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AD641S Datasheet(PDF) 9 Page - Analog Devices |
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AD641S Datasheet(HTML) 9 Page - Analog Devices |
9 / 16 page REV. C AD641 –9– Note that this lower limit is not determined by the intercept voltage, VX; it can occur either above or below VX, depending on the design. When using two AD641s in cascade, input offset voltage and wideband noise are the major limitations to low level accuracy. Offset can be eliminated in various ways. Noise can only be reduced by lowering the system bandwidth, using a filter between the two devices. EFFECT OF WAVEFORM ON INTERCEPT The absolute value response of the AD641 allows inputs of either polarity to be accepted. Thus, the logarithmic output in response to an amplitude-symmetric square wave is a steady value. For a sinusoidal input the fluctuating output current will usually be low-pass filtered to extract the baseband signal. The unfiltered output is at twice the carrier frequency, simplifying the design of this filter when the video bandwidth must be maxi- mized. The averaged output depends on waveform in a roughly analogous way to waveform dependence of rms value. The effect is to change the apparent intercept voltage. The intercept volt- age appears to be doubled for a sinusoidal input, that is, the averaged output in response to a sine wave of amplitude (not rms value) of 20 mV would be the same as for a dc or square wave input of 10 mV. Other waveforms will result in different inter- cept factors. An amplitude-symmetric-rectangular waveform has the same intercept as a dc input, while the average of a base- band unipolar pulse can be determined by multiplying the response to a dc input of the same amplitude by the duty cycle. It is important to understand that in responding to pulsed RF signals it is the waveform of the carrier (usually sinusoidal) not the modulation envelope, that determines the effective intercept voltage. Table I shows the effective intercept and resulting deci- bel offset for commonly occurring waveforms. The input wave- form does not affect the slope of the transfer function. Figure 22 shows the absolute deviation from the ideal response of cascaded AD641s for three common waveforms at input levels from –80 dBV to –10 dBV. The measured sine wave and triwave responses are 6 dB and 8.7 dB, respectively, below the square wave response—in agreement with theory. Table I. Input Peak Intercept Error (Relative Waveform or rms Factor to a DC Input) Square Wave Either 1 0.00 dB Sine Wave Peak 2 –6.02 dB Sine Wave rms 1.414 ( √2) –3.01 dB Triwave Peak 2.718 (e) –8.68 dB Triwave rms 1.569 (e/ √3) –3.91 dB Gaussian Noise rms 1.887 –5.52 dB Logarithmic Conformance and Waveform The waveform also affects the ripple, or periodic deviation from an ideal logarithmic response. The ripple is greatest for dc or square wave inputs because every value of the input voltage maps to a single location on the transfer function and thus traces out the full nonlinearities in the logarithmic response. 2 0 –2 –4 –6 –8 –10 –70 –60 –50 –40 –30 –20 –10 –80 INPUT AMPLITUDE IN dB ABOVE 1V, AT 10kHz SQUARE WAVE INPUT SINE WAVE INPUT TRIWAVE INPUT Figure 22. Deviation from Exact Logarithmic Transfer Function for Two Cascaded AD641s, Showing Effect of Waveform on Calibration and Linearity By contrast, a general time varying signal has a continuum of values within each cycle of its waveform. The averaged output is thereby “smoothed” because the periodic deviations away from the ideal response, as the waveform “sweeps over” the transfer function, tend to cancel. This smoothing effect is greatest for a triwave input, as demonstrated in Figure 22. The accuracy at low signal inputs is also waveform dependent. The detectors are not perfect absolute value circuits, having a sharp “corner” near zero; in fact they become parabolic at low levels and behave as if there were a dead zone. Consequently, the output tends to be higher than ideal. When there are enough stages in the system, as when two AD641s are connected in cascade, most detectors will be adequately loaded due to the high overall gain, but a single AD641 does not have sufficient gain to maintain high accuracy for low level sine wave or triwave inputs. Figure 23 shows the absolute deviation from calibration for the same three waveforms for a single AD641. For inputs between –10 dBV and –40 dBV the vertical displacement of the traces for the various waveforms remains in agreement with the predicted dependence, but significant calibration errors arise at low signal levels. 4 2 0 –2 –4 –6 –8 –10 –70 INPUT AMPLITUDE IN dB ABOVE 1V, AT 10kHz –60 –50 –40 –30 –20 –10 –12 SQUARE WAVE INPUT SINE WAVE INPUT TRIWAVE INPUT Figure 23. Deviation from Exact Logarithmic Transfer Function for a Single AD641, Compare Low Level Response with That of Figure 22 |
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