AD8370 gain modes

Hello

How to calculate the point where AD8370 has the same gain in both low gain mode and high gain mode. I need to switch smoothly between the two modes (for AGC (automatic gain control) construction). In the datasheet there is only a formula for calculating gain in high gain mode.

Thanks

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  • Hello,

    The formula on p. 15 of the datasheet describes the gain difference being 17 dB (i.e.,  ~ 7.08 X) when flipped between the low and high gain ranges (MSB= 0 or 1).  I low gain mode, PreGain = 1, and in high gain mode, PreGain = 7.08 V/V.

    The theoretical linear voltage gain can be expressed with respect to the gain code as:


    AV = GainCode Vernier (1 + (PreGain − 1) MSB) where:

    AV is the linear voltage gain.

    GainCode is the digital gain control word minus the MSB
    (the final 7 bits).

    Vernier = 0.055744 V/V


    PreGain = 7.079458 V/V


    MSB is the most significant bit of the 8-bit gain control word. The MSB sets the device in either high gain mode (MSB = 1)
    or low gain mode (MSB = 0).


    For example, a gain control word of HG45 (or 10101101 binary) results in a theoretical linear voltage gain of 17.76 V/V,
    calculated as:

    45 × 0.055744 × (1 + (7.079458 − 1) × 1)

    The correct equations are :

    High gain mode:

     

    Av = GainCode x Vernier x (1 + (PreGain-1) x MSB) 

     

    Low gain mode:  (PreGain = 0)

     

    Av = GainCode x Vernier x (1 + (1-1) x MSB)

    Thanks,

    Darrell

Reply
  • Hello,

    The formula on p. 15 of the datasheet describes the gain difference being 17 dB (i.e.,  ~ 7.08 X) when flipped between the low and high gain ranges (MSB= 0 or 1).  I low gain mode, PreGain = 1, and in high gain mode, PreGain = 7.08 V/V.

    The theoretical linear voltage gain can be expressed with respect to the gain code as:


    AV = GainCode Vernier (1 + (PreGain − 1) MSB) where:

    AV is the linear voltage gain.

    GainCode is the digital gain control word minus the MSB
    (the final 7 bits).

    Vernier = 0.055744 V/V


    PreGain = 7.079458 V/V


    MSB is the most significant bit of the 8-bit gain control word. The MSB sets the device in either high gain mode (MSB = 1)
    or low gain mode (MSB = 0).


    For example, a gain control word of HG45 (or 10101101 binary) results in a theoretical linear voltage gain of 17.76 V/V,
    calculated as:

    45 × 0.055744 × (1 + (7.079458 − 1) × 1)

    The correct equations are :

    High gain mode:

     

    Av = GainCode x Vernier x (1 + (PreGain-1) x MSB) 

     

    Low gain mode:  (PreGain = 0)

     

    Av = GainCode x Vernier x (1 + (1-1) x MSB)

    Thanks,

    Darrell

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