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Comments on How can I fix this 4-20 mA current loop to source current that matches current sink on its input?

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How can I fix this 4-20 mA current loop to source current that matches current sink on its input?

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Building on a previous question about terminology, I am trying to arrive at a solution to connect a digital air pressure regulator that sinks current as part of its 4-20 mA current feedback loop to an ADC that also sinks current.

Block Diagram:

Block Diagram

The pressure regulator has two 4-20 mA loops, and sinks current for both of them. The control loop (sets the desired pressure) is handled by an Analog Devices AD74413R (Datasheet) and is working without a problem. The feedback loop (reports actual pressure) is intended to be handled by an Analog Devices AD4111 (Datasheet), but since both endpoints sink current, I need to create a bridge or adapter circuit.

A colleague and I used this TI Application Note as the basis for the following schematics:

Schematic 1:
Schematic 1

Schematic 2: (connects at right of schematic 1)
Schematic 2

We first created a simulation in Falstad.

We have been struggling because our knowledge of opamps is a bit rusty/weak. I will explain the thinking, as well as what is happening. This circuit has been built on a prototype PCB. Initial tests indicated it would work, but connecting it to the AD4111 yielded different results.

PREG_N connects to the pressure regulator. The amount of current it sinks to effectively report pressure is measured by the voltage drop across R1. The right half of schematic 1 is based on the TI application note and is a voltage-to-current converter. The left half of schematic 1 is a current-to-voltage converter which was using the opamp as a differential amplifier with unity gain. IIN_DP connects to a current input channel of the AD4111.

The ratio between the two voltage dividers (R21-22, R23-24) determines how much current is driven based on the input voltage.

We failed to include the load resistance in the simulation, however. There is 50 Ω load resistance internal to the AD4111, and an additional 180 Ω in series which was used based on the AD4111 evaluation board. With a 5 V source, there can only be a maximum of 250 Ω resistance and still allow 20 mA.

In troubleshooting the physical circuit, we replaced the fuse and 180 Ω resistor with 0 Ω jumpers and removed the TVS diode and capacitor to eliminate variables.

If a 50 Ω load resistor is added in the simulation, we see the same as what we observe with the physical circuit: that for a 10 mA input current, only 8 mA is driven at the output.

Changing the 26.7 Ω resistor to 20 Ω resolves this issue and appears to work for the full range of 4 to 20 mA input in simulation. However, it then becomes dependent on the load resistance, which will vary between devices, and defeats the purpose of the current source.

I'm starting to feel that this solution is the wrong approach. I've tested a variety of different component values in simulation but output current always varies by load resistance.

I'd prefer to tweak this design since we already have some PCBs that use the schematic, but if a completely new circuit is warranted, I'm open to suggestions on what to use. I've been researching current mirrors and current sources, but I don't really know if those are the hammer for this nail.

Can I modify this circuit to get the desired result or should I scrap it and start over?

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1 comment thread

More popups when clicking on the links. No thanks. I probably don't need the datasheet if you just ... (2 comments)
More popups when clicking on the links. No thanks. I probably don't need the datasheet if you just ...
Olin Lathrop‭ wrote 12 months ago

More popups when clicking on the links. No thanks. I probably don't need the datasheet if you just said what these parts are. Are they opamps, A/Ds, D/As, etc?

Also, how does your circuit fit into the larger block diagram? What exactly is driving PREG_N? Is that one of the current sinks you are trying to get the value of? What format of signal do you want as a result? Is that going into a microcontroller, or driving further analog circuitry? If into a micro, then polarity doesn't matter because that can be compensated for in firmware.

Basically give proper spec for your circuit. We can't tell whether it does what it's supposed top without knowing what it's supposed to do.

JYelton‭ wrote 12 months ago

I added direct links to the datasheets. We can probably ignore the AD74413R, which is a DAC that's sourcing 4-20 mA for the control loop. The feedback loop is driven by 5V (left side of schematic 1) through the 200 Ω and out PREG_N where the pressure regulator sinks it. On the other side, IIN1_DP connects to the AD4111, which is an ADC. The goal is to copy the value of the current driving PREG_N and source the same amount of current to IIN1_DP. There is a microcontroller talking to both the DAC and ADC via SPI. I also added a block diagram if that helps.