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AD640BD 数据表(PDF) 9 Page - Analog Devices |
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AD640BD 数据表(HTML) 9 Page - Analog Devices |
9 / 16 page AD640 REV. C –9– (see Figure 20). For the AD640, VX is calibrated to exactly 1 mV. The slope of the line is directly proportional to VY. Base 10 logarithms are used in this context to simplify the relation- ship to decibel values. For VIN = 10 VX, the logarithm has a value of 1, so the output voltage is VY. At VIN = 100 VX, the output is 2 VY, and so on. VY can therefore be viewed either as the Slope Voltage or as the Volts per Decade Factor. 0 VY 2VY VIN = VX VIN = 10VX VIN = 100VX SLOPE = VY ACTUAL IDEAL INPUT ON LOG SCALE ACTUAL IDEAL VYLOG (VIN/VX) Figure 20. Basic DC Transfer Function of the AD640 The AD640 conforms to Equation (1) except that its two out- puts are in the form of currents, rather than voltages: IOUT = IY LOG (VIN/VX) Equation (2) IY the Slope Current, is 1 mA. The current output can readily be converted to a voltage with a slope of 1 V/decade, for example, using one of the 1 k Ω resistors provided for this purpose, in conjunction with an op amp, as shown in Figure 21. 11 15 14 13 12 6 7 8 9 10 SIG +OUT LOG COM LOG OUT +VS –VS ITC BL2 SIG –OUT AD640 C1 330pF AD844 R1 48.7 R2 1mA PER DECADE OUTPUT VOLTAGE 1V PER DECADE FOR R2 = 1k 100mV PER dB for R2 = 2k Figure 21. Using an External Op Amp to Convert the AD640 Output Current to a Buffered Voltage Output Intercept Stabilization Internally, the intercept voltage is a fraction of the thermal volt- age kT/q, that is, VX = VXOT/TO, where VXO is the value of VX at a reference temperature TO. So the uncorrected transfer function has the form IOUT = IY LOG (VIN TO/VXOT) Equation (3) Now, if the amplitude of the signal input VIN could somehow be rendered PTAT, the intercept would be stable with tempera- ture, since the temperature dependence in both the numerator and denominator of the logarithmic argument would cancel. This is what is actually achieved by interposing the on-chip attenuator, which has the necessary temperature dependence to cause the input to the first stage to vary in proportion to abso- lute temperature. The end limits of the dynamic range are now totally independent of temperature. Consequently, this is the preferred method of intercept stabilization for applications where the input signal is sufficiently large. When the attenuator is not used, the PTAT variation in VX will result in the intercept being temperature dependent. Near 300K (27 °C) it will vary by 20 LOG (301/300) dB/°C, about 0.03 dB/ °C. Unless corrected, the whole output function would drift up or down by this amount with changes in temperature. In the AD640 a temperature compensating current IYLOG(T/TO) is added to the output. This effectively maintains a constant intercept VXO. This correction is active in the default state (Pin 8 open circuited). When using the attenuator, Pin 8 should be grounded, which disables the compensation current. The drift term needs to be compensated only once; when the outputs of two AD540s are summed, Pin 8 should be grounded on at least one of the two devices (both if the attenuator is used). Conversion Range Practical logarithmic converters have an upper and lower limit on the input, beyond which errors increase rapidly. The upper limit occurs when the first stage in the chain is driven into limit- ing. Above this, no further increase in the output can occur and the transfer function flattens off. The lower limit arises because a finite number of stages provide finite gain, and therefore at low signal levels the system becomes a simple linear amplifier. 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 AD640s 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 AD640 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 baseband 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 AD640s 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. |
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类似说明 - AD640BD |
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