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I have been working on 4-20mA transmitter project and stuck with a problem. My design is based on CN0319 evaluation board but I changed some of it, to make it works with other part of the design.

Below is my design, I want it to transmit 4-20mA current when the input of op-amp is PWM 0-100% duty cycle. It uses power supply 24V and drop it to 5.7V to supply entired circuit.

4-20mA_design

My problem is when I try to drive 4-20mA output current with 0-500 ohms load resistor. At around 400-500 ohms load, the circuit sense output current as desired. but when load resistor is 0-400 ohms. the current goes a little bit higher than it should be. For example, if I want to drive 4 mA, my circuit can drive 4 mA at around 420-500 ohms. But at 0 ohms, it senses about 4.2mA and it slowly goes down to 4mA when I increase load resistor to 420 ohms.

To drive 4-20mA, input voltage at V- of op-amp must be 0-0.6V. So I measure V- and V+ compared to DGND, V- voltage is constant at any load resistance but V+ is goes lower when load resistance is 0-400 ohms. What am I missing if I want to make the current consistant at load 0-500 ohms.

I measured voltage across Rload using oscilloscope and start to see ripple on it when I decrease the Rload. the more Rload goes low the more ripple voltage goes high. So, I measured voltage at Pin3 and see the ripple. But did not see that on input voltage at pin2. Input voltage at Pin2 is from PWM and pass through the low pass filter to make it DC.

This is the voltage at Pin3 at Rload = 356 Ohms enter image description here

This is the voltage at Pin3 at Rload = 445 Ohms enter image description here

LazyLabor
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  • Is your OPA244 rail to rail on the output? – winny Aug 01 '18 at 07:35
  • @winny Thank you for checking out my question. I'm pretty newbie and my english is not good. so sorry about that. Anyway, I have checked its datasheet, it says output voltage can go from 0.1V to (Vs+) -0.75V (SPECIFICATIONS: VS = +2.6V to +36V). So, I think it is rail-to-rail. – LazyLabor Aug 01 '18 at 07:53
  • What winny is asking, is if the voltage at Pin 6 of the Op-Amp is swinging between 0v and 5V properly. – MIL-SPEC Aug 01 '18 at 08:03
  • 0.1 V is not rail to rail (but almost) and could explain why you are seeing problems at low impedance and thus low output voltage. Can you try with a R2R-advertised one? Could you tolerate a ballast resistor in series? – winny Aug 01 '18 at 08:04
  • Oh, thanks. Let me try to replace with the new one. – LazyLabor Aug 01 '18 at 08:08
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    Have you checked the output with an oscilloscope? – Spehro Pefhany Aug 01 '18 at 08:16
  • @winny Did you mean put R in serie with Rload?. I already tried to put 500 ohms and then I can consistantly drive 4-20mA current with Rload around 0-250Ohms, above that, my circuit is not working properly (i assumed its power is not enought). My target is to reach 500 ohms at 24V supply. – LazyLabor Aug 01 '18 at 08:22
  • The output voltage of the op-amp only has to cover the range from 450 mV to 900 mV relative to ground (look at the base-emitter voltage as a function of collector current [in the BC847 datasheet](https://assets.nexperia.com/documents/data-sheet/BC847_SER.pdf)). It's the *input* voltage range of the op-amp you need to be concerned about, as the input voltage ranges between 0 mV and 600 mV. Your op-amp input voltage range does go all the way down to 0, so it should be fine. – jms Aug 01 '18 at 08:38
  • @SpehroPefhany I checked voltage across Rload as your advice. Please see from the question. – LazyLabor Aug 01 '18 at 09:57
  • Question, are L4 and L5 a common-mode suppression choke, or are they separate inductors? I can't find any information on the part number to say what their inductance is, but I kind of doubt that's the problem anyway. – Hearth Aug 01 '18 at 14:50
  • Looks like you have a stability problem. The frequency looks suspiciously low (aliasing?). C38 could be contributing, try setting your scope to microseconds not ms in the horizontal sweep. – Spehro Pefhany Aug 01 '18 at 15:35
  • @Felthry It is a common-mode chokes whose inductance is 2.2mH. I can't find the datasheet either, I just found it among old parts. I tried removing them out, then I did not see ripple on both Rload and pin 3 of op-amp anymore. So, I think this is the problem. Thanks for pointing it out. – LazyLabor Aug 02 '18 at 04:29

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As Feilthry pointed out my problem might be commom-mode chokes L4 and L5. So, I tried replacing them with new ones (new part is RL822-3R3K-RC), then my problem was solved. It now can drive 4-20mA current consistantly on load resister at 0-500ohms.

Sorry that I can't find a datasheet for KV03405 to compare parameters. I think it's because the new chokes got a highter Self Resonant frequency than last one.

LazyLabor
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