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Data Sheet

AD603

Rev. K | Page 15 of 24

USING THE AD603 IN CASCADE

Two or more AD603s can be connected in series to achieve

higher gain. Invariably, ac coupling must be used to prevent the

dc offset voltage at the output of each amplifier from overloading

the following amplifier at maximum gain. The required high-

pass coupling network is usually just a capacitor, chosen to set

the desired corner frequency in conjunction with the well-

defined 100 Ω input resistance of the following amplifier.

For two AD603s, the total gain control range becomes 84 dB

(2 × 42.14 dB); the overall −3 dB bandwidth of cascaded stages

is somewhat reduced. Depending on the pin strapping, the gain

and bandwidth for two cascaded amplifiers can range from

−22 dB to +62 dB (with a bandwidth of about 70 MHz) to

+22 dB to +102 dB (with a bandwidth of about 6 MHz).

There are several ways of connecting the gain control inputs

in cascaded operation. The choice depends on whether it is

important to achieve the highest possible instantaneous signal-

to-noise ratio (ISNR), or, alternatively, to minimize the ripple

in the gain error. The following examples feature the AD603

programmed for maximum bandwidth; the explanations apply

to other gain/bandwidth combinations with appropriate

changes to the arrangements for setting the maximum gain.

SEQUENTIAL MODE (OPTIMAL SNR)

In the sequential mode of operation, the ISNR is maintained at

its highest level for as much of the gain control range as possible.

Figure 37 shows the SNR over a gain range of −22 dB to +62 dB,

assuming an output of 1 V rms and a 1 MHz bandwidth. Figure 38,

Figure 39, and Figure 40 show the general connections to

accomplish this. Here, both the positive gain control inputs

(GPOS) are driven in parallel by a positive-only, ground-referenced

source with a range of 0 V to 2 V, while the negative gain

control inputs (GNEG) are biased by stable voltages to provide

the needed gain offsets. These voltages may be provided by

resistive dividers operating from a common voltage reference.

VC (V)

2.2

–0.2

0.6

0.2

1.4

1.0

1.8

Figure 37. SNR vs. Control Voltage, Sequential Control (1 MHz Bandwidth)

31.07dB

–42.14dB

GPOS

GNEG

31.07dB

–42.14dB

GPOS

GNEG

–40.00dB

–51.07dB

–8.93dB

INPUT

0dB

VC = 0V

VG1

VG2

VO1 = 0.473V

VO2 = 1.526V

OUTPUT

–20dB

Figure 38. AD603 Gain Control Input Calculations for Sequential Control Operation VC = 0 V

31.07dB

–42.14dB

GPOS

GNEG

31.07dB

0dB

GPOS

GNEG

0dB

–11.07dB

31.07dB

INPUT

0dB

VC = 1.0V

VG1

VG2

VO1 = 0.473V

VO2 = 1.526V

OUTPUT

20dB

Figure 39. AD603 Gain Control Calculations for Sequential Control Operation VC = 1.0 V

31.07dB

–2.14dB

GPOS

GNEG

31.07dB

0dB

GPOS

GNEG

0dB

–28.93dB

31.07dB

INPUT

0dB

VC = 2.0V

VG1

VG2

VO1 = 0.473V

VO2 = 1.526V

OUTPUT

60dB

Figure 40. AD603 Gain Control Input Calculations for Sequential Operation VC = 2.0 V

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