AD603
USING THE AD603 IN CASCADE
Two or more AD603s can be connected in series to achieve
higher gain. Invariably, ac coupling must be used to prevent the
dc offset voltage at the output of each amplifier from
overloading the following amplifier at maximum gain. The
required high-pass coupling network will usually be just a
capacitor, chosen to set the desired corner frequency in
conjunction with the well-defined 100 Ω input resistance of the
following amplifier.
For two AD603s, the total gain control range becomes 84 dB
(2 × 42.14 dB); the overall −3 dB bandwidth of cascaded stages
will be somewhat reduced. Depending on the pin strapping, the
gain and bandwidth for two cascaded amplifiers can range from
−22 dB to +62 dB (with a bandwidth of about 70 MHz) to
+22 dB to +102 dB (with a bandwidth of about 6 MHz).
There are several ways of connecting the gain control inputs in
cascaded operation. The choice depends on whether it is
important to achieve the highest possible instantaneous signal-
to-noise ratio (ISNR), or, alternatively, to minimize the ripple in
the gain error. The following examples feature the AD603
programmed for maximum bandwidth; the explanations apply
to other gain/bandwidth combinations with appropriate
changes to the arrangements for setting the maximum gain.
be provided by resistive dividers operating from a common
voltage reference.
90
85
80
S/N RATIO (dB)
75
70
65
60
55
00539-035
50
–0.2
0.2
0.6
1.0
V
C
(V)
1.4
1.8
2.2
Figure 35. SNR vs. Control Voltage–Sequential Control (1 MHz Bandwidth)
SEQUENTIAL MODE (OPTIMAL S/N RATIO)
In the sequential mode of operation, the ISNR is maintained at
its highest level for as much of the gain control range as
possible. Figure 35 shows the SNR over a gain range of −22 dB
to +62 dB, assuming an output of 1 V rms and a 1 MHz
bandwidth; Figure 36, Figure 37, and Figure 38 show the general
connections to accomplish this. Here, both the positive gain
control inputs (GPOS) are driven in parallel by a positive-only,
ground-referenced source with a range of 0 V to +2 V, while the
negative gain control inputs (GNEG) are biased by stable
voltages to provide the needed gain offsets. These voltages may
A1
–40.00dB
The gains are offset (Figure 39) such that A2’s gain is increased
only after A1’s gain has reached its maximum value. Note that
for a differential input of –600 mV or less, the gain of a single
amplifier (A1 or A2) will be at its minimum value of −11.07 dB;
for a differential input of +600 mV or more, the gain will be at
its maximum value of 31.07 dB. Control inputs beyond these
limits will not affect the gain and can be tolerated without
damage or foldover in the response. This is an important aspect
of the AD603’s gain control response. (See the Specifications
section for more details on the allowable voltage range.) The
gain is now
Gain
(dB) = 40
V
G
+
G
O
(3)
where V
G
is the applied control voltage and G
O
is determined by
the gain range chosen. In the explanatory notes that follow, it is
assumed the maximum bandwidth connections are used, for
which G
O
is −20 dB.
A2
–51.07dB
INPUT
0dB
–42.14dB
GPOS
V
G1
GNEG
–8.93dB
31.07dB
–42.14dB
GPOS
V
G2
GNEG
31.07dB
V
C
= 0V
V
O1
= 0.473V
V
O2
= 1.526V
Figure 36. AD603 Gain Control Input Calculations for Sequential Control Operation V
C
= 0 V
0dB
–11.07dB
INPUT
0dB
0dB
GPOS
V
G1
GNEG
31.07dB
31.07dB
–42.14dB
GPOS
V
G2
GNEG
31.07dB
V
C
= 1.0V
V
O1
= 0.473V
V
O2
= 1.526V
Figure 37. AD603 Gain Control Calculations for Sequential Control Operation V
C
= 1.0 V
Rev. G | Page 14 of 20
00539-037
OUTPUT
20dB
00539-036
OUTPUT
–20dB
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