Datasheet AD640 (Analog Devices) - 9

AD640
(see Figure 20). For the AD640, VX is calibrated to exactly When the attenuator is not used, the PTAT variation in VX 1 mV. The slope of the line is directly proportional to VY. Base will result in the intercept being temperature dependent. Near 10 logarithms are used in this context to simplify the relation- 300K (27°C) it will vary by 20 LOG (301/300) dB/°C, about ship to decibel values. For VIN = 10 VX, the logarithm has a 0.03 dB/°C. Unless corrected, the whole output function would value of 1, so the output voltage is VY. At VIN = 100 VX, the drift up or down by this amount with changes in temperature. In output is 2 VY, and so on. VY can therefore be viewed either as the AD640 a temperature compensating current IYLOG(T/TO) the Slope Voltage or as the Volts per Decade Factor. is added to the output. This effectively maintains a constant intercept VXO. This correction is active in the default state (Pin
V IDEAL YLOG (VIN /VX)
8 open circuited). When using the attenuator, Pin 8 should be
ACTUAL
grounded, which disables the compensation current. The drift
2VY
term needs to be compensated only once; when the outputs of
SLOPE = V
two AD540s are summed, Pin 8 should be grounded on at least
Y
one of the two devices (both if the attenuator is used).
VY 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-
0
ing. Above this, no further increase in the output can occur and
ACTUAL V INPUT ON IN = VX VIN = 10VX VIN = 100VX LOG SCALE
the transfer function flattens off. The lower limit arises because
IDEAL
a finite number of stages provide finite gain, and therefore at low signal levels the system becomes a simple linear amplifier. Figure 20. Basic DC Transfer Function of the AD640 Note that this lower limit is not determined by the intercept The AD640 conforms to Equation (1) except that its two out- voltage, V puts are in the form of currents, rather than voltages: X; it can occur either above or below VX, depending on the design. When using two AD640s in cascade, input offset IOUT = IY LOG (VIN/VX) Equation (2) voltage and wideband noise are the major limitations to low I level accuracy. Offset can be eliminated in various ways. Noise Y 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, can only be reduced by lowering the system bandwidth, using a using one of the 1 kΩ resistors provided for this purpose, in filter between the two devices. conjunction with an op amp, as shown in Figure 21.
EFFECT OF WAVEFORM ON INTERCEPT 1mA PER R2
The absolute value response of the AD640 allows inputs of
DECADE R1
either polarity to be accepted. Thus, the logarithmic output in
48.7
V
AD844
response to an amplitude-symmetric square wave is a steady
C1 330pF
value. For a sinusoidal input the fluctuating output current will
OUTPUT VOLTAGE
usually be low-pass filtered to extract the baseband signal. The
1V PER DECADE 15 14 13 12 11
unfiltered output is at twice the carrier frequency, simplifying the
FOR R2 = 1k
V
LOG LOG +V SIG S 100mV PER dB
design of this filter when the video bandwidth must be maxi-
OUT COM +OUT for R2 = 2k
V mized. The averaged output depends on waveform in a roughly
AD640
analogous way to waveform dependence of rms value. The effect
SIG –VS ITC BL2 –OUT
is to change the apparent intercept voltage. The intercept volt-
6 7 8 9 10
age appears to be doubled for a sinusoidal input, that is, the averaged output in response to a sine wave of amplitude (not rms Figure 21. Using an External Op Amp to Convert the value) of 20 mV would be the same as for a dc or square wave AD640 Output Current to a Buffered Voltage Output input of 10 mV. Other waveforms will result in different inter-
Intercept Stabilization
cept factors. An amplitude-symmetric-rectangular waveform Internally, the intercept voltage is a fraction of the thermal volt- has the same intercept as a dc input, while the average of a age kT/q, that is, VX = VXOT/TO, where VXO is the value of VX baseband unipolar pulse can be determined by multiplying the at a reference temperature TO. So the uncorrected transfer response to a dc input of the same amplitude by the duty cycle. function has the form It is important to understand that in responding to pulsed RF I signals it is the waveform of the carrier (usually sinusoidal) not OUT = IY LOG (VIN TO/VXOT) Equation (3) the modulation envelope, that determines the effective intercept Now, if the amplitude of the signal input VIN could somehow be voltage. Table I shows the effective intercept and resulting deci- rendered PTAT, the intercept would be stable with tempera- bel offset for commonly occurring waveforms. The input wave- ture, since the temperature dependence in both the numerator form does not affect the slope of the transfer function. Figure 22 and denominator of the logarithmic argument would cancel. shows the absolute deviation from the ideal response of cascaded This is what is actually achieved by interposing the on-chip AD640s for three common waveforms at input levels from attenuator, which has the necessary temperature dependence to –80 dBV to –10 dBV. The measured sine wave and triwave cause the input to the first stage to vary in proportion to abso- responses are 6 dB and 8.7 dB, respectively, below the square lute temperature. The end limits of the dynamic range are now wave response—in agreement with theory. totally independent of temperature. Consequently, this is the preferred method of intercept stabilization for applications where the input signal is sufficiently large. REV. D –9– 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