AD603
1
2
3
4
8
7
6
5
Optionally, when a resistor is placed from FDBK to COMM,
VC1
VC2
GPOS VPOS
VPOS
VNEG
AD603
GNEG VOUT
higher gains can be achieved. This fourth mode is of limited
value because of the low bandwidth and the elevated output
offsets; it is thus not included in Figure 30, Figure 31, or
Figure 32.
VOUT
VIN
VINP
VNEG
2.15kΩ
COMM FDBK
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 20ꢀ. Thus, when an external resistor is
connected in parallel with the nominal 6.44 kΩ 20ꢀ internal
resistor, the overall gain accuracy is somewhat poorer. The
worst-case error occurs at about 2 kΩ (see Figure 34).
5.6pF
Figure 31. 0 dB to 40 dB; 30 MHz Bandwidth
1
2
3
4
8
7
6
5
VC1
VC2
GPOS VPOS
VPOS
AD603
GNEG VOUT
VOUT
1.2
VIN
VINP
VNEG
VNEG
18pF
–1:VdB (OUT) – (–1):VdB (O
)
REF
1.0
0.8
COMM FDBK
0.6
0.4
Figure 32. 10 dB to 50 db; 9 MHz to Set Gain
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
52
50
48
46
44
42
40
38
36
34
32
30
–1:VdB (OUT)
VdB (OUT) – VdB (O
)
REF
10
100
1k
10k
100k
1M
VdB (OUT)
R
(Ω)
EXT
–2:VdB (OUT)
Figure 34. Worst-Case Gain Error, Assuming Internal Resistors 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 30,
Figure 31, and Figure 32 show how an optional capacitor may
be added to extend the frequency response in high gain modes.
10
100
1k
10k
100k
1M
R
(Ω)
EXT
Figure 33. Gain vs. REXT, Showing Worst-Case Limits Assuming Internal
Resistors have a Maximum Tolerance of 20%
Rev. G | Page 13 of 20
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