Advice on Implementing a Precision Full Wave Rectifier

Hi - the attached AD paper describes building a precision full wave rectifier with a difference amp such as the AD8277.  I like this solution as it does not have the diodes typical of such a configuration which can lead to anomalies in the output, which this application cannot tolerate.  I have three questions regarding this:

1. There are no starting or recommended values for the resistors in Figure 3.  Can anyone provide me with same?

2. The difference amp allows me to implement an output offset using the Ref pin, which is important in this application (about 100mV positive for zero input).  Will using the Ref input cause any anomalous behavior in this application?

3. The input to this circuit comes off a divider network across an AC line, so by definition the source impedance is a few hundred K which will result in a high impedance input.  Will this setup tolerate such or do I need to buffer the input between the AC input dividers and the full wave rectifier circuit?

You can probably tell I am not an analog expert, and hence looking for some guidance from the community.  Thanks.

ADFullWavePrecisionRecifierPaper.pdf
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  • 0
    •  Analog Employees 
    on Jun 28, 2016 6:41 PM

    Hi David,

    Sorry if we missed this thread. Please see below for my answers.

    1. The AD8277 is a difference amplifier with four on-chip resistors that are laser trimmed for excellent gain accuracy and CMRR. The "R": in figure 3 is the on-chip resistor (40kohms) of the AD8277. Therefore if you want to build the same circuit with figure 3, there is no additional external resistor.

    2. I believe that the reference pin in figure 3 is tied up with +IN and therefore you can't use this for injecting offset voltage. Offset voltage should be implemented after the AD8277 output (A2). Note that in order this circuit to work properly, you should use only a single positive supply for the amplifiers and input signal should be centered to 0V.

    3. You should used buffer since any impedance on the input could easily degrade the gain accuracy and CMRR of the device especially on your suggested setup condition.

    Let me know if you need anything else.

    Best regareds,

    Emman

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  • 0
    •  Analog Employees 
    on Jun 28, 2016 6:41 PM

    Hi David,

    Sorry if we missed this thread. Please see below for my answers.

    1. The AD8277 is a difference amplifier with four on-chip resistors that are laser trimmed for excellent gain accuracy and CMRR. The "R": in figure 3 is the on-chip resistor (40kohms) of the AD8277. Therefore if you want to build the same circuit with figure 3, there is no additional external resistor.

    2. I believe that the reference pin in figure 3 is tied up with +IN and therefore you can't use this for injecting offset voltage. Offset voltage should be implemented after the AD8277 output (A2). Note that in order this circuit to work properly, you should use only a single positive supply for the amplifiers and input signal should be centered to 0V.

    3. You should used buffer since any impedance on the input could easily degrade the gain accuracy and CMRR of the device especially on your suggested setup condition.

    Let me know if you need anything else.

    Best regareds,

    Emman

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