AD8330 Analog Devices, AD8330 Datasheet - Page 13

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

CIRCUIT DESCRIPTION
Many monolithic variable-gain amplifiers use techniques that share
common principles that are broadly classified as translinear, a
term referring to circuit cells whose functions depend directly on
the very predictable properties of bipolar junction transistors,
notably the linear dependence of their transconductance on collec-
tor current. Since the discovery of these cells in 1967, and their
commercial exploitation in products developed during the early 1970s,
accurate wide bandwidth analog multipliers, dividers, and variable-
gain amplifiers have invariably employed translinear principles.
While these techniques are well understood, the realization of a
high performance variable-gain amplifier (VGA) requires special
technologies and attention to many subtle details in its design.
The AD8330 is fabricated on a proprietary silicon-on-insulator,
complementary bipolar IC process and draws on decades of
experience in developing many leading-edge products using trans-
linear principles to provide an unprecedented level of versatility.
Figure 2 shows a basic representative cell comprising just four
transistors. This, or a very closely related form, is at the heart of most
translinear multipliers, dividers, and VGAs. The key concepts
are as follows: First, the ratio of the currents in the left-hand and
right-hand pairs of transistors are identical; this is represented
by the modulation factor, x, which may have values between –1
and +1. Second, the input signal is arranged to modulate the fixed
tail current I
left-hand pair to be replicated in the right-hand pair, and thus
generate the output by modulating its nominally fixed tail current
I
over many decades of variable bias current. In practice, the
realization of the full potential of this circuit involves many other
factors, but these three elementary ideas remain essential.
By varying I
analog multiplier, exhibiting a linear relationship to both the signal
modulation factor x and this numerator current. On the other
hand, by varying I
having a hyperbolic gain function with respect to the input
factor x, controlled by this denominator current. The AD8330
exploits both modes of operation. However, since a hyperbolic
gain function is generally of less value than one in which the
decibel gain is a linear function of a control input, a special interface
is included to provide either increasing or decreasing exponential
control of I
REV. A
N
. Third, the current-gain of this cell is very exactly G = I
(1–x) I
INPUT IS xl
Figure 2. The Basic Core of the AD8330
2
D
N
D
.
D
, the overall function is that of a two-quadrant
Q1
to cause the variable value of x introduced in the
D
D
Q2
, a two-quadrant analog divider is realized,
I
D
(1–x) I
DENOMINATOR
BIAS CURRENT
2
D
G = I
+
N
/I
D
–
LOOP
AMPLIFIER
BIAS CURRENT
NUMERATOR
(1–x) I
2
OUTPUT IS xl
N
Q3
Q4
I
N
(1+x) I
N
2
N
N
/I
D
–13–
Overall Structure
Figure 3 shows a block schematic of the AD8330 in which the key
sections are located. More detailed discussions of its structure
and features are provided later; this figure provides a general
overview of its capabilities.
The VGA core contains a much elaborated version of the cell shown
in Figure 2. The current called I
(linear in decibels) through the decibel gain interface at the pin
VDBS and its local common CMGN. The gain span (that is,
the decibel difference between maximum and minimum values)
provided by this control function is slightly more than 50 dB.
The absolute gain from input to output is a function of source
and load impedance and also depends on the voltage on a second
gain-control pin, VMAG, as will be explained in a moment.
Normal Operating Conditions
To minimize confusion, we define these normal operating condi-
tions: the input pins are voltage driven (the source impedance is
assumed to be zero); the output pins are open circuited (the load
impedance is assumed to be infinite); pin VMAG is unconnected,
which sets up the output bias current (I
cell) to its nominal value; pin CMGN is grounded; and MODE
is either tied to a logic high or left unconnected, to set the UP
gain mode. The effects of other operating conditions can then be
considered separately.
Throughout this data sheet, the end-to-end voltage gain for the
normal operating conditions will be referred to as the Basic
Gain. Under these conditions, it runs from 0 dB when V
(where this voltage is more exactly measured with reference to
pin CMGN, which may not necessarily be tied to ground) up to
50 dB for V
the VDBS pin is driven below ground or above its nominal full-
scale value.
The input is accepted at the differential port INHI/INLO. These
pins are internally biased to roughly the midpoint of the supply
V
= 3 V), but the AD8330 is able to accept a forced common-
mode value, from zero to V
provides good common-mode rejection up to high frequencies
(see TPC 13) and thus can be driven in either a single-sided or
differential manner. However, operation using a differential drive
is preferable, and this is assumed in the specifications, unless
otherwise stated.
S
(it is actually ~2.75 V for V
Figure 3. Block Schematic of the AD8330
DBS
VPSI
INHI
INLO
MODE
ENBL
VDBS
= 1.5 V. The gain does not “fold-over” when
BIAS AND
V
AD8330
VGA CORE
REF
GAIN INTERFACE
OFST
S
CMGN
, with certain limitations. This interface
S
= 5 V, V
CM MODE AND
OFFSET CONTROL
D
VPOS
is controlled exponentially
COMM
OUTPUT
STAGES
N
OUTPUT
CONTROL
in the four-transistor gain
DBS
= 0, and 1.5 V for V
CNTR
VMAG
VPSO
OPHI
OPLO
CMOP
AD8330
DBS
= 0
S