Precision Rectifier Problem

[garg29]

hi,
I am trying to rectify a AC voltage ranging from 0-3 volts using a precision rectifier.

As shown in the diagram the final output is full wave rectified signal i want to know how can I feed this signal directly to ADC I mean the voltage is going up & Down in the curve. For proper reading I think we need pure DC, but , in this case the output is full wave, how will be the value (that will be displayed) be stable?

If I use a resisor & condensor (100 ohm + 1uF) on output the change in voltage is reflected after a long time. How can I use this precisiion rectifier ? I'm using PIC microcontoller 16F877 to display the voltage on lcd & It's ADC to read the analog input .

Please help me out.

Thanks.With best Regards,
Garg

 

[WimRFP]

Hello,

Before adding any HW, you should ask the question what you want to know from this waveform (amplitude [peak value], average, RMS) and what are the limits of the signal (amplitude, lowest value, frequency, distortion, etc).

When the signal is always sinusoidal, you can add a peak hold circuit with reset so you have a valid reading within 1 period. Note that a short noise spike will trigger your peak hold circuit also. As you might know, RMS value for pure sinusoidal signals is amplitude*sqrt(0.5)

When you want to determine more properties, and the frequency of the AC signal can be handled by your ADC (with processor), you might do average or RMS in the digital domain. When you determine T, you can make the averaging time equal to a (multiple of) periods, this will give you a fast result.

When you use an averaging scheme with exponential decay (like the analog circuit, or digitally), you need to observe many periods before having a stable output (as you mentioned).

When you don't have processing power to do this, you may built a sample and hold circuit that triggers itself. This is used frequently when the frequency of the signal is known or limited to a narrow range.