AD600/AD602

Rev. E | Page 12 of 28

ACHIEVING 80 dB GAIN RANGE

The two amplifier sections of the X-AMP can be connected in

series to achieve higher gain. In this mode, the output of A1

(A1OP and A1CM) drives the input of A2 via a high-pass

network (usually just a capacitor) that rejects the dc offset.

The nominal gain range is now –2 dB to +82 dB for the AD600

or −22 dB to +62 dB for the AD602.

There are several options in connecting the gain-control inputs.

The choice depends on the desired SNR and gain error (output

ripple). The following examples feature the AD600; the

arguments generally apply to the AD602, with appropriate

changes to the gain values.

SEQUENTIAL MODE (MAXIMUM SNR)

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

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

possible, as shown in Figure 22. Note here that the gain range is

0 dB to 80 dB. Figure 23, Figure 24, and Figure 25 show the

general connections to accomplish this. Both gain-control

inputs, C1HI and C2HI, are driven in parallel by a positive-only,

ground-referenced source with a range of 0 V to 2.5 V.

85

30

3.0

45

35

0

40

–0.5

60

50

55

65

70

75

80

2.5

2.0

1.5

1.0

0.5

Figure 22. SNR vs. Control Voltage Sequential Control (1 MHz Bandwidth)

An auxiliary amplifier that senses the voltage difference

between input and output commons is provided to reject this

common voltage.

41.07dB

INPUT

0dB

–40.00dB

–40.00dB

C1HI

C1LO

1.07dB

OUTPUT

0dB

41.07dB

–41.07dB

–42.14dB

C2HI

C2LO

A1

A2

Figure 23. AD600 Gain Control Input Calculations for Sequential Control Operation (A)

–0.51dB

OUTPUT

40dB

A1

41.07dB

–0.51dB

C1HI

C1LO

41.07dB

–1.07dB

–41.63dB

C2HI

C2LO

40.56dB

A2

INPUT

0dB

Figure 24. AD600 Gain Control Input Calculations for Sequential Control Operation (B)

38.93dB

A1

41.07dB

0dB

0dB

C1HI

C1LO

41.07dB

–2.14dB

C2HI

C2LO

41.07dB

A2

INPUT

0dB

OUTPUT

80dB

Figure 25. AD600 Gain Control Input Calculations for Sequential Control Operation (C)