AD600JRZ-RL (Analog) PDF资料下载 Datasheet(20/33 页)

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AD600/AD602

Rev. F | Page 26 of 32

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 SNR

progressively decreases.

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

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

presented 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 SNR continues to decrease. Figure 56

clearly shows this because the maximum SNR 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. 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) explain the rather odd values of

the offset values that are used.

The optimization of the output SNR is of obvious value in AGC

systems. However, in applications where these circuits are

considered for their wide range logarithmic measurement

capabilities, the inevitable degradation of the SNR at high gains

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

the circuit shown in Figure 47 was specifically chosen to improve

measurement accuracy by altering the shape of the log error

curve at low signal levels (see Figure 53).

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