AD600/AD602

Rev. E | Page 25 of 28

5

3

–5

4

2

0

1

–1

–3

–2

–4

1µ

10µ

10

1

100m

10m

1m

100µ

INPUT SIGNAL (V rms)

Figure 53 shows the error ripple due to the individual gain

functions bounded by ±0.2 dB (dotted lines) from 6 μV to 2 V.

The small perturbations at about 200 μV and 20 mV, caused by

the impracticality of matching the gain functions perfectly, are

the only sign that the gains are now sequential. Figure 54 is a

rms over the full 120 dB range.

To compare the SNRs in the simultaneous and sequential

modes of operation more directly, all interstage attenuation was

eliminated (R2 and R3 in Figure 47 and R2 in Figure 51), the

input of U1A was shorted, R5 was selected to provide a 20 kHz

bandwidth (R5 = 7.87 kΩ), and only the gain control was

varied, using an external source. The rms value of the noise was

0 dBV, which is almost the maximum output capability of the

AD600. Results for the simultaneous mode can be seen in

Figure 55. The SNR degrades uniformly as the gain is increased.

Note that since the inverting gain control was used, the gain in

this curve and in Figure 56 decreases for more positive values of

the gain-control voltage.

2.0

–2.0

0.5

1.0

1.5

–1.5

–1.0

–0.5

–0.2

0.2

0

1µ

10µ

10

1

100m

10m

1m

100µ

INPUT SIGNAL (V rms)

Figure 53. Error Ripple due to the Individual Gain Functions

400

300

200

350

250

1µ

10µ

10

1

100m

10m

1m

100µ

INPUT SIGNAL (V rms)

316 mV rms over the Full 120 dB Range

90

0

833.2

20

10

–625.0

–833.2

30

40

50

60

70

80

625.0

416.6

208.3

0

–208.3

–416.6

Figure 55. SNR vs. Control Voltage for Parallel Gain Control (See Figure 47)

In contrast, the SNR for the sequential mode is shown in Figure 56.

U1A always acts as a fixed noise source; varying its gain has no

influence on the output noise. This is a feature of the X-AMP

technique. Therefore, for the first 40 dB of control range

(actually slightly more, as is explained later), when only this

VCA section has its gain varied, the SNR remains constant.

During this time, the gains of U1B and U2A are at their

minimum value of −1.07 dB.

90

0

3.817

20

10

–0.558

–1.183

30

40

50

60

70

80

3.192

2.567

1.942

1.317

0.692

0.067

Figure 56. SNR vs. Control Voltage for Sequential Gain Control (See Figure 51)