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전자부품 데이터시트 검색엔진 |
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전자부품 데이터시트 검색엔진 |
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AD8310ARMZ-REEL7 데이터시트(PDF) 10 Page - Analog Devices |
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AD8310ARMZ-REEL7 데이터시트(HTML) 10 Page - Analog Devices |
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10 / 24 page  AD8310 Rev. E | Page 10 of 24 SLOPE AND INTERCEPT CALIBRATION All monolithic log amps from Analog Devices use precision design techniques to control the logarithmic slope and intercept. The primary source of this calibration is a pair of accurate voltage references that provide supply- and temperature-independent scaling. The slope is set to 24 mV/dB by the bias chosen for the detector cells and the subsequent gain of the postdetector output interface. With this slope, the full 95 dB dynamic range can be easily accommodated within the output swing capacity, when operating from a 2.7 V supply. Intercept positioning at −108 dBV (−95 dBm re 50 Ω) has likewise been chosen to provide an output centered in the available voltage range. Precise control of the slope and intercept results in a log amp with stable scaling parameters, making it a true measurement device as, for example, a calibrated received signal strength indicator (RSSI). In this application, the input waveform is invariably sinusoidal. The input level is correctly specified in dBV. It can alternatively be stated as an equivalent power, in dBm, but in this case, it is necessary to specify the impedance in which this power is presumed to be measured. In RF practice, it is common to assume a reference impedance of 50 Ω, in which 0 dBm (1 mW) corresponds to a sinusoidal amplitude of 316.2 mV (223.6 mV rms). However, the power metric is correct only when the input impedance is lowered to 50 Ω, either by a termination resistor added across INHI and INLO, or by the use of a narrow-band matching network. Note that log amps do not inherently respond to power, but to the voltage applied to their input. The AD8310 presents a nominal input impedance much higher than 50 Ω (typically 1 kΩ at low frequencies). A simple input matching network can considerably improve the power sensitivity of this type of log amp. This increases the voltage applied to the input and, therefore, alters the intercept. For a 50 Ω reactive match, the voltage gain is about 4.8, and the whole dynamic range moves down by 13.6 dB. The effective intercept is a function of wave- form. For example, a square-wave input reads 6 dB higher than a sine wave of the same amplitude, and a Gaussian noise input reads 0.5 dB higher than a sine wave of the same rms value. OFFSET CONTROL In a monolithic log amp, direct coupling is used between the stages for several reasons. First, it avoids the need for coupling capacitors, which typically have a chip area at least as large as that of a basic gain cell, considerably increasing die size. Second, the capacitor values predetermine the lowest frequency at which the log amp can operate. For moderate values, this can be as high as 30 MHz, limiting the application range. Third, the parasitic back-plate capacitance lowers the bandwidth of the cell, further limiting the scope of applications. However, the very high dc gain of a direct-coupled amplifier raises a practical issue. An offset voltage in the early stages of the chain is indistinguishable from a real signal. If it were as high as 400 μV, it would be 18 dB larger than the smallest ac signal (50 μV), potentially reducing the dynamic range by this amount. This problem can be averted by using a global feedback path from the last stage to the first, which corrects this offset in a similar fashion to the dc negative feedback applied around an op amp. The high frequency components of the feedback signal must, of course, be removed to prevent a reduction of the HF gain in the forward path. An on-chip filter capacitor of 33 pF provides sufficient suppres- sion of HF feedback to allow operation above 1 MHz. The − 3 dB point in the high-pass response is at 2 MHz, but the usable range extends well below this frequency. To further lower the frequency range, an external capacitor can be added at OFLT (Pin 3). For example, 300 pF lowers it by a factor of 10. Operation at low audio frequencies requires a capacitor of about 1 μF. Note that this filter has no effect for input levels well above the offset voltage, where the frequency range would extend down to dc (for a signal applied directly to the input pins). The dc offset can optionally be nulled by adjusting the voltage on the OFLT pin (see the Applications section). |
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