AD603 Analog Devices, Inc., AD603 Datasheet - Page 15

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

The gains are offset (Figure 39) such that the gain of A2 is
increased only after the gain of A1 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) is at its minimum value of
−11.07 dB; for a differential input of 600 mV or more, the gain
is at its maximum value of 31.07 dB. Control inputs beyond
these limits do not affect the gain and can be tolerated without
damage or foldover in the response. This is an important aspect
of the gain control response of the AD603. (See the Specifications
section for more details on the allowable voltage range.) The
gain is now
where:
V
G
In the explanatory notes that follow, it is assumed that
the maximum bandwidth connections are used, for which
G
With reference to Figure 36, Figure 37, and Figure 38, note that
V
refers to the differential gain control input to A2. When V
0 V, V
that the gain of each individual amplifier in the maximum
bandwidth mode is –10 dB for V
= 0 V); meanwhile, V
at −11.07 dB. The overall gain is therefore –20 dB (see Figure 36).
When V
the gain of A1 to nearly its maximum value of +31.07 dB, while
V
nearly its minimum value of −11.07 dB. Close analysis shows
that the degree to which neither AD603 is completely pushed to
its maximum nor minimum gain exactly cancels in the overall
gain, which is now 20 dB (see Figure 37).
When V
of A2 is near its maximum value of 28.93 dB, resulting in an
overall gain of 60 dB (see Figure 38). This mode of operation is
further clarified in Figure 40, which is a plot of the separate
gains of A1 and A2 and the overall gain vs. the control voltage.
GAIN
*GAIN OFFSET OF 1.07dB, OR 26.75mV.
(dB)
G
O
O
G1
G2
–8.93dB
is the applied control voltage.
is determined by the gain range chosen.
is −20 dB.
refers to the differential gain control input to A1, and V
= 1.00 V − 1.526 V = 0.526 V, which sets the gain of A2 to
Figure 39. Explanation of Offset Calibration for Sequential Control
Gain (dB) = 40 V
–22.14
G1
= −473 mV and thus the gain of A1 is −8.93 dB (recall
G
G
= 1.00 V, V
= 2.0 V, the gain of A1 is pinned at 31.07 dB and that
+31.07dB
+10dB
–20
0
0.473
A1
0.5
G1
G2
0
G
–11.07dB
= 1.00 V − 0.473 V = 0.526 V, which sets
+ G
= −1.908 V so the gain of A2 is pinned
*
O
1.0
20
G
= −500 mV and 10 dB for V
+31.07dB
*
A2
1.50
40
1.526
2.0
60
–11.07dB
+28.96dB
V
62.14
C
(V)
G
G2
is
Rev. H | Page 15 of 24
(3)
G
Figure 41 is a plot of the SNR of the cascaded amplifiers vs. the
control voltage. Figure 42 is a plot of the gain error of the
cascaded stages vs. the control voltages.
Figure 40. Plot of Separate and Overall Gains in Sequential Control
–0.5
–1.0
–1.5
–2.0
–10
–20
–30
2.0
1.5
1.0
0.5
70
60
50
40
30
20
10
90
80
70
60
50
40
30
20
10
Figure 42. Gain Error for Cascaded Stages–Sequential Control
0
–0.2
–0.2
0
–0.2
Figure 41. SNR for Cascaded Stages—Sequential Control
0
0.2
0.2
0.2
0.4
0.6
0.6
0.6
A1
0.8
V
V
V
C
C
C
1.0
1.0
1.0
(V)
(V)
(V)
COMBINED
1.2
1.4
1.4
1.4
A2
1.6
1.8
1.8
1.8
2.0
AD603
2.0
2.0
2.2