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| 1∝V | 10∝V 100∝V 1mV 10mV 100mV | 1V | 10V |
INPUT SIGNAL – V RMS
Figure 27. Gain Error for Figure 25 Without the 2 dB Offset Modification
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INPUT SIGNAL – V RMS
Figure 28. Adding the 2 dB Offsets Improves the Linearization
The maximum gain of this circuit is 120 dB. If no filtering were used, the noise spectral density of the AD600 (1.4 nV/√Hz) would amount to an input noise of 8.28 μV rms in the full band- width (35 MHz). At a gain of one million, the output noise would dominate. Consequently, some reduction of bandwidth is mandatory, and in the circuit of Figure 25 it is due mostly to a
(at VAGC) to about 100 mV rms at a gain of 100 dB. Of course, the bandwidth (and hence output noise) could be easily reduced further, for example, in audio applications, merely by increasing C3. The value chosen for this application is optimal in minimiz- ing the error in the VLOG output for small input signals.
The AD600 is
AD600/AD602
The rms value of VLOG is generated at Pin 8 of the AD636; the averaging time for this process is determined by C5, and the
value shown results in less than 1% rms error at 20 Hz. The slowly varying V rms is compared with a fixed reference of
316 mV, derived from the positive supply by R10/R11. Any dif- ference between these two voltages is integrated in C6, in con- junction with op amp U3C, the output of which is VLOG. A fraction of this voltage, determined by R12 and R13, is returned to the gain control inputs of all AD600 sections. An increase in VLOG lowers the gain, because this voltage is connected to the inverting polarity control inputs.
Now, in this case, the gains of all three VCA sections are being varied simultaneously, so the scaling is not 32 dB/V but 96 dB/
V, or 10.42 mV/dB. The fraction of VLOG required to set its scaling to 50 mV/dB is therefore 10.42/50, or 0.208. The result-
ing
±5 V then requires the use of supply voltages of at least ± 7.5 V.
A simple attenuator of 16.6 ± 1.25 dB is formed by R2/R3 and the 100 Ω input resistance of the AD600. This allows the refer- ence level of the decibel output to be precisely set to zero for an input of 3.16 mV rms, and thus center the 100 dB range be- tween 10 μV and 1 V. In many applications R2/R3 may be re- placed by a fixed resistor of 590 Ω. For example, in AGC applications, neither the slope nor the intercept of the logarith- mic output is important.
A few additional components
accuracy of VLOG at the top end of the signal range (that is, for small gains). The gain starts rolling off when the input to the
first amplifier, U1A, reaches 0 dB. To compensate for this non- linearity, Q1 turns on at VLOG ~ +1.5 V and increases the feed- back to the control inputs of the AD600s, thereby needing a smaller voltage at VLOG to maintain the input to the AD636 to the setpoint of 316 mV rms.
A 120 dB RMS/AGC System with Optimal S/N Ratio (Sequential Gain)
In the last case, all gains were adjusted simultaneously, resulting in an output
Figure 29 shows the circuit for the sequential control scheme. R6 to R9 with R16 provide offsets of 42.14 dB between the individual amplifiers to ensure smooth transitions between the gain of each successive
REV. A |
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