RCV420 alternate wiring of common terminal for isolation

Sean Bigley

Other Parts Discussed in Thread: RCV420, XTR105

Team,

I'm using the RCV420 to receive a 4-20mA output current from a pressure sensor, and I want to isolate the sensor input stage. I am using a fuse and a zener between the sensor and In+ (pin 3) of the RCV420. I want to know if it is acceptable to connect the common terminal (pin 2) to the negative lead of the source used to power the sensor, as opposed to the common of the power source for the RCV420. Figure 1 on page 5 of the datasheet shows pin 2 connected to the common ground, but I want to connect it as shown below:

Will this be an acceptable way to wire the circuit?

-Sean

over 8 years ago

0 Collin Wells over 8 years ago

TI__Guru 62035 points

Hi Sean,

The input common-mode voltage range of the RCV420 is +/-40V relative to the RCV420 power-supply GND and RCV_COM connections. Therefore, the circuit will function as you've drawn it as long as the loop-supply GND of the 2-wire transmitter does not float away from the RCV420 by more than +/-40V.

That said, you've mentioned "isolating" the sensor input stage which is typically done on the transmitter side by creating an "input isolated 2-wire transmitter" which I've described in the following blog post: e2e.ti.com/.../2-wire-4-20ma-sensor-transmitters-background-and-common-issues-part-5

Without knowing more about your transmitter, the circuit connections with the RCV420 will only achieve a +/-40V "isolation" between the two grounds which I'm not sure will be sufficient, but that's up to you to determine.

0 Randy Adams80 over 8 years ago

Intellectual 345 points

I have the RCV420 connected as described in the datasheet. I am getting -0.15v out at 3.99ma in and 0.046v at 5ma. I should be getting 0 volt at 4 ma?

0 Collin Wells over 8 years ago in reply to Randy Adams80

TI__Guru 62035 points

Hi Randy,

Is your post related to Sean's post, or is this an unrelated question?

0 Randy Adams80 over 8 years ago in reply to Collin Wells

Intellectual 345 points

It is related Sean Posted this for me. I am trying to get this to work. I am also not getting the correct output. I get 0.046v at 13psi I am reading 5.9ma and should be getting about 1.5v out here with a 0-100psi 4-20am loop transducer/

0 Sean Bigley over 8 years ago in reply to Randy Adams80

TI__Genius 17175 points

Collin,

Randy and I have noted that at 0V out from the pressure bridge, the RCV420 draws 5.81mA current, not the expected 4mA. Would it be possible to setup a call or Webex to go over our circuit and see if there are any issues that may be causing this increased current draw at 0V?

Sean

0 Randy Adams80 over 8 years ago in reply to Sean Bigley

Intellectual 345 points

Sean,

I have found a better solution. I am using a 0-5v output Meas pressure transducer. solves all the problems without any circuit.

I am now working on the Interface for the PT-100

Thanks

Randy

0 Collin Wells over 8 years ago in reply to Randy Adams80

TI__Guru 62035 points

I'm glad you found a solution, I'm not sure the RCV420 would have provided the "isolation" you were looking for.

Regarding a PT-100 design, take a look at TIPD161: Analog Linearized 3-wire PT-100 RTD to 4-20mA Current Loop Transmitter Reference Design.

It's a reference design with complete calculations and measurements for configuring a PT-100 RTD over a desired temperature range to a 4-20mA output.

0 Randy Adams80 over 8 years ago in reply to Collin Wells

Intellectual 345 points

I have this chip. But need 0-3.3v to go into a msp430 ADC

0 Collin Wells over 8 years ago in reply to Randy Adams80

TI__Guru 62035 points

Hi Randy,

So you already have the PT100 to 4-20mA circuitry working and you're only looking for a 4-20mA to 0-3.3V receiver circuit? Let me know and I'll get something your way.

0 Randy Adams80 over 8 years ago in reply to Collin Wells

Intellectual 345 points

Collin,
I have the XTR105 chip. but the min voltage is 7v. I can't use it. when I will be reading temp the device will be running on battery. Only 3.3v. I need an RTD circuit that will give me -40 -160 deg F from the RTD in voltage at -2.4-2.4v for my ADC in the msp430.
Thanks

0 Luis Cortez over 7 years ago in reply to Collin Wells

Prodigy 10 points

Hi Collin

I am trying to interface the RCV420 to and Arduino uno where i can read the 0 - 5 V, However i am having issues getting it to work. I have only a 24V PSU and have tried wiring the RCV420 as per the above schematic.

My question is how can i wire the RCV420 using single 24PSU ? I know the datasheets specify +15 and -15V to power the chip.

Regards

Luis

0 Collin Wells over 7 years ago in reply to Luis Cortez

TI__Guru 62035 points

Hi Luis,

The RCV420 is not designed to operate on a single-supply voltage and requires a dual-supply configuration for proper operation. This information is provided in detail in the following post:

You could also consider a simple circuit with a non-inverting op-amp and shunt resistor as shown in the post below. For a +24V supply, consider the OPA192 or OPA188 instead of the OPA333.

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RCV420 alternate wiring of common terminal for isolation