AD600 datasheet(17/20 Pages) AD

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AD600/AD602

REV. A

–17–

2.0

–2.0

µV10µV

10V

100mV

10mV

1mV

100

µV

0.5

1.0

1.5

–1.5

–1.0

–0.5

INPUT SIGNAL – V RMS

0.2

–0.2

Figure 31. The Error Ripple Due to the Individual Gain

Functions

400

300

200

µV10µV

10V

100mV

10mV

1mV

100

µV

350

250

INPUT SIGNAL – V RMS

Figure 32. VAGC Remains Nose to Its Setpoint of

316 mV RMS Over the Full 120 dB Range

833.2

–625.0

–833.2

625.0

416.6

208.3

–208.3

–416.6

CONTROL VOLTAGE, V

(10.417mV/dB) – mV

Figure 33. S/N Ratio vs. Control Voltage for Parallel Gain

Control (Figure 25)

In contrast, the S/N ratio for the sequential mode is shown in

Figure 34. 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.) Thus, for the first 40 dB of control

range (actually slightly more, as explained below), when only

this VCA section has its gain varied, the S/N ratio remains con-

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

minimum value of –1.07 dB.

3.817

–0.558

–1.183

3.192

2.567

1.942

1.317

0.692

0.067

CONTROL VOLTAGE, V

(31.25mV/dB) – Volts

Figure 34. S/N Ratio vs. Control Voltage for Sequential

Gain Control (Figure 29)

For the next 40 dB of control range, the gain of U1A remains

fixed at its maximum value of 41.07 dB and only the gain of

U1B is varied, while that of U2A remains at its minimum value

of –1.07 dB. In this interval, the fixed output noise of U1A is

amplified by the increasing gain of U1B and the S/N ratio pro-

gressively decreases.

Once U1B reaches its maximum gain of 41.07 dB, its output

also becomes a gain independent noise source; this noise is pre-

sented to U2A. As the control voltage is further increased, the

gains of both U1A and U1B remain fixed at their maximum

value of 41.07 dB, and the S/N ratio continues to decrease. Fig-

ure 34 clearly shows this, because the maximum S/N ratio of

90 dB is extended for the first 40 dB of input signal before it

starts to roll off.

This arrangement of staggered gains can be easily implemented

because, when the control inputs of the AD600 are overdriven,

the gain limits to its maximum or minimum values without side

effects. This eliminates the need for awkward nonlinear shaping

circuits that have previously been used to break up the gain

range of multistage AGC amplifiers. It is the precise values of

the AD600’s maximum and minimum gain (not 0 dB and

40 dB but –1.07 dB and 41.07 dB) that explain the rather odd

values of the offset values that are used.

The optimization of the output S/N ratio is of obvious value in

AGC systems. However, in applications where these circuit are

considered for their wide range logarithmic measurements capa-

bilities, the inevitable degradation of the S/N ratio at high gains

need not seriously impair their utility. In fact, the bandwidth of

the circuit shown in Figure 25 was specifically chosen so as to

improve measurement accuracy by altering the shape of the log

error curve (Figure 31) at low signal levels.

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