AD8306 Logarithmic Amplifier Noise

Hi,

I'm analyzing the signal to noise ratio of a system which includes an AD8306. I'm using the logarithmic amplifier output VLOG, and I'm interested in the noise contribution og the RSSI path. So basically I'm interested in the SNR as a function of the input voltage.

The noise contribution of the input stage (Inputs INHI, INLO) is stated in the datasheet. So the additive noise contribution for the "linear" path can be calculated straightforward.

To realize the logarithmic behaviour of the RSSI output, the detector cells have to be weighted/limited, leading in a different noise contribution of the single stages (depending on the input voltage). So I'm interested in the wheights of the detector cells. Further the ladder attenuation network at the beginning of the RSSI path and the corresponding four detector cells are not described closer.

In gerneral, I'm interested in the realization of the detector cells itself. According to the datasheet they are realized ba a full-wave detector, but how does this detector looks like (schematic). An ordinary full-wave rectifier (without additional capacitor) would end up in a time varing signal. So how is the DC signal generated.

BR

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  • 0
    •  Analog Employees 
    on Sep 5, 2017 7:52 PM

    Concerning the 23 Aug. 2017 post:

    The detector cell is a proprietary design, but can be thought of as rectifying the limited signal of the 12 dB limiting amplifiers.  The basic reason that the noise at VLOG goes down with input signal level is that as input signal level increase, the gain through the limiting amp chain decreases.  I think that is what you were trying to say in your second bullet point.

    Concerning the 4 Sept. 2017 post: 

    You are mostly observing the part-to-part variation of the intercept.  The specs table lists how much variation you can expect over part-to-part (16 dB) and temperature (18 dB).  You can easily calibrated this variation out with a simple 2-point calibration.

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  • 0
    •  Analog Employees 
    on Sep 5, 2017 7:52 PM

    Concerning the 23 Aug. 2017 post:

    The detector cell is a proprietary design, but can be thought of as rectifying the limited signal of the 12 dB limiting amplifiers.  The basic reason that the noise at VLOG goes down with input signal level is that as input signal level increase, the gain through the limiting amp chain decreases.  I think that is what you were trying to say in your second bullet point.

    Concerning the 4 Sept. 2017 post: 

    You are mostly observing the part-to-part variation of the intercept.  The specs table lists how much variation you can expect over part-to-part (16 dB) and temperature (18 dB).  You can easily calibrated this variation out with a simple 2-point calibration.

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