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AD600 Datasheet PDF : 32 Pages
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The gains are offset such that the gain of A2 is increased only
after the gain of A1 has reached its maximum value (see Figure 26).
Note that, for a differential input of −700 mV or less, the gain of
a single amplifier (A1 or A2) is at its minimum value of −1.07 dB;
for a differential input of +700 mV or more, the gain is at its
maximum value of +41.07 dB. Control inputs beyond these
limits do not affect the gain and can be tolerated without damage or
foldover in the response. See the Specifications section for more
details on the allowable voltage range. The gain is now
Gain (dB) = 32 VC
(3)
where VC is the applied control voltage.
+41.07dB
+20dB
+1.07dB
A1
*
–0.56dB
+40.56dB
A2
*
+38.93dB
–1.07dB
GAIN
(dB)
–2.14
0.592
1.908
0
0.625
1.25
1.875
2.5 VC (V)
0
20
40
60
80 82.14
*GAIN OFFSET OF 1.07dB, OR 33.44mV
Figure 26. Explanation of Offset Calibration for Sequential Control
When VC is set to zero, VG1 = −0.592 V and the gain of A1 is
1.07 dB (recall that the gain of each amplifier section is 0 dB for
VG = 625 mV); meanwhile, VG2 = −1.908 V, so the gain of A2 is
−1.07 dB. The overall gain is thus 0 dB (see Figure 23). When
VC = 1.25 V, VG1 = 1.25 V – 0.592 V = 0.658 V, which sets the
gain of A1 to 40.56 dB, while VG2 = 1.25 V – 1.908 V = −0.658 V,
which sets the gain of A2 at −0.56 dB. The overall gain is now
40 dB (see Figure 24). When VC = 2.5 V, the gain of A1 is 41.07 dB
and the gain of A2 is 38.93 dB, resulting in an overall gain of 80
dB (see Figure 25). This mode of operation is further clarified
by Figure 27, which is a plot of the separate gains of A1 and A2
and the overall gain vs. the control voltage. Figure 28 is a plot of
the gain error of the cascaded amplifiers vs. the control voltage.
AD600/AD602
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
voltage, and C1LO and C2LO are optionally connected to an
offset voltage of 0.625 V. The gain scaling is then doubled to
64dB/V, requiring only 1.25 V for an 80 dB change of gain. In
this case, the amplitude of the gain ripple is also doubled, as is
shown in Figure 29, and the instantaneous SNR at the output of
A2 decreases linearly as the gain is increased (see Figure 30).
LOW RIPPLE MODE (MINIMUM GAIN ERROR)
As shown in Figure 28 and Figure 29, the output ripple is
periodic. By offsetting the gains of A1 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 31 shows the much
lower gain ripple when configured in this manner. Figure 32
plots the SNR as a function of gain; it is very similar to that in
the parallel mode.
Rev. F | Page 13 of 32
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