Datasheet AD641 (Analog Devices) - 9

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AD641
Note that this lower limit is not determined by the intercept
2
voltage, VX; it can occur either above or below VX, depending on the design. When using two AD641s in cascade, input offset
0
voltage and wideband noise are the major limitations to low
SQUARE WAVE INPUT
level accuracy. Offset can be eliminated in various ways. Noise
–2
can only be reduced by lowering the system bandwidth, using a filter between the two devices.
–4 EFFECT OF WAVEFORM ON INTERCEPT SINE WAVE INPUT
The absolute value response of the AD641 allows inputs of
–6
either polarity to be accepted. Thus, the logarithmic output in
TRANSFER FUNCTION – dB TRIWAVE
response to an amplitude-symmetric square wave is a steady
–8 INPUT
value. For a sinusoidal input the fluctuating output current will
DEVIATION FROM EXACT LOGARITHMIC
usually be low-pass filtered to extract the baseband signal. The
–10 –80 –70 –60 –50 –40 –30 –20 –10
unfiltered output is at twice the carrier frequency, simplifying the
INPUT AMPLITUDE IN dB ABOVE 1V, AT 10kHz
design of this filter when the video bandwidth must be maxi- Figure 22. Deviation from Exact Logarithmic Transfer mized. The averaged output depends on waveform in a roughly Function for Two Cascaded AD641s, Showing Effect of analogous way to waveform dependence of rms value. The effect Waveform on Calibration and Linearity is to change the apparent intercept voltage. The intercept volt- age appears to be doubled for a sinusoidal input, that is, the By contrast, a general time varying signal has a continuum of averaged output in response to a sine wave of amplitude (not rms values within each cycle of its waveform. The averaged output is value) of 20 mV would be the same as for a dc or square wave thereby “smoothed” because the periodic deviations away from input of 10 mV. Other waveforms will result in different inter- the ideal response, as the waveform “sweeps over” the transfer cept factors. An amplitude-symmetric-rectangular waveform has function, tend to cancel. This smoothing effect is greatest for a the same intercept as a dc input, while the average of a base- triwave input, as demonstrated in Figure 22. band unipolar pulse can be determined by multiplying the The accuracy at low signal inputs is also waveform dependent. response to a dc input of the same amplitude by the duty cycle. The detectors are not perfect absolute value circuits, having a It is important to understand that in responding to pulsed RF sharp “corner” near zero; in fact they become parabolic at low signals it is the waveform of the carrier (usually sinusoidal) not levels and behave as if there were a dead zone. Consequently, the modulation envelope, that determines the effective intercept the output tends to be higher than ideal. When there are enough voltage. Table I shows the effective intercept and resulting deci- stages in the system, as when two AD641s are connected in bel offset for commonly occurring waveforms. The input wave- cascade, most detectors will be adequately loaded due to the form does not affect the slope of the transfer function. Figure 22 high overall gain, but a single AD641 does not have sufficient shows the absolute deviation from the ideal response of cascaded gain to maintain high accuracy for low level sine wave or triwave AD641s for three common waveforms at input levels from inputs. Figure 23 shows the absolute deviation from calibration –80 dBV to –10 dBV. The measured sine wave and triwave for the same three waveforms for a single AD641. For inputs responses are 6 dB and 8.7 dB, respectively, below the square between –10 dBV and –40 dBV the vertical displacement of the wave response—in agreement with theory. traces for the various waveforms remains in agreement with the predicted dependence, but significant calibration errors arise at
Table I.
low signal levels.
Input Peak Intercept Error (Relative 4 Waveform or rms Factor to a DC Input) 2 SQUARE
Square Wave Either 1 0.00 dB
WAVE INPUT
Sine Wave Peak 2 –6.02 dB
0
Sine Wave rms 1.414 (√2) –3.01 dB
–2
Triwave Peak 2.718 (e) –8.68 dB Triwave rms 1.569 (e/√3) –3.91 dB
–4 SINE WAVE
Gaussian Noise rms 1.887 –5.52 dB
INPUT –6 Logarithmic Conformance and Waveform
The waveform also affects the ripple, or periodic deviation from
–8 TRANSFER FUNCTION – dB
an ideal logarithmic response. The ripple is greatest for dc or
TRIWAVE –10 INPUT DEVIATION FROM EXACT LOGARITHMIC
square wave inputs because every value of the input voltage maps to a single location on the transfer function and thus traces
–12 –70 –60 –50 –40 –30 –20 –10
out the full nonlinearities in the logarithmic response.
INPUT AMPLITUDE IN dB ABOVE 1V, AT 10kHz
Figure 23. Deviation from Exact Logarithmic Transfer Function for a Single AD641, Compare Low Level Response with That of Figure 22 REV. D –9– Document Outline FEATURES PRODUCT DESCRIPTION PIN CONFIGURATIONS AD641--SPECIFICATIONS ELECTRICAL CHARACTERISTICS THERMAL CHARACTERISTICS ABSOLUTE MAXIMUM RATINGS ESD CAUTION REVISION HISTORY AD641--TYPICAL DC PERFORMANCE CHARACTERISTICS TYPICAL AC PERFORMANCE CHARACTERISTICS CIRCUIT DESCRIPTION CIRCUIT OPERATION FUNDAMENTALS OF LOGARITHMIC CONVERSION INTERCEPT STABILIZATION CONVERSION RANGE EFFECT OF WAVEFORM ON INTERCEPT LOGARITHMIC CONFORMANCE AND WAVEFORM SIGNAL MAGNITUDE INTERCEPT AND LOGARITHMIC OFFSET OPERATION OF A SINGLE AD641 ACTIVE CURENT-TO-VOLTAGE CONVERSION EFFECT OF FREQUENCY ON CALIBRATION SOURCE RESISTANCE AND INPUT OFFSET USING HIGHER SUPPLY VOLTAGES USING THE ATTENUATOR OPERATION OF CASCADED AD641s ELIMINATING THE EFFECT OF FIRST STAGE OFFSET PRACTICAL APPLICATIONS RSSI APPLICATIONS 250 MHz RSSI CONVERTER WITH 58 dB DYNAMIC RANGE OUTLINE DIMENSIONS ORDERING GUIDE