AD600/AD602

REV. A

–6–

41.07dB

–40.00dB

C1HI

C1LO

V

= 0.592V

O1

1.07dB

41.07dB

–42.14dB

C1HI

C1LO

V

= 1.908V

O2

OUTPUT

0dB

–41.07dB

–40.00dB

INPUT

0dB

V = 0V

C

41.07dB

–0.51dB

C1HI

C1LO

V

= 0.592V

O1

40.56dB

41.07dB

–41.63dB

C1HI

C1LO

V

= 1.908V

O2

OUTPUT

40dB

–1.07dB

–0.51dB

INPUT

0dB

V = 1.25V

C

41.07dB

0dB

C1HI

C1LO

V

= 0.592V

O1

41.07dB

41.07dB

–2.14dB

C1HI

C1LO

V

= 1.908V

O2

OUTPUT

80dB

38.93dB

0dB

INPUT

0dB

V = 25V

C

A1

A2

(a)

(b)

(c)

Figure 3. AD600 Gain Control Input Calculations for Sequential Control Operation

+1.07 dB (recall that the gain of each amplifier section is 0 dB

A2 is –1.07 dB. The overall gain is thus 0 dB (see Figure 3a).

–0.658 V, which sets A2’s gain at –0.56 dB. The overall gain is

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.”

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

(3)

+41.07dB

20dB

+1.07dB

–0.56dB

–1.07dB

40.56dB

+38.93dB

0.592

1.908

A1

A2

0

0.625

1.25

1.875

2.5

020

40

60

80

–2.14

82.14

GAIN

(dB)

*

*

C

V (V)

Figure 4. Explanation of Offset Calibration for Sequential

Control