AD603
Optionally, when a resistor is placed from FDBK to COMM,
higher gains can be achieved. This fourth mode is of limited
value because of the low bandwidth and the elevated output off-
sets; it is thus not included in Figure 2.
The gain of this amplifier in the first two modes is set by the
ratio of on-chip laser-trimmed resistors. While the ratio of these
resistors is very accurate, the absolute value of these resistors
can vary by as much as
±
20%. Thus, when an external resistor
is connected in parallel with the nominal 6.44 kΩ
±
20% inter-
nal resistor, the overall gain accuracy is somewhat poorer. The
worst-case error occurs at about 2 kΩ (see Figure 4).
1.2
1.0
0.8
0.6
DECIBELS
0.4
0.2
0.0
–0.2
–0.4
–0.6
–0.8
80
–1.0
10
100
1k
R
EXT
10k
100k
1M
90
85
VdB (OUT) – VdB (O
REF
)
–1:VdB (OUT) – (–1):VdB (O
REF
)
There are several ways of connecting the gain-control inputs in
cascaded operation. The choice depends on whether it is impor-
tant 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 arrange-
ments for setting the maximum gain.
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 possible.
Figure 5 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
6 shows 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) arc biased
by stable voltages to provide the needed gain-offsets. These volt-
ages may be provided by resistive dividers operating from a
common voltage reference.
S/N RATIO – dB
75
70
65
60
55
50
–0.2
Figure 4. Worst-Case Gain Error, Assuming Internal Resis-
tors Have a Maximum Tolerance of –20% (Top Curve) or
+20% (Bottom Curve)
While the gain-bandwidth product of the fixed-gain amplifier is
about 4 GHz, the actual bandwidth is not exactly related to the
maximum gain. This is because there is a slight enhancing of the
ac response magnitude on the maximum bandwidth range, due
to higher order poles in the open-loop gain function; this mild
peaking is not present on the higher gain ranges. Figure 2 shows
how optional capacitors may be added to extend the frequency
response in high gain modes.
CASCADING TWO AD603S
0.2
0.6
1.0
V
C
1.4
1.8
2.2
Figure 5. SNR vs. Control Voltage—Sequential Control
(1 MHz Bandwidth)
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 overload-
ing 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
(two times 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).
The gains are offset (Figure 7) 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 dam-
age or foldover in the response. This is an important aspect of
the AD603’s gain-control response. (See the Specifications sec-
tion of this data sheet 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,
we assume the maximum-bandwidth connections are used, for
which G
O
is –20 dB.
–6–
REV. C
AD [ ANALOG DEVICES ]
