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VCA810 Driving with a DAC to get linear output

dan kantorski
Intellectual 910 points
Other Parts Discussed in Thread: VCA810, VCA822, VCA824

I am using the VCA810 for a gain control circuit.  I have it in an AGC loop and it works well.  As another option from my multipurpose board, I wish to drive the control input directly from a DAC and get linear gain which is not possible as the VCA is a logarithmic type gain.  In my prototype I have already scaled and negated the output of the DAC (0->3V to -2->0)  but did not take into account the VCA logarithmic output.  Is there an interface circuit which can take the linear output of the DAC and transform it to get a linear gain control from the output of the VCA?

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    TI__Genius 17905 points

    Hi Dan,

    I do not know of a simple circuit that will readily convert the linear, inverted DAC output into the logarithmic gain control voltage that would be required to get a linear gain out of the VCA810. I think the simplest approach would be to implement the log scaling in your DAC, i.e. map the desired linear gain transfer function to a corresponding logarithmic transfer function in your software driving the DAC. Would that be possible?

    Otherwise, you may want to take a look at the VCA822 and VCA824 (higher bandwidth) which are similar to the VCA810 but not pin compatible. The VCA822/24 have gain control that is linear in V/V but the gain adjust range is only 40dB versus the 80dB of the VCA810.