AD8367S datasheet(17/19 Pages) AD

17 / 19 page

Figure 10. Basic connections for AGC operation.

A valuable feature of using a square law detector is that the RSSI voltage is a true reflection of signal power and can

be converted to an absolute power measurement for any given source impedance. The AD8367S can thus be

employed as a true-power meter, or decibel-reading ac voltmeter, as distinct from its basic amplifier function.

The AGC mode of operation requires that the correct gain direction is chosen. Specifically, the gain must fall as VAGC

increases to restore the needed balance against the setpoint. Therefore, the MODE pin must be pulled LO. This

accurate leveling function is shown in Figure 11, where the rms output is held to within 0.1 dB of the setpoint for >35

dB range of input levels.

The dynamics of this loop are controlled by CAGC acting in conjunction with an on-chip equivalent resistance, RAGC, of

10 kΩ which form an effective time-constant TAGC = RAGC x CAGC. The loop thus operates as a single-pole system with

a loop bandwidth of 1/(2πTAGC). Because the gain control function is linear in decibels, this bandwidth is independent

of the absolute signal level. Figure 12 illustrates the loop dynamics for a 30 dB change in input signal level with CAGC =

100 pF.

Figure 11: Leveling Accuracy of the AGC function.

AD8367S Datasheet(PDF) 17 Page - Analog Devices