AD602 AD [Analog Devices], AD602 Datasheet - Page 6

• Manufacturer Part Number: AD602
• Description: Dual, Low Noise, Wideband Variable Gain Amplifiers
• Manufacturer: AD [Analog Devices]

AD600/AD602
The gains are offset (Figure 4) such that A2’s gain is increased
only after A1’s gain has reached its maximum value. Note that
for a differential input of –700 mV or less, the gain of a single
amplifier (A1 or A2) will be at its minimum value of –1.07 dB;
for a differential input of +700 mV or more, the gain will be at
its maximum value of 41.07 dB. Control inputs beyond these
limits will not affect the gain and can be tolerated without dam-
age or foldover in the response. See the Specifications Section of
this data sheet for more details on the allowable voltage range.
The gain is now
where V
Figure 4. Explanation of Offset Calibration for Sequential
Control
GAIN
(dB)
+1.07dB
–2.14
C
+41.07dB
20dB
is the applied control voltage.
Gain (dB) = 32 V
0
0
*
0.592
GAIN OFFSET OF 1.07dB, OR 33.44mV
V = 1.25V
Figure 3. AD600 Gain Control Input Calculations for Sequential Control Operation
0.625
C
V = 25V
20
V = 0V
C
A1
INPUT
INPUT
INPUT
C
–0.56dB
0dB
0dB
0dB
*
1.25
40
C
40.56dB
C1HI
C1HI
C1HI
*
–40.00dB
–0.51dB
1.875
0dB
V
V
V
60
G1
G1
G1
1.908
C1LO
C1LO
C1LO
–40.00dB
V
V
V
A2
–0.51dB
O1
O1
O1
A1
0dB
= 0.592V
= 0.592V
= 0.592V
2.5
80
41.07dB
41.07dB
41.07dB
V (V)
82.14
C
+38.93dB
–1.07dB
(3)
(a)
(b)
(c)
40.56dB
41.07dB
1.07dB
–6–
When V
+1.07 dB (recall that the gain of each amplifier section is 0 dB
for V
A2 is –1.07 dB. The overall gain is thus 0 dB (see Figure 3a).
When V
sets the gain of A1 to 40.56 dB, while V
–0.658 V, which sets A2’s gain at –0.56 dB. The overall gain is
now 40 dB (see Figure 3b). When V
is 41.07 dB and that of A2 is 38.93 dB, resulting in an overall
gain of 80 dB (see Figure 3c). This mode of operation is further
clarified by Figure 5, which is a plot of the separate gains of A1
and A2 and the overall gain versus the control voltage. Figure 6
is a plot of the gain error of the cascaded amplifiers versus the
control voltage.
Parallel Mode (Simplest Gain-Control Interface)
In this mode, the gain-control voltage is applied to both inputs
in parallel—C1HI and C2HI are connected to the control volt-
age, and C1LO and C2LO are optionally connected to an offset
voltage of +0.625 V. The gain scaling is then doubled to 64 dB/
V, requiring only 1.25 V for an 80 dB change of gain. The am-
plitude of the gain ripple in this case is also doubled, as shown
in Figure 7, and the instantaneous signal-to-noise ratio at the
output of A2 decreases linearly as the gain is increased (Figure 8).
Low Ripple Mode (Minimum Gain Error)
As can be seen in Figures 6 and 7, the output ripple is periodic.
By offsetting the gains of Al and A2 by half the period of the
ripple, or 3 dB, the residual gain errors of the two amplifiers
can be made to cancel. Figure 9 shows the much lower gain rip
ple when configured in this manner. Figure 10 plots the S/N
ratio as a function of gain; it is very similar to that in the “Par-
allel Mode.”
G
C1HI
C1HI
C1HI
= 625 mV); meanwhile, V
–42.14dB
–41.63dB
–2.14dB
C
C
V
V
V
is set to zero, V
= +1.25 V, V
G2
G2
G2
C1LO
V
C1LO
V
C1LO
V
–41.07dB
O2
O2
O2
A2
–1.07dB
38.93dB
= 1.908V
= 1.908V
= 1.908V
41.07dB
41.07dB
41.07dB
G1
G1
= 1.25 V– 0.592 V = +0.658 V, which
= –0.592 V and the gain of A1 is
G2
OUTPUT
OUTPUT
OUTPUT
40dB
80dB
= –1.908 V so the gain of
0dB
C
= +2.5 V, the gain of A1
G2
= 1.25 V – 1.908 V =
REV. A