Datasheet

Data Sheet AD8338
Rev. A | Page 15 of 20
AGC Circuit, VAGC Pin
The AD8338 includes a current output RMS detector that can
be used to configure the AD8338 as an AGC amplifier (see
Figure 46).
In this configuration, the AGC circuit compares the RMS
output amplitude of the VGA with the desired RMS output
amplitude (voltage at Pin VAGC with respect to the voltage at
Pin VREF), and drives Pin GAIN to minimize their difference.
Therefore, in steady state conditions, the circuit forces the RMS
output amplitude of the AD8338 to be the voltage at Pin VAGC
with respect to the voltage at Pin VREF. Because the AGC
circuit uses negative feedback, the gain slope of the AD8338
needs to be inverted by connecting Pin MODE to ground.
The bandwidth of the AGC circuit is dictated by Capacitor
C
DETO
. Choose this capacitor carefully to ensure that the
bandwidth of the AGC circuit is high enough so that it can
react to fast changes in the input signal’s amplitude, but low
enough to prevent the AGC circuit from distorting the signal
of interest. For example, in an on-off keying (OOK) application
with a carrier frequency of 6.8 MHz and a bit rate of 10 kb/s, it
is recommended to use a 0.01 µF capacitor. This capacitor value
ensures that the gain reacts to the bit energy but does not react
to the carrier signal.
As mentioned previously, the AGC circuit forces the RMS
output amplitude of the AD8338 to be the voltage at Pin VAGC
with respect to the voltage at Pin VREF. Furthermore, the input
to the AGC circuit, Pin VAGC, is symmetrical with respect to
Pin VREF. In other words, the AGC circuit responds to the
absolute value of the difference in voltage between Pin VAGC
and Pin VREF (see Figure 45).
Figure 45. Output RMS Voltage and VGAIN vs. VAGC – VREF
Not all applications require the AGC circuit. Therefore, the
AGC circuit can be disabled by connecting Pin DETO to
ground, and connecting Pin VAGC to Pin VREF.
Internal Reference, Pin VREF
The AD8338 includes an internal 1.5 V voltage reference that is
used to set the quiescent bias voltages of many key nodes in the
VGA. These nodes include inputs pins (INPR, INMR, INPD,
and INMD), output pins (OUTP and OUTM), and feedback
pins (FKBP and FBKM). The output voltage of the internal
reference, Pin VREF, maybe bypassed with a 0.1 µF capacitor
to Pin COMM, but do not force it externally.
Figure 46. AD8338 Configured as an AGC Amplifier
1200
0
200
400
600
800
1000
–1.0 –0.8 –0.6 –0.4 –0.2 0 0.2 0.4 0.6 0.8 1.0
OUTPUT AMPLITUDE (mV rms)
VAGC – VREF (V)
11279-147
1200
0
200
400
600
800
1000
V
GAIN
(mV)
INPR
INPD
INMD
INMR
MODE
COMM
GAIN
FBKM
OUTP
OUTM
DETO
VAGC
FBKP
VREF
VGA CORE
0dB TO 80dB
OFFSET NULL
OUTPUT
STAGE
0dB
OFSN
VREF
VBAT
AUTOMATIC
GAIN
CONTROL
GAIN INTERFACE
AD8338
AGC SETPOINT
0.01µF
C
AGC
11279-148