Hi all.

I've a general question regarding the output of a filter. I've a circuit that outputs an AC voltage which I need to apply a long pass filter to and then read into an ADC.

The ADC reads values from 0-5V, but the signal swings both positive and negative. What is the best way to filter this signal so I only get positive voltages?

I've looked at a few active filters (eg Sallen-Key) and considered just running the -Vcc rail of the op-amp to ground, which clips the output to positive. However, in the modelling I've done this appears to change the overall response of the filter and the output becomes attenuated. Ideally I'd like to be a unity gain response. This is also a portable device so I need to be as economic as possible with power draw.

Any suggestions would be hugely appreciated. Thanks in advance.

Stuart

 

you can use a diode in forward biased...

 

I've looked at a few active filters (eg Sallen-Key)

Tell us about the Sallen Key. I have books on this one.

 

Do you want the output to be equal to the peak or the average of the AC signal?

If you want the peak, then you can run the signal through a precision peak detector and than a LP filter.

If you want the average, then you can run the signal through a precision full-wave rectifier and than a LP filter.

An easy way to design active filters is the free FilterPro software from TI.

 

To me it sounds like you need a precision rectifieer. This is a good discussion of the topic http://sound.westhost.com/appnotes/an001.htm It may also be an idea to an amplifier before the precision rectifieer. If the input signal is low. Also remember that all cicuits in the link. Will require dual supply voltages for the OPAMPs.

 

I would try a precision rectifier circuit that looks like this--driving the input below common does unpredictable things and it ceases to be high impedance, so buffer the input via a series resistor and limit negative voltage at the input via a shottky diode.

In this way, the filter can do anything is has to do including going negative without causing other problems with your A-D converter.

 

Thanks for all the suggestions people - much appreciated!