Tool/software:
Hi TI experts,
We aim to control the output current via the input voltage, as illustrated in the figure.
While the ideal output current should have no DC offset, an offset current is introduced due to the input offset voltage.
We are looking for effective methods to compensate for this offset.
If you have any suggestions or ideas, we would greatly appreciate your input.
Input voltage:
- Sinusoidal
- 0.1Hz~20Hz
- up to 4V
best regards,
Takuya
Hi Takuya-san,
Could you let me know what the minimum current level you wish to output and the maximum offset error allowable? Am I correct in assuming that since your diagram has a 1 ohm resistor, you are wishing to drive a max of 4A at the output? Also, do you know the power supply values you will be using?
As far as voltage offset is concerned, depending on the precision required at the minimum current level you wish to drive, the offset voltage introduced by the op amp might be a very small fraction of the minimum voltage you wish to output, so for some applications offset adjustment isn't necessary. Also, the offset voltage does drift over temperature and power supply voltage, as shown in the "Offset Voltage" section of the "Specifications" table in the datasheet. You can learn more about the different offset error sources via our TI Precision Labs training series on op amps.
This part has a short circuit current limit at 4A, but that can vary over temperature and decrease to less than 4A, so I would advise against using this part if you wish to drive 4V across the 1 ohm load since temperature rise can often be high for high power applications. I would advise using something like the OPA549, where the output current can be set externally, and can support 4A across the entire temperature range if needed.
For adjusting offset voltage, there are a variety of techniques, but the one below is the most simple. The potentiometer essentially works as a variable voltage reference, where the reference voltage can be adjusted between +/-Vs and can be used to cancel out the offset created by the op amp. This must be manually calibrated, so this is a simple option for just initial offset. Below R1 would equal R3 for your case.
If you wish to cancel out any drift, I would go another direction and use a servo loop. This works by integrating the offset seen by the output and "correcting it" via the feedback loop, essentially removing any DC offset. The downside to this method is that you would have to make sure that the RC circuit doesn't attenuate your actual signal, which is pretty close to DC at 0.1Hz.
Best Regards,
Taylor Allan
