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VCA810 Automatic Gain Control Circuit

Monse Lozano
Prodigy 80 points
Other Parts Discussed in Thread: VCA810, VCA822, LMH6505, VCA820, VCA821, PGA281, PGA280

Hello,

I'm looking for help in designing an Automatic Gain Control Circuit for slow random wave forms ranging from 0.1Hz to 100Hz. The input signals vary from 50mVpk to 4Vpk. I would like the circuit to recreate these signals normalized to 4.8Vpk.

Figure 33 in the VCA810 datasheet looks like what I'm trying to accomplish.

I have a few questions:

1. Will the circuit shown in the datasheet work for slower signals, in the range I'm looking for. 

2. Do you recommend another part for this application, I've looked at VCA820, VCA821, VCA822, and LMH6505, they all have some AGC circuit in their datasheets, I'm not sure which is best suited for my application.

much thanks!

  • 0
    TI__Genius 15975 points

    Hello Monse,

    With the VCA810, you would be reaching output swing limit at 40dB while normalizing a 50mVpp input signal to 4.8Vpp output.

    This limitation is not present with the VCA820, VCA821, VCA822 and the LMH6505. However, for these parts, the input dynamic range is limited to +1.6V to -2.1V on a +/-5V supply. So, while normalizing a 4Vpp signal to 4.8Vpp output signal, you would end up distorting the output signal. You could stick with adding attenuators at the input of these devices, but might need to amplify these signals using a second stage fixed gain op-amp. 

    I think for your application for signal swing from 50mVp to 4Vp and signal frequency from 0.1Hz to 100Hz, the PGA280 and PGA281 would be better choices.

    Best Regards,

    Rohit

  • 0 in reply to Rohit Bhat
    Prodigy 80 points

    Hi Rohit,

    Thank you for your response. I need the gain to be controlled by a voltage, I don't think the PGA solutions will work for me. I can bring the my 50mV lower input signal requirement higher in order to not come so close to the 40dB limit. My concern is the lower frequencies I'm dealing with, is there an issue with the lower 1Hz - 100Hz frequency range? 

    Thanks!

  • 0 in reply to Monse Lozano
    TI__Genius 15975 points
    Hi Monse,

    I don't see an issue operating these amplifiers in the lower 0.1Hz-100Hz frequency range as they can be dc-coupled at the inputs. In that case, I would recommend any of the VCA82x family amplifiers or the LMH6505. They would provide the necessary output swing of 4.8Vpp at maximum gain. However, you would have to use an attenuator stage before the VCA82x to limit the maximum input signal swing to less than 1Vpp and amplify the signal further to 4.8Vpp in the second stage.

    Best Regards,
    Rohit