AD600/AD602

Rev. E | Page 21 of 28

This system can, of course, be used as an AGC amplifier in

which the rms value of the input is leveled. Figure 43 shows the

decibel output voltage. More revealing is Figure 44, which

shows that the deviation from the ideal output predicted by

Equation 1 over the input range 80 μV to 500 mV rms is within

±0.5 dB, and within ±1 dB for the 80 dB range from 80 μV to

800 mV. By suitable choice of the input attenuator R1 + R2, this

can be centered to cover any range from a low of 25 mV to

250 mV to a high of 1 mV to 10 V, with appropriate correction

changes in the range. The gain ripple of ±0.2 dB seen in this

curve is the result of the finite interpolation error of the

X-AMP. Note that it occurs with a periodicity of 12 dB, twice

the separation between the tap points (because of the two

cascaded stages).

5

0

–5

1

2

3

4

–4

–3

–2

–1

10µ

100µ

10

1

100m

10m

1m

INPUT SIGNAL (V rms)

Figure 43. The dB Output of Figure 41’s Circuit is Linear over an 80 dB Range

2.5

0

–2.5

0.5

1.0

1.5

2.0

–2.0

–1.5

–1.0

–0.5

10µ

100µ

10

1

100m

10m

1m

INPUT SIGNAL (V rms)

Figure 44. Data from Figure 42 Presented as the Deviation

from the Ideal Output Given in Equation 4

This ripple can be canceled whenever the X-AMP stages are

cascaded by introducing a 3 dB offset between the two pairs of

control voltages. A simple means to achieve this is shown in

Figure 45: the voltages at C1HI and C2HI are split by

±46.875 mV, or ±1.5 dB. Alternatively, either one of these pins

can be individually offset by 3 dB and a 1.5 dB gain adjustment

made at the input attenuator (R1 + R2).

16

15

14

13

12

11

10

9

U1

AD600

C1HI

A1CM

A1OP

VPOS

VNEG

A2OP

A2CM

C2HI

+6V DEC

–6V DEC

C2

2µF

1

2

3

4

5

6

7

VINP

VNEG

CAVG

VLOG

BFOP

BFIN

U2

AD636

NC

NC

NC

–6V DEC

–46.875mV

NC = NO CONNECT

10kΩ

10kΩ

+6V

DEC

–6V

DEC

78.7Ω 78.7Ω

3dB OFFSET

MODIFICATION

+46.875mV

Figure 45. Reducing the Gain Error Ripple

The error curve shown in Figure 46 demonstrates that over the

central portion of the range the output voltage can be maintained

close to the ideal value. The penalty for this modification is the

higher errors at the extremities of the range. The next two

applications show how three amplifier sections can be cascaded

to extend the nominal conversion range to 120 dB, with the

inclusion of simple LP filters of the type shown in Figure 37.

Very low errors can then be maintained over a 100 dB range.

2.5

0

–2.5

0.5

1.0

1.5

2.0

–2.0

–1.5

–1.0

–0.5

10µ

100µ

10

1

100m

10m

1m

INPUT SIGNAL (V rms)

Figure 46. Using a 3 dB Offset Network Reduces Ripple

100 dB TO 120 dB RMS RESPONDING CONSTANT

BANDWIDTH AGC SYSTEMS WITH HIGH

ACCURACY dB OUTPUTS

The next two applications double as both AGC amplifiers and

measurement systems. In both, precise gain offsets are used to

achieve either a high gain linearity of ±0.1 dB over the full

100 dB range or the optimal SNR at any gain.