Datasheet

Data Sheet AD8330
Rev. F | Page 25 of 32
APPLICATIONS INFORMATION
The versatility of the AD8330, its very constant ac response over
a wide range of gains, the large signal dynamic range, output
swing, single supply operation, and low power consumption
commend this VGA to a diverse variety of applications. Only a
few can be described here, including the most basic uses and some
unusual ones.
ADC DRIVING
The AD8330 is well-suited to drive a high speed converter.
There are many high speed converters available, but to illustrate
the general features, the example in this data sheet uses one of
the least expensive, the AD9214. This is available in three
grades for operation at 65 MHz, 80 MHz, and 105 MHz; the
AD9214BRS-80 is a good complement to the general capabili-
ties of this VGA.
Figure 63 shows the connections to drive an ADC. A 3.3 V
supply is used for both parts. The ADC requires that its input
pins be positioned at one third of the supply, or 1.1 V. Given
that the default output level of the VGA is one-half the supply
or 1.65 V, a small correction is introduced by the 8 kΩ resistor
from CNTR to ground. The ADC specifications require that the
common-mode input be within ±0.2 V of the nominal 1.1 V;
variations of up to ±20% in the AD8330 on-chip resistors change
this voltage by only ±70 mV. With the connections shown in
Figure 63, the AD9214 is able to receive an input of 2 V p-p; the
peak output of the AD8330 can be reduced if desired by adding
a resistor
from VMAG to ground. An overrange condition is signaled by a
high state on Pin OR of the AD9214. DFS/GAIN is unconnected
in this example producing an offset-binary output. To provide a
twos complement output, it should be connected to the REF pin.
For ADCs running at sampling rates substantially below the
bandwidth of the AD8330, an intervening noise filter is
recommended to limit the noise bandwidth. A one-pole filter
can easily be created with a single differential capacitor between
the OPHI and OPLO outputs. For a corner frequency of f
C
, the
capacitor should have a value of
C
FILT
= 1/942 f
C
(26)
For example, a 10 MHz corner requires about 100 pF.
SIMPLE AGC AMPLIFIER
Figure 64 illustrates the use of the inverted gain mode and the
offset gain range (0.2 V < V
DBS
< 1.7 V) in supporting a low cost
AGC loop. Q1 is used as a detector. When OPHI is sufficiently
higher than CNTR, due to the signal swing, it conducts and
charges C1. This raises V
DBS
and rapidly lowers the gain. Note
that MODE is grounded (see Figure 48). The minimum voltage
needed across R1 to set up the full gain is 0.2 V because CMGN
is dc open-circuited (this does not alter V
MAG
) and the maxi-
mum voltage is 1.7 V.
COMM
OPHI
INLO
OPLO
INHI
VPSI
VPSO
CMOP
MODE
VDBS CMGN
OFST CNTRENBL VPOS
BIAS AND
V-REF
CM MODE AND
OFFSET CONTROL
OUTPUT
STAGES
OUTPUT
CONTROL
VGA CORE
AD9214BRS-80
DATA OUTPUTS
D0
D1
D2
D3
D4
D5
D6
D7
D8
D9
DGND
CLK
AGND
CLOCK
REF
0.1µF
REFSENSE
NC
ANALOG GROUND
A
IN
A
IN
DFS/GAIN
PWRDN
DIGITAL
GROUND
GAIN BIAS,
V
DBS
, 0V–1.5V
NC
INPUT,
±2V MAX
0.1µF
10Ω
GAIN INTERFACE
CHPF
8kΩ
0.1µF 0.1µF
AV
DD
OR
DrV
DD
3.3Ω
OVER-
RANGE
3.3Ω
0.1µF
V
S
, 3.3V
03217-064
VMAG
Figure 63. Driving an Analog-to-Digital Converter (Preliminary)