AD603

Rev. G | Page 14 of 20

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 will usually be 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

will be 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 S/N RATIO)

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 35 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 36, Figure 37, and Figure 38 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.

2.2

–0.2

0.6

0.2

1.4

1.0

1.8

90

80

85

70

75

65

55

60

50

Figure 35. SNR vs. Control Voltage–Sequential Control (1 MHz Bandwidth)

The gains are offset (Figure 39) such that A2’s gain is increased

only after A1’s gain 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) will be at its minimum value of −11.07 dB;

for a differential input of +600 mV or more, the gain will be at

its maximum value of 31.07 dB. Control inputs beyond these

limits will not affect the gain and can be tolerated without

damage or foldover in the response. This is an important aspect

of the AD603’s gain control response. (See the Specifications

section for more details on the allowable voltage range.) The

gain is now

(3)

the gain range chosen. In the explanatory notes that follow, it is

assumed the maximum bandwidth connections are used, for

31.07dB

–42.14dB

GPOS

GNEG

31.07dB

–42.14dB

GPOS

GNEG

–40.00dB

–51.07dB

–8.93dB

INPUT

0dB

OUTPUT

–20dB

A2

A1

31.07dB

–42.14dB

GPOS

GNEG

31.07dB

0dB

GPOS

GNEG

0dB

–11.07dB

31.07dB

INPUT

0dB

OUTPUT

20dB