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AD603

Data Sheet

Rev. K | Page 16 of 24

The gains are offset (Figure 41) 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

Gain (dB) = 40 VG + GO

(3)

where:

VG is the applied control voltage.

GO is determined by the gain range chosen.

In the explanatory notes that follow, it is assumed that

the maximum bandwidth connections are used, for which

GO is −20 dB.

+31.07dB

+10dB

+31.07dB

+28.96dB

–11.07dB

0.473

1.526

–8.93dB

–20

0.5

1.0

1.50

2.0

VC (V)

62.14

–22.14

GAIN

(dB)

*GAIN OFFSET OF 1.07dB, OR 26.75mV.

Figure 41. Explanation of Offset Calibration for Sequential Control

With reference to Figure 38, Figure 39, and Figure 40, note that

VG1 refers to the differential gain control input to A1, and VG2

refers to the differential gain control input to A2. When VG is

0 V, VG1 = −473 mV and thus the gain of A1 is −8.93 dB (recall

that the gain of each individual amplifier in the maximum

bandwidth mode is –10 dB for VG = −500 mV and 10 dB for VG

= 0 V); meanwhile, VG2 = −1.908 V so the gain of A2 is pinned

at −11.07 dB. The overall gain is therefore –20 dB (see Figure 38).

When VG = 1.00 V, VG1 = 1.00 V − 0.473 V = 0.526 V, which sets

the gain of A1 to nearly its maximum value of +31.07 dB, while

VG2 = 1.00 V − 1.526 V = 0.526 V, which sets the gain of A2 to

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 39).

When VG = 2.0 V, the gain of A1 is pinned at 31.07 dB and that

of A2 is near its maximum value of 28.93 dB, resulting in an

overall gain of 60 dB (see Figure 40). This mode of operation is

further clarified in Figure 42, which is a plot of the separate

gains of A1 and A2 and the overall gain vs. the control voltage.

Figure 43 is a plot of the SNR of the cascaded amplifiers vs. the

control voltage. Figure 44 is a plot of the gain error of the

cascaded stages vs. the control voltages.

VC (V)

2.0

–0.2

0.2

0.6

1.0

1.4

1.8

–10

–20

–30

COMBINED

Figure 42. Plot of Separate and Overall Gains in Sequential Control

VC (V)

2.0

–0.2

0.2

0.6

1.0

1.4

1.8

Figure 43. SNR for Cascaded Stages—Sequential Control

VC (V)

2.2

–0.2

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

2.0

1.5

1.0

0.5

–0.5

–1.0

–1.5

–2.0

Figure 44. Gain Error for Cascaded Stages–Sequential Control

PARALLEL MODE (SIMPLEST GAIN CONTROL

INTERFACE)

In this mode, the gain control of voltage is applied to both

inputs in parallel: the GPOS pins of both A1 and A2 are

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