AD600ARZ-R7 datasheet(13/28 Pages) AD

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AD600/AD602

Rev. E | Page 13 of 28

The gains are offset such that A2’s gain is increased only after

A1’s gain 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

–0.56dB

–1.07dB

+40.56dB

+38.93dB

GAIN

(dB)

*GAIN OFFSET OF 1.07dB, OR 33.44mV

2.5

1.875

1.25

0.625

–2.14

VC (V)

82.14

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 A2’s gain 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.

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 can be seen 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.

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