurement? I would appreciate your help..

thanks.. best regards..

Fabián Britez

Hi IAN,

Could you please answer my doubt please, I have a RCV420 and my pressure sensor are supplied with 24 volt, so the current loop powered with this supply, and my rcv420 is supplied with +-15 V with another power supply (different ground of the 24 v).

 Is it posible join both grounds without affecting the measurement? I would appreciate your help..

thanks.. best regards..

Fabián Britez

hello sir.

i read your post.

i also trying to use RCV420 to convert 4 to 20ma to 0 to 5v. i have 4to20ma source which i will give to my circuit.

what i want to ask u that if internal resistor is provided then it is necessary to connect one more resistor? we can not give directly input to 1 and 3 pin?

in my circuit i have shorted the 2nd and 1st pin . i have given +input to 3rd pin if RCV420 and negative wore coming from source to 1+2 pin of rcv420.

10 and 12 pin is shorted.13pin is grounded.4pin =-15v and 16pin= +15v. output voltage taken from 14 pin w.r.t gnd.

Is it ok?

i get a output with 0.36v error.

i dont understand what i do with ref in,rcv com,rcv fb,pin and minimize this error?

thanks and regards

swapnali

Hello Swapnali,

Although you've described your circuit in detail, would you please share a schematic so I can analyze all of your connections exactly as you have them?

Once we see the schematic we'll get to the bottom of this issue.