Datasheet AD640 (Analog Devices) - 10

AD640 Table I.
The accuracy at low signal inputs is also waveform dependent. The detectors are not perfect absolute value circuits, having a
Input Peak Intercept Error (Relative
sharp “corner” near zero; in fact they become parabolic at low
Waveform or RMS Factor to a DC Input)
levels and behave as if there were a dead zone. Consequently, Square Wave Either 1 0.00 dB the output tends to be higher than ideal. When there are enough Sine Wave Peak 2 –6.02 dB stages in the system, as when two AD640s are connected in Sine Wave rms 1.414(√2) –3.01 dB cascade, most detectors will be adequately loaded due to the Triwave Peak 2.718 (e) –8.68 dB high overall gain, but a single AD640 does not have sufficient Triwave rms 1.569(e/√3) –3.91 dB gain to maintain high accuracy for low level sine wave or triwave Gaussian Noise rms 1.887 –5.52 dB inputs. Figure 23 shows the absolute deviation from calibration for the same three waveforms for a single AD640. For inputs
Logarithmic Conformance and Waveform
between –10 dBV and –40 dBV the vertical displacement of the The waveform also affects the ripple, or periodic deviation from traces for the various waveforms remains in agreement with the an ideal logarithmic response. The ripple is greatest for dc or predicted dependence, but significant calibration errors arise at square wave inputs because every value of the input voltage low signal levels. maps to a single location on the transfer function and thus traces out the full nonlinearities in the logarithmic response.
SIGNAL MAGNITUDE
AD640 is a calibrated device. It is, therefore, important to be By contrast, a general time varying signal has a continuum of clear in specifying the signal magnitude under all waveform values within each cycle of its waveform. The averaged output is conditions. For dc or square wave inputs there is, of course, no thereby “smoothed” because the periodic deviations away from ambiguity. Bounded periodic signals, such as sinusoids and the ideal response, as the waveform “sweeps over” the transfer triwaves, can be specified in terms of their simple amplitude function, tend to cancel. This smoothing effect is greatest for a (peak value) or alternatively by their rms value (which is a mea- triwave input, as demonstrated in Figure 22. sure of power when the impedance is specified). It is generally bet-
2
ter to define this type of signal in terms of its amplitude because the AD640 response is a consequence of the input voltage, not
SQUARE WAVE INPUT 0
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
–2
value because their peak value may be unbounded, as is the case for Gaussian noise. These must be treated on a case-by-case
–4
basis. The effective intercept given in Table I should be used for
SINE WAVE INPUT
Gaussian noise inputs.
–6 TRANSFER FUNCTION – dB
On the other hand, for bounded signals the amplitude can be
–8
expressed either in volts or dBV (decibels relative to 1 V). For
DEVIATION FROM EXACT LOGARITHMIC
example, a sine wave or triwave of 1 mV amplitude can also be
TRIWAVE INPUT –10
defined as an input of –60 dBV, one of 100 mV amplitude as
–80 –70 –60 –50 –40 –30 –20 –10
–20 dBV, and so on. RMS value is usually expressed in dBm
INPUT AMPLITUDE IN dB ABOVE 1V, AT 10kHz
(decibels above 1 mW) for a specified impedance level. Through- Figure 22. Deviation from Exact Logarithmic Transfer out this data sheet we assume a 50 Ω environment, the customary Function for Two Cascaded AD640s, Showing Effect of impedance level for high speed systems, when referring to signal power Waveform on Calibration and Linearity in dBm. Bearing in mind the above discussion of the effect of waveform on the intercept calibration of the AD640, it will be
4
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
2
same power. Thus, a sine wave at a power level of –10 dBm has
SQUARE WAVE INPUT 0
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
–2
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.
–4 SINE WAVE INPUT “Intercept” and “Logarithmic Offset” –6
If the signals are expressed in dBV, we can write the output in a
TRANSFER FUNCTION – dB –8
simpler form, as
TRIWAVE INPUT
I
–10
OUT = 50 µA (InputdBV – XdBV) Equation (4)
DEVIATION FROM EXACT LOGARITHMIC
where InputdBV is the input voltage amplitude (not rms) in dBV
–12–70 –60 –50 –40 –30 –20 –10
and XdBV is the appropriate value of the intercept (for a given
INPUT AMPLITUDE IN dB ABOVE 1V, AT 10kHz
waveform) in dBV. This form shows more clearly why the intercept Figure 23. Deviation from Exact Logarithmic Transfer is often referred to as the logarithmic offset. For dc or square Function for a Single AD640; Compare Low Level wave inputs, VX is 1 mV so the numerical value of XdBV is –60, Response with that of Figure 22 and Equation (4) becomes –10– REV. D Document Outline AD640-SPECIFICATIONS DC Specifications AC Specifications Thermal Characteristics ABSOLUTE MAXIMUM RATINGS TYPICAL DC PERFORMANCE CHARACTERISTICS TYPICAL AC PERFORMANCE CHARACTERISTICS OUTLINE DIMENSIONS ORDERING GUIDE