DAC161S997: Inquiry on Loop and Ground Configuration in the Isolated Power Supply Design for DAC161S997

Hyejun,

I'm not sure I understand the need for the isolation between the loop and the DAC161S997 but I think it should be ok. In the transmitter, I generally see the device operating at the same voltage as the loop, without the need of an isolated DC DC converter. In your schematic, this is the ME124S-24E, and I would not think this is necessary. I'm not sure if there would be issues measuring the loop current because of the DC DC conversion.

Other than that, I think that +VOUT of the ME124S-24E would be the LOOP+ node that sources the loop current going to node A of SD4. Then -VOUT of the ME124S-24E would be the LOOP- node going to node C of SD5.

Joseph Wu

Joseph Wu,

Thank you for your response.

My intention was to generate the loop power directly from my board so that an external loop power supply would not be required. I was concerned that using the main power supply (EXT_24V), which is also used for other circuits on the board, as both the loop power and the power supply for the DAC might cause issues with loop operation. For this reason, I wanted to isolate the power used for the loop and the DAC from the main power supply.

However, I don’t fully understand the details regarding the connection of the LOOP node. I assumed that the terminal connected to the external resistor would ultimately be -VOUT of the ME124S-24E and the output of the DAC.

Is there something I am missing? I would appreciate your help.

Hyejun,

I wasn't able to look at this question today. I might have a chance to look at this later tonight, but for now, expect an answer tomorrow.

Joseph Wu

Hyejun,

I'm still not sure how you intend on connecting this device. I generally see this device set up like a field transmitter (2-wire). It's basically like the diagram that you see in the data sheet:

With the 2-wire transmitter you have a loop with a power supply and a transmitter that runs off of the loop power. There's a sense resistor near the supply that you would use to measure the loop current. This transmitter is remote and the DAC161S997 controls the current pulled through the loop. There's only the LOOP+ and the LOOP- and the DAC161S997 needs a variable ground for the COMA/COMD line. The COMA/COMD is not isolated from LOOP-, but needs to be able to float with respect to each other. This is important for the control of the current in the loop.

In your case, you have a main power supply and it sets the power in the loop. If your circuit uses HART, I've seen DC-DC converters used for generating the supply for some controller used for programming a HART-enabled transmitter outside the normal loop.

Regardless, I'm still unsure what you're trying to create. Can you please put up a basic block diagram of your circuit, and what you want to do?

Joseph Wu

Hello Joseph Wu,

I hope this message finds you well. I have drawn a block diagram to illustrate my intended design. The original DAC chip was meant for a 2-wire setup, where the loop power supply is located externally. However, what I want to do is use a DC-DC converter on my own board to create an independent power source and supply the loop power directly, so that external devices (such as PLCs) only need a resistor and a receiver circuit to read the current—no additional power supply required. Is this what you would call a 4-wire design?

Under this approach, the negative output (-Vout) of the DC-DC converter is routed externally through the LOOP+ terminal, while the DAC’s OUT pin serves as LOOP-. This line connects through the external resistor (for example, 250 Ω) in parallel at the external device, then returns via the loop wiring, passes through the DAC’s internal 40 Ω resistor, and finally arrives at COMA and COMD. In other words, the connection sequence is:

DC-DC (-Vout) → external resistor (e.g., 250 Ω) → DAC OUT → internal 40 Ω resistor → COMA/COMD

The key idea is that the loop is fully powered from my board, eliminating the need for an external power supply. All the external device needs is a resistor and a receiver (ADC) to measure the current.

I look forward to hearing your thoughts or any feedback you might have.