AD603 Analog Devices, Inc., AD603 Datasheet - Page 16

• Manufacturer Part Number: AD603
• Description: Low Noise, 90 Mhz Variable-gain Amplifier
• Manufacturer: Analog Devices, Inc.

AD603
PARALLEL MODE (SIMPLEST GAIN CONTROL
INTERFACE)
In this mode, the gain control of voltage is applied to both
inputs in parallel: the GPOS pins of both A1 and A2 are
connected to the control voltage and the GNEW inputs are
grounded. The gain scaling is then doubled to 80 dB/V,
requiring only a 1.00 V change for an 80 dB change of gain
where, as before, G
in the maximum bandwidth mode, G
the GNEG pins may be connected to an offset voltage of
0.500 V, in which case G
The amplitude of the gain ripple in this case is also doubled, as
shown in Figure 43, while the ISNR at the output of A2 now
decreases linearly as the gain increases, as shown in Figure 44.
–0.5
–1.0
–1.5
–2.0
Gain = (dB) = 80 V
2.0
1.5
1.0
0.5
90
85
80
75
70
65
60
55
50
0
–0.2
–0.2
Figure 43. Gain Error for Cascaded Stages—Parallel Control
Figure 44. ISNR for Cascaded Stages—Parallel Control
0
0
0.2
0.4
O
depends on the range selected; for example,
0.2
0.6
G
O
+ G
is −20 dB.
0.8
0.4
O
V
V
C
C
1.0
(V)
(V)
0.6
1.2
O
1.4
is 20 dB. Alternatively,
0.8
1.6
1.8
1.0
2.0
2.2
1.2
Rev. H | Page 16 of 24
(4)
LOW GAIN RIPPLE MODE (MINIMUM GAIN ERROR)
As can be seen in Figure 42 and Figure 43, the error in the gain
is periodic, that is, it shows a small ripple. (Note that there is
also a variation in the output offset voltage, which is due to the
gain interpolation, but this is not exact in amplitude.) By
offsetting the gains of A1 and A2 by half the period of the ripple,
that is, by 3 dB, the residual gain errors of the two amplifiers
can be made to cancel. Figure 45 shows much lower gain ripple
when configured in this manner. Figure 46 plots the ISNR as a
function of gain; it is very similar to that in the parallel mode.
–0.5
–1.0
–1.5
–2.0
–2.5
–3.0
3.0
2.5
2.0
1.5
1.0
0.5
90
85
80
75
70
65
60
55
50
0
–0.1
Figure 45. Gain Error for Cascaded Stages—Low Ripple Mode
–0.2
Figure 46. ISNR vs. Control Voltage—Low Ripple Mode
0
0
0.1
0.2
0.2
0.3
0.4
0.4
V
V
C
0.5
C
(V)
(V)
0.6
0.6
0.7
0.8
0.8
0.9
1.0
1.0
1.1
1.2