user-1: I am trying to control the speed of motor through VFD using 4-20mA signal.
I generate the 4-20mA signal using Arduino.I use low pas filter to obtain the voltages from Arduino PWM pins as shown in the circuit.
Their is continuous deviation at the output.But the deviation is very small.
Example: 
when Input is 1V(from 20% PWM in Arduino Code)--> Then output continuous to vary between 3mA to 4mA.
when Input is 5V(from 100% PWM in Arduino Code)--> Then output continuous to vary  between 19mA to 20mA
Can I implement this circuit practically with VFD?
Will this deviation create problem for VFD to control the speed?
Will this very fast continuous deviation is dangerous for

user2967: The sawtooth pattern you're showing looks like what I would expect from incomplete filtering of the PWM signal from the Arduino. 
I don't know if the variation is going to affect your motor controller or not. 
I suggest using a different low pass filter with a much lower pass frequency. That will slow down the response to changes in speed slightly, but produce smoother output. I'd suggest lowering the pass frequency of your filter by 10X or more.
(The good news is that you want to filter the hell out of your PWM signal. You want it to only pass like 10Hz or so. Using a very low cut-off frequency for your filter will get rid of that sawtooth. The result of that would be an output voltage that would be slower to respond, but that should be fine. If your motor takes 1/10th of a second to adjust speeds, is that a problem in your application?)

-1: <p>I am trying to control the speed of motor through VFD using 4-20mA signal.</p>

<p>I generate the 4-20mA signal using Arduino.I use low pas filter to obtain the voltages from Arduino PWM pins as shown in the circuit.</p>

<p>Their is continuous deviation at the output.But the deviation is very small.</p>

<p>Example: 
when Input is 1V(from 20% PWM in Arduino Code)--> Then output continuous to vary between 3mA to 4mA.</p>

<p>when Input is 5V(from 100% PWM in Arduino Code)--> Then output continuous to vary  between 19mA to 20mA</p>

<p>Can I implement this circuit practically with VFD?</p>

<p>Will this deviation create problem for VFD to control the speed?</p>

<p>Will this very fast continuous deviation is dangerous for</p>
 2967: <p>The sawtooth pattern you're showing looks like what I would expect from incomplete filtering of the PWM signal from the Arduino. </p>

<p>I don't know if the variation is going to affect your motor controller or not. </p>

<p>I suggest using a different low pass filter with a much lower pass frequency. That will slow down the response to changes in speed slightly, but produce smoother output. I'd suggest lowering the pass frequency of your filter by 10X or more.</p>

<p>(The good news is that you want to filter the hell out of your PWM signal. You want it to only pass like 10Hz or so. Using a very low cut-off frequency for your filter will get rid of that sawtooth. The result of that would be an output voltage that would be slower to respond, but that should be fine. If your motor takes 1/10th of a second to adjust speeds, is that a problem in your application?)</p>
