Converting 1 - 5V signal to 4 - 20mA signal

Question

reference: https://www.analog.com/media/en/technical-documentation/technical-articles/D61_EN-Convert.pdf

I am trying to convert a 1-5V signal to a 4-20mA signal using this circuit but it was not successful, using the exact components mentioned in the schematic. When I was testing the circuit, I kept getting 46mA on the output no matter what the input voltage was.

The output of the first op amp would keep giving me 2.5mV, regardless of the input voltage. Can anyone help me with this?

I had to change M1 (not in my database), but it works very well (simulated). — Antonio51, Jun 28 '22 at 09:24
You have probably swapped the Drain and Source of M1. Found ~ 60 mA. Or M2 ... found 46 mA — Antonio51, Jun 28 '22 at 09:39

score 3 · Answer 1 · answered Jun 28 '22 at 11:38

Output of the first op-amp is railed at the negative supply so it is likely getting imbalance on the inputs with inverting input higher than non-inverting.

So likely either M2 has drain-source swapped or it is shorted (or there is a problem with the op-amp).

NDS7002A has the standard pinout for MOSFETs in SOT23, however, curiously, the TO-92 version has the opposite (drain/source swapped) pinout of the (otherwise very similar) 2N7000, so that is a possible source of error.

Thank you, the pins swapped was the problem. – Karl Jun 29 '22 at 01:00 — Karl, Jun 29 '22 at 01:00

score 2 · Answer 2 · answered Jun 28 '22 at 08:23

Debug your circuit logically.

Check that you have the correct op-amp.
Check your 24 V supply voltage.
Check that the supply is reaching the op-amp pins.
The left op-amp will be balanced when V_- = V₊. That will happen when V_R1 = V_IN. Apply a known voltage to V_IN and check that you get the same voltage across R1. If you fail this test then the problem is in this area.
Note that 1 to 5 V across R1 will cause a current of 0.1 mA to 0.5 mA to flow through R2, M2, R1. Check that the voltage across R2 is the same as R1 since they're the same value, V_R2 = V_R1.

Report back in your question. (Answer all the points.)

Thank you for your time typing up this checklist. This can be the checklist for my future problem solving. :) — Karl, Jun 29 '22 at 01:04

Antonio51 · Accepted Answer · 2022-06-28T11:20:23.220

2

I simulated this circuit with the idea of a "swapping" of drain and source of the two MOSFETs.

Found that swapping D & S of your M2 leads to a current of ~ 46 mA.
Swapping D & S of your M1 lead to a current of ~ 60 mA.

edited Jun 28 '22 at 11:20

answered Jun 28 '22 at 09:59

Antonio51

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Thank you for the simulation. very clear as to where the problem lays. – Karl Jun 29 '22 at 01:01

Converting 1 - 5V signal to 4 - 20mA signal

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