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AD640TD 数据表(PDF) 10 Page - Analog Devices |
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AD640TD 数据表(HTML) 10 Page - Analog Devices |
10 / 16 page AD640 REV. C –10– 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. 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. INPUT AMPLITUDE IN dB ABOVE 1V, AT 10kHz 2 –10 –80 –8 –6 –4 –2 0 –70 –60 –50 –40 –30 –20 –10 SQUARE WAVE INPUT SINE WAVE INPUT TRIWAVE INPUT Figure 22. Deviation from Exact Logarithmic Transfer Function for Two Cascaded AD640s, Showing Effect of Waveform on Calibration and Linearity INPUT AMPLITUDE IN dB ABOVE 1V, AT 10kHz 2 –10 –8 –6 –4 –2 0 –70 –60 –50 –40 –30 –20 –10 SQUARE WAVE INPUT SINE WAVE INPUT TRIWAVE INPUT –12 4 Figure 23. Deviation from Exact Logarithmic Transfer Function for a Single AD640; Compare Low Level Response with that of 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 AD640s are connected in cascade, most detectors will be adequately loaded due to the high overall gain, but a single AD640 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 AD640. 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. SIGNAL MAGNITUDE AD640 is a calibrated device. It is, therefore, important to be clear in specifying the signal magnitude under all waveform conditions. For dc or square wave inputs there is, of course, no ambiguity. Bounded periodic signals, such as sinusoids and triwaves, can be specified in terms of their simple amplitude (peak value) or alternatively by their rms value (which is a mea- sure of power when the impedance is specified). It is generally bet- ter to define this type of signal in terms of its amplitude because the AD640 response is a consequence of the input voltage, not power. However, provided that the appropriate value of inter- cept for a specific waveform is observed, rms measures may be used. Random waveforms can only be specified in terms of rms value because their peak value may be unbounded, as is the case for Gaussian noise. These must be treated on a case-by-case basis. The effective intercept given in Table I should be used for Gaussian noise inputs. On the other hand, for bounded signals the amplitude can be expressed either in volts or dBV (decibels relative to 1 V). For example, a sine wave or triwave of 1 mV amplitude can also be defined as an input of –60 dBV, one of 100 mV amplitude as –20 dBV, and so on. RMS value is usually expressed in dBm (decibels above 1 mW) for a specified impedance level. Through- out this data sheet we assume a 50 Ω environment, the customary impedance level for high speed systems, when referring to signal power in dBm. Bearing in mind the above discussion of the effect of waveform on the intercept calibration of the AD640, it will be apparent that a sine wave at a power of, say, –10 dBm will not produce the same output as a triwave or square wave of the same power. Thus, a sine wave at a power level of –10 dBm has an rms value of 70.7 mV or an amplitude of 100 mV (that is, √2 times as large, the ratio of amplitude to rms value for a sine wave), while a triwave of the same power has an amplitude which is √3 or 1.73 times its rms value, or 122.5 mV. “Intercept” and “Logarithmic Offset” If the signals are expressed in dBV, we can write the output in a simpler form, as IOUT = 50 µA (Input dBV – XdBV) Equation (4) where InputdBV is the input voltage amplitude (not rms) in dBV and XdBV is the appropriate value of the intercept (for a given waveform) in dBV. This form shows more clearly why the intercept is often referred to as the logarithmic offset. For dc or square wave inputs, VX is 1 mV so the numerical value of XdBV is –60, and Equation (4) becomes |
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