Original question:
Part Number: RCV420
Hi,
I am getting + and - (4-20mA) output from a sensor which need to be converted to 0-3.3V. So, I am using two external resistors of value 83.5ohms connecting CT (pin2), +IN (pin 3) and -IN (pin1). What other changes are need to be done and what should be connected to +IN (pin 3) and -IN (pin 1) to get 0-3.3V output as I am getting + and - (4-20mA) output from the sensor.
Hi,
This is one possible solution using the RCV420 internal 10V reference pin through a voltage divider and op-amp. This circuit accepts a 4-20mA input current producing a 0V to 3.3V output.
There are other possible discrete 4-20mA receiver circuits (without the RCV420) that could be used as well to produce a 0-3.3V output.
Thank you and Regards,
Luis
Edit 7/26/2021: Adjusted Iin pin 1 connection with 145-Ohm to GND, Adjusted slighlty Rx to 145-ohm.
As mentioned in the datasheet I'm connecting two Rx resistors of value 83.5 ohms. This value came from the given formula.
Hi K.R.,
In order to convert the signal 4-20mA input and obtain a 0V to 3.3V output, you require an overall gain of:
Vout/Iin = 3.3V/16mA = 0.20625V/mA.
Then, use the equation on the datasheet:
Vout/Iin = 0.3125 x Rx / (Rx +Rs)
On the equation above, substitute Vout/Iin = 0.20625V/mA, Rs =75Ω, and solve for Rx, this yields 145.58Ω, choose a standard value resistor of ~145Ω; then offset the REFIN pin to the correct offset potential to support 0-3.3V output using the 10V reference, a voltage divider and a buffer amplifier.
One easy way to verify the transfer function circuit is to use TINA SPICE. Although, there is no TINA model for this legacy device from 1988, you can approximate the expected transfer function using the block diagram of page 1 with an ideal op-amp to build an "approximate" model. This model will not simulate the frequency response and all the AC/DC performance parameters of the RCV420 accurately, but this approximate model is plenty good to verify the transfer function of the circuit when selecting the component values.
Below is the circuit that supports the 4-20mA input with 0-3.3V output. I adjusted the resistor just slightly to Rx=145Ω to follow the equation and procedure recommended on the datasheet, but the circuit has almost the same output transfer function as the circuit suggested on the previous post. The plot shows the simulated input current 4-20mA input and the resulting 0-3.3V output:
A TINA simulation for the DC transfer characteristic sweep for Iin 4mA to 20mA produces the required output 0V to 3.3V output.
Here is the simulation file I used to check the transfer function. Since there is no model for the RCV420 legacy device, I used a generic op-amp model only to verify the expected "ideal" transfer function:
As Kai had mentioned, the E2E post below suggest other possible receiver circuits ideas. You can modify/adjust the op-amp per the supply available and precision requirements specific to your application.
Thank you and Regards,
Luis
