Datasheet

AD8367
Rev. A | Page 13 of 24
RMS DETECTION
The AD8367 contains a square-law detector that senses
the output signal and compares it to a calibrated setpoint of
354 mV rms, which corresponds to a 1 V p-p sine wave. This
setpoint is internally set and cannot be modified to change the
AGC setpoint and the resulting VOUT level without using
additional external components. This is described in the
Modifying the AGC Setpoint section.
Any difference between the output and setpoint generates
a current that is integrated by an external capacitor, C
AGC
,
connected from the DETO pin to ground, to provide an AGC
control voltage. There is also an internal 5 pF capacitor on the
DETO pin.
The resulting voltage is used as an AGC bias. For this
application, the MODE pin is pulled low and the DETO pin is
tied to the GAIN pin. The output signal level is then regulated
to 354 mV rms. The AGC bias represents a calibrated rms
measure of the received signal strength (RSSI). Since in AGC
mode the output signal is forced to the 354 mV rms setpoint
(−9.02 dBV rms), Equation 2 can be recast to express the
strength of the received signal, V
IN-RMS
, in terms of the AGC
bias V
DETO
.
V
IN − RMS
(dBV rms) = 54.02 + 50 × V
DETO
(4)
where −54.02 dBV rms = −45 dB − 9.02 dBV rms.
For small changes in input signal level, V
DETO
responds with a
characteristic single-pole time constant, τ
AGC
, which is
proportional to C
AGC
.
τ
AGC
(μs) = 10 × C
AGC
(nf) (5)
where the internal 5 pF capacitor is lumped with the external
capacitor to give C
AGC
.