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VGA optimized for 14bit

Andy4678
Prodigy 130 points
Other Parts Discussed in Thread: VCA2617

Hi,

I am looking for a VGA that I can use for a 14bit ADC. Some DS (eg. VCA2617) mention "Suitable for 10-Bit and 12-Bit Systems".

What makes this device suitable for 10 or 12bit?

Can I use it for 14bit as well, or is the SNR not sufficient?

Which parameter do I have to look for, to find a better suited device (Output Noise Level?)?

 

Thank you,

 

ND

 

  • 0
    TI__Genius 17905 points

    Hello ND,

    I see this post was recently moved to this forum.

    Yes, noise and SNR performance is the basic metric used for the 10- to 12-bit classification, but the classification makes some assumptions about the signal bandwidth and gain. For example, to do a rough calculation of the output referred noise in VRMS, you can take the input-referred voltage noise of the device (VN), specified in nV/rt(Hz), multiply by the gain of the amplifier (G), and multiply by the square root of the equivalent brickwall noise bandwidth (ENB). In equation form,

    Voutputnoise_RMS = VN*G*sqrt(ENB)

    Let's say you take the VCA2617, which has an input-referred voltage noise of 4.1nV/rt(Hz), use the amplifier in a gain of 20dB=10V/V, and have a 2nd-order post amplifier filter response with -3dB bandwidth of 5MHz. The equivalent output noise will be the following:

    Voutputnoise_RMS = 4.1nV/rt(Hz) * 10V/V * sqrt(5MHz * 1.1) = 96.5uVRMS,

    where I have multiplied the filter cutoff frequency by a conversion factor of 1.1 to get the equivalent noise bandwidth.

    Assuming the signal source noise is negligible compared to the amplifier noise, you can divide the intended output signal by the calculated output noise to get the amplifier output SNR. For example, 2Vpp signal output is 0.707VRMS, resulting in an SNR of S/N = 0.707/0.0000965 = 7326, or 77.3dB given the noise calculated previously.

    Using these calculations, you can back-calculate what amplifier input-referred voltage noise spec is required based on your desired SNR, bandwidth, and gain.

    Some app notes you may find informative are listed below:
    Noise Analysis for High Speed Op Amps - sboa066
    Noise Analysis in Operational Amplifier Circuits - slva043b

    What is your signal bandwidth/signal frequencies, target SNR, and required gain? Any other constraints, such as supply voltage, single-ended or differential input/output?