AD600 datasheet(13/20 Pages) AD

13 / 20 page

AD600/AD602

REV. A

–13–

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

which the rms value of the input is leveled.) Figure 21 shows the

“decibel” output voltage. More revealing is Figure 22, 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

450

300

150

µV

100

µV

10V

100mV

10mV

1mV

225

375

350

200

275

425

325

175

250

400

INPUT SIGNAL –V RMS

Figure 20. The RMS Output of A2 Is Held Close to the

“Setpoint” 316 mV for an Input Range of Over 80 dB

–5

µV

100

µV

10V

100mV

10mV

1mV

–4

–3

–2

–1

INPUT SIGNAL – V RMS

Figure 21. The dB Output of Figure 19’s Circuit Is Linear

Over an 80 dB Range

2.5

–2.5

µV

100

µV

10V

100mV

10mV

1mV

0.5

1.0

1.5

2.0

–2.0

–1.5

–1.0

–0.5

INPUT SIGNAL – V RMS

Figure 22. Data from Figure 20 Presented as the Deviation

from the Ideal Output Given in Equation 4

±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 could be centered to cover any range from 25 mV to 250 mV

to, say, 1 mV to 10 V, with appropriate correction to the value

of VREF. (Note that VSCALE is not affected by the changes in the

range.) The gain ripple of

±0.2 dB seen in this curve is the re-

sult of the finite interpolation error of the X-AMP. Note that it

occurs with a periodicity of 12 dB—twice the separation be-

tween the tap points (because of the two cascaded stages).

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 23: 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).

C1HI

A1CM

A1OP

VPOS

VNEG

A2OP

A2CM

C2HI

+6V DEC

–6V DEC

µF

–6V

DEC

VINP

VNEG

CAVG

VLOG

BFOP

BFIN

AD636

AD600

–46.875mV

+46.875mV

NC = NO CONNECT

10k

Ω

78.7

Ω

78.7

Ω

10k

Ω

+6V

DEC

3dB OFFSET

MODIFICATION

–6V

DEC

Figure 23. Reducing the Gain Error Ripple

The error curve shown in Figure 24 demonstrates that over the

central portion of the range the output voltage can be main-

tained very close to the ideal value. The penalty for this modifi-

cation 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 Fig-

ure 15. Very low errors can then be maintained over a 100 dB

range.

2.5

–2.5

µV

100

µV

10V

100mV

10mV

1mV

0.5

1.0

1.5

2.0

–2.0

–1.5

–1.0

–0.5

INPUT SIGNAL – V RMS

Figure 24. Using the 3 dB Offset Network, the Ripple

Is Reduced

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