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Rectifier Problem, Analog to Digital Converter Please Help! Kind of Urgent.

Discussion in 'General Electronics Discussion' started by Nevermoed, Jun 1, 2012.

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  1. Nevermoed


    Jun 1, 2012

    I'm using the ADC on an ATtiny24 to record a AC signal. The ADC has a range from 0-3.3V. I'm using a precision rectifier to get rid of the negative portion of the signal. I've attached a picture of the circuit I'm using for the rectifier. I'm using an LMC6484 as the op-amp with 3.3V to the V+ rail and 0 to the V- rail. I'm using 1N4004's as the diodes and 10k for the resistors.

    Image.jpg shows a signal on an oscope after being sent through the data logger. Rect.jpg shows how my logger interprets the signal. It follows the function generator pretty well. BUT the oscope shows that the signal varies from -0.5 to 1V. But my micro shows a signal with the same waveform that goes from 0-1.5V! If I replace the precision rectifier with a simple diode/resistor combination, I get a waveform that varies from -0.X volts to Y volts (depending on the input through the function generator). The micro logs the waveform correctly but the voltage range is from 0.X to Y volts.

    I always have a little bit of a negative signal even with the precision rectifier and when I graph the results through my logger, it appears that the largest negative value has been moved up to zero, so I seem to get a larger range than the input.

    Is this a result of putting in negative voltages into my ADC? Does it take the negative voltages, set that as 'ground' with respect to other voltages?

    I've input positive voltages into the logger through a power supply, and it reads those voltages perfectly!

    All my grounds are connected together, so I know that isn't a problem. Honestly, I'm at my wit's end! :confused::confused:

    Attached Files:

  2. duke37


    Jan 9, 2011
    The rectifier is an inverting circuit. With two equal resistors, it will output the same as the input voltage but with the polarity inverted.

    When the amp is supplied with 3.3V, it can manage an output of 3.3 - 0.6 = 2.7V so the measuring input range is 0V to -2.7V.

    1N4004 diodes are a bit over the top, they are slow and have considerable capacitance which may be significant depending on your input frequency. It would be better to use 1N4148.
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