AC current sensing (rectify, amplify + levelshift)

[chrisw1990]

yes sorry the schematic was wrong, and when i put it on pcb, that was all corrected.. for some reason i hadnt connected the power together for the hall sensor and the trannie and opamp. nor outputted the signal.
my direction towards that thread was for removing the dc offset. which was the use of the transistor in the middle.
sorry if you think i am over my head. trying to help a colleague was my aim, and in my eyes i did that by pointing to the thread.

[Col]

Implementing the simpler circuit... I have a rectified output signal which is offset by a reference of 2.5V. MY microcontroller has a 3.3V supply, (my ADC reference voltage). This is leaving me with a small 0.8V effective signal range... just too little in my opinion.

Ron... is there a way to remove the 2.5V offset from my DC signal? I never really considered I could use opamps for DC signals. Again, putting in negative supply voltage is not an option

[Ron H]

Originally Posted by Col

Implementing the simpler circuit... I have a rectified output signal which is offset by a reference of 2.5V. MY microcontroller has a 3.3V supply, (my ADC reference voltage). This is leaving me with a small 0.8V effective signal range... just too little in my opinion.

Ron... is there a way to remove the 2.5V offset from my DC signal? I never really considered I could use opamps for DC signals. Again, putting in negative supply voltage is not an option

Are you wanting to digitize the instantaneous half-wave rectified output, or do you want a precision peak rectifier, where the peak value is converted to DC (with a little ripple)?

[Col]

I only want to measure a DC value representing my current. No need to digitise the 50Hz

[Col]

Is it really bad to just let the amplifier saturate?... since its only 50Hz I dont imagine it would be much affected by the saturated amps recovery time

[Ron H]

Here is a circuit that works well in simulation. You should be able to use any op amp whose input and output can go to the negative rail (GND). On most such amps, the input must never exceed -0.3V, although LMV321 is rated at -0.5V max.
I set the gain at 100. You can adjust it as required. Keep in mind that the op amp's input offset voltage will be amplified by the gain. You can't put a cap in series with R2 to reduce the DC gain to 1, because the cap would charge up to the point where the circuit doesn't work.
You can disconnect the input and replace it with a short, then measure the zero-signal output offset voltage.
Keep in mind that the output time constant is about 100 seconds, so sudden large decreases in the input signal will take a long time to show up on the output. You can reduce the output cap at the cost of more ripple.

EDIT: You can do this with a gain of 50 preamp with AC coupling, which will reduce the gain requirement for the rectifier, thereby reducing the output offset. It would require a dual op amp (same approximate footprint), and two more resistors and a 1uF capacitor. Let me know if you're interested.

[Col]

You've been quite helpful,
Thanks Ron