This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

RCV420: Reference Circuit Configuration

Part Number: RCV420
Other Parts Discussed in Thread: XTR105, ADS1115

I am using the RCV420JP IC to take my 4-20mA signal to 0-5VDC, but am having some difficulties understanding the reference circuit Figure8 on the datasheet.  I connected 12VDC+ to the V+ of the IC and GND to the V-.  But with this configuration I am not able to read from my pressure transmitter at all so something must be wrong.  I also posted about this on the electrical engineering stack overflow forum (post), but didn't receive a satisfactory answer because it seems the datasheet is unclear to most about what to connect to V+ and V-.

Below is my attempt at the circuit side-by-side with the reference circuit in the datasheet.  Could someone tell me if I've made a mistake?

  • Hi Alex.

    Could you provide me more information about your XTR105,  RCV420 and post-RCV420 Configuration? There are several ways to configure the RCV420 in 4-20mA to 0-5V conversion. I enclosed two configurations below. 

    Per Figure 3, dual supply ±12V supply is shown (onboard reference is 10Vdc, So V+ needs to be approx. 12V or higher. Negative supply may not require to be the same or symmetric.). 

    The positive power rail needs to be > 11Vdc for dual supply configuration. The negative power rail need to be <= -5Vdc per RCV420's recommendation for dual supply configuration.   Enclosed is a Tina simulation of the above circuit. 

    RCV420 4-20mA 0-5V dual Vs 10252021.TSC

    If you only have single supply rail, the input voltage rail needs to be >= 15Vdc. One of following circuit may be configured. 

    https://www.ti.com/lit/an/sboa023/sboa023.pdf?ts=1635182316441&ref_url=https%253A%252F%252Fwww.google.com%252F

    Please let me know your system configuration and I will recommend a setup per your available supply rails. 

    Best,

    Raymond

  • Raymond,  thank you for the reply.  From what you mentioned it is clear why my current set-up with 12V as V+ and GND as V- doesn't work.  But fortunately I do have a lot of freedom with this PCB.  The major constraint here is accuracy and precision.

    The transmitter I have is not the XTR105 I was just using this figure from the ICs datasheet as an example.  But my transmitter does require a supply between 8-30VDC and scales its output to 4-20mA.  Available rails on the board are 24, 12, 5, 3.3VDC.  The voltage output of the RCV420JP is connected to a TI ADC (ADS1115) channel.  This IC is supplied with 5VDC so an analog channel input cannot have more than voltage supply (5VDC) +0.3V referenced to GND.

    Could you recommend a set-up based on this information?

  • Hi Alex,

    Could you recommend a set-up based on this information?

    Since you have on the board are 24, 12, 5, 3.3VDC power rails, I would recommend to create a negative -5V supply rail from switched-capacitor voltage inverter, such as  LM2776LM27761LM2662 ot  LMC7660. These are charge-pump voltage converter inverters and it will generate negative voltage rail from positive supply voltage. 

    With dual supply voltage rails, RCV420 may have wider options in configuring the input and output. 

    The major constraint here is accuracy and precision.

    Here is how RCV420 works, and by selecting precision resistor, you will achieve the accuracy and precision that are looking for. 

    The working principle in RCV420 is based on Howland current pump configuration in difference amplifier, see one of the application note below. 

    https://www.ti.com/lit/an/sboa441/sboa441.pdf?ts=1635177352328&ref_url=https%253A%252F%252Fwww.ti.com.cn%252Fzh-cn%252Fdesign-resources%252Fdesign-tools-simulation%252Fanalog-circuits%252Famplifier-circuits.html

    https://www.ti.com/lit/an/snoa474a/snoa474a.pdf?ts=1635189251617&ref_url=https%253A%252F%252Fwww.google.com%252F

    I am going to capture series of simulation plots and you may check out the Howland Current Pump properties from the attached simulation file. 

    The captured image file is with input current of 20mA, where the input voltage Vin+ = 20mA*75Ω =1.49972V. The difference amplifier has a gain 1.3333V/V, therefore Vout = 1.33333*(100k/400k)*1.49972 = 0.499894V,  If 1.01kΩ load is applied to the output, it will generate a constant current of 493.281uA  (0.499894V/1.01kΩ) at Is = 20mA. 

    The remaining circuit is utilizing the property of difference amplifier. When Vref of 10V and R6 (92k) resistor is placed at the output, it will not change the Vout level with the simulation below, where Vp, Vout and Iout through R5 or 1.01kΩ load is unchanged.  Keep in mind that the current through R1 (300k) is unchanged due to Vin- = Vin+ = 374.93016mV.

    If we insert R7 in the feedback return loop, Vp and Iout are still unchanged, but Vout and current through R7 will change due to (Vout - Vp)/R7. When R7 is increased to 11.50631kΩ  level, the Vout will change to 5.0Vdc, which it is level shifted from 0.5V to 5.0V.

    If you are able to keep the accuracy in 4-20mA current loop and R7 resistors as shown above, you should get Vout with high accuracy and precision that application is required. RCV420 is trimmed with matching and load resistors internally (CMRR >70dB minimum, or R matching is equivalent 0.01% or better), which is typically better than your standard discrete resistors used in the application. 

    RCV420 4-20mA 0-5V Howland Current Pump-3 10262021.TSC

    If you have additional questions, please let me know. 

    Best,

    Raymond

  • Raymond great explanation of the Howland, I am not as familiar so I need to review your answer a few times to understand the concepts.  If I used a voltage inverter as suggested and supplied -12V to V- and 12V+ to V+ as shown in the figure below, I could connect Vo directly to the analog channel of ADS1115?  But I see you have suggested -5V instead of -12V for V-, does this mean that I would still connect 12V+ to V+ and would get 0-5VDC on the Vo?

      

  • Hi Alex,

    You have options to create -12V from 24Vdc supply rail from listed charge-pump converter. Since the operating current is low, you should have minimum power dissipation in the application. It is your choices to create -5V or -12V for the negative supply rail.  

    I could connect Vo directly to the analog channel of ADS1115?

    If ADS1115 is configured as singled ended input, then RCV420's 0-5V output will work for the ADC. If ADS1115's input is configured in differential mode, then we need to config RCV420 differently, similar to the figure below. 

    Currently, I assumed that the ADC input is configured as single ended in ADS1115. 

    Best,

    Raymond

  • You are correct, single-ended configuration is what I'm using.  I think the -5V ICs are more available so if a -12V rail isn't necessary then that's what I'll do since I'll still get the GND to 5V Vo.  I am still using 12V+ as V+

  • Hi Alex,

    I am going to close this inquiry. If you have additional questions, you may still re-open the thread, or you may create a new one via E2E Forum. 

    Best,

    Raymond