AD8367S datasheet(16/19 Pages) AD

16 / 19 page

Figure 8. Basic connections for Voltage Controlled Gain mode.

Figure 9. Gain vs. Frequency for Multiple Values of HPFL Capacitor ag VGAIN = 500mV.

The AD8367S can be used as an AGC amplifier, as shown in Figure 10. For this application, the accurate internal,

square-law detector is employed. The output of this detector is a current that varies in polarity, depending on whether

the rms value of the output is greater to or less than its internally-determined setpoint of 354 mVrms. This is 1 Vp-p for

sine-wave signals, but the peak amplitude for other signals, such as Gaussian noise, or those carrying complex

modulation, is invariably somewhat greater. However, for all waveforms having a crest factor of <5, and when using a

supply voltage of 4.5 V to 5.25V, the rms value is correctly measured and delivered at VOUT. When using lower

supplies, the rms value of VOUT is unaffected (the setpoint is determined by a band gap reference), but the peak crest

factor capacity is reduced.

The GAIN pin is connected to the base of a transistor internally and thus requires less than 2 uA of current drive. The

output of the detector is delivered to the DETO pin. The detector can source up to 60uA and can sink up to 11 uA. For

a sine-wave output signal, and under conditions where the AGC loop is settled, the detector output also takes the

form of a sine-wave, but at twice the frequency and having a mean value of 0. If the input to the amplifier increases,

the mean of this current also increases and charges the external loop filter capacitor, CAGC, toward more positive

voltages. Conversely, a reduction in VOUT below the setpoint of 354 mVrms causes this voltage to fall toward ground.

The capacitor voltage is the AGC bias; this can be used as a received signal strength indicator (RSSI) output and is

scaled exactly as VGAIN, that is, 20 mV/dB.

AD8367S Datasheet(PDF) 16 Page - Analog Devices