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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    • DC output from the detector cells are generated with an averaging cap.
    • We don't have data on VLOG Noise vs. Input Signal level on the AD8306, but in general for log amps of this sort, noise on VLOG decreases as you increase signal level to a certain point (different on each log amp), then does not change as you increase the input signal level.  An example of this can be seen in the ADL5506 datasheet on Figure 31.
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