AD8330 Analog Devices, AD8330 Datasheet - Page 15

• Manufacturer Part Number: AD8330
• Description: Low Cost DC-150MHz Variable Gain Amplifier
• Manufacturer: Analog Devices

Gain Magnitude Control (V
In addition to the basic linear-in-dB control, two more gain-
control features are provided. The voltage applied to pin VMAG
provides accurate linear-in-magnitude gain control with a very
rapid response. The bandwidth of this interface is >100 MHz.
When this pin is unconnected, V
of 500 mV (see Figure 4) to set up the basic 0 dB–50 dB range.
But any voltage from ~15 mV to 5 V may be applied to either
lower the gain by up to 30 dB or to raise it by 20 dB. The com-
bined gain span is thus 100 dB, that is, the 50 dB Basic Gain
span provided by V
provided by V
G
Using this to calculate the output voltage, we can write:
from which it is apparent that the AD8330 implements a linear,
two-quadrant multiplier with a bipolar V
Since the AD8330 is a dc-coupled system (the management of
dc offsets at high gains is discussed later), it may be used in many
applications where a wideband two-quadrant multiplier function
is required, from dc up to about 100 MHz from either input
(V
As V
point where this is limited by the absolute output limit imposed
by the supply voltage. In the absence of the latter effect, the
peak output into an open circuited load is just:
while for a load resistance of R
These capabilities are illustrated, first in Figure 6, where
V
+2.5 VDC and V
Except for the last value of V
tion 5; this exceeds the supply-limited value when V
and the peak output is 5.65 V, that is, 6 V–0.35 V. Figure 7
demonstrates the high speed multiplication capability. The
signal input is a 100 MHz, 0.1 V sine wave, V
and V
1 V. The output is ideally a sine wave switching in amplitude
between 0.5 V and 2 V.
REV. A
S
BN
IN
= 6 V, R
V
V
V
to generate a total gain, expressed here in magnitude terms:
G
MAG
or V
Figure 6. Effect of V
MAG
OUT
OUT_PK
OUT_PK
T
–2
–4
–6
–8
MAG
is varied, so also is the peak output magnitude, up to a
8
6
0
4
2
–3
is a square wave at 5 MHz alternating from 0.25 V to
G
L
BN
2G
).
MAG
= O/C, V
0 5 .
V
BN
MAG
–2
. The latter modifies the basic numerical gain
MAG
2V
R
2
DBS
V V
L
V
V
IN
MAG
MAG
was set to 0.25 V, 0.5 V, 1 V, and 2 V.
plus a 60 dB linear-in-magnitude span
150
DBS
MAG
–1
R
MAG
L
L
= 0 V, V
MAG
MAG
directly across OPHI and OPLO, it is:
V
on Gain and Peak Output
MAG
)
MAG
, the peak output follows Equa-
0
– V
IN
assumes its default value
IN
was swept from –2.5 to
and a unipolar V
1
DBS
V
MAG
2
is set to 0.6 V,
0.25V
= 2V
0.5V
1V
MAG
= 2 V
3
MAG
(3)
(4)
.
(5)
(6)
–15–
Another gain-related feature allows both of the gain control ranges
to be accurately raised by 200 mV. To enable this offset, open-
circuit Pin 6 (CMGN) and add a 0.1 F capacitor to ground. In
use, the nominal range for V
and V
supply voltage. This allows the use of DACs whose output range
does not include ground, as sources for the gain control function(s).
Note that the 200 mV that appears on this pin will affect the
response to an externally-applied V
unconnected, the internally set default value of 0.5 V still applies.
Furthermore, the pin CMGN can, if desired, be driven by a user
supplied voltage to reposition the baseline for V
externally applied V
all cases, the gain scaling, its law conformance, and temperature
stability are unaffected.
Two Classes of Variable-Gain Amplifiers
It may be noted at this point that there are two broad classes of
VGA. The first type is designed to cope with a very wide range of
input amplitudes and, by virtue of its gain control function, com-
press this range down to an essentially constant output. This is
the function needed in an AGC system. Such a VGA is called an
IVGA, referring to a structure optimized to address a wide range
of input amplitudes. By contrast, an OVGA is optimized to deliver
a wide range of output values while operating with an essentially
constant input amplitude. This is the function that might be needed,
for example, in providing a variable drive to a power amplifier.
It will be apparent from the foregoing that the AD8330 is both an
IVGA and an OVGA in the one package. This is an unusual and
possibly confusing degree of versatility for a VGA; consequently we
will generally discuss these two distinct control functions at separate
points throughout this data sheet, in explaining the operation and
applications of this product. It is nevertheless useful to briefly
demonstrate the capabilities of these features when used together.
MAG
–0.05
–0.05
–0.10
0.10
0.05
–0.5
–1.0
–1.5
–2.0
1.2
1.0
0.8
0.6
0.4
0.2
2.5
2.0
1.5
1.0
0.5
Figure 7. Using VMAG in Modulation Mode
–400
0
0
from 0.2 V to 5.2 V. These specifications apply for any
–300
MAG
–200
) to any other voltage up to 500 mV. In
DBS
–100
TIME – ns
V
now extends from 0.2 V to 1.7 V
V
V
MAG
OUT
IN
MAG
0
, but when pin VMAG is
100
AD8330
DBS
200
(or for an
300