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ADL5902 Arkusz danych(PDF) 2 Page - Analog Devices |
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ADL5902 Arkusz danych(HTML) 2 Page - Analog Devices |
2 / 5 page CN-0178 Circuit Note Rev. A| Page 2 of 5 Data is shown for the two devices operating over a −40°C to +85°C temperature range. CIRCUIT DESCRIPTION The RF signal being measured is applied to the input of the ADL5902, a linear-in-dB rms-responding rms detector. The external 60.4 Ω resistor, R3, combined with the relatively high input impedance of the ADL5902 ensures a broadband 50 Ω match to the RF input. The ADL5902 is configured in its so-called “measurement mode,” with the VSET and VOUT pins connected together. In this mode the output voltage is proportional to the logarithm of the rms value of the input. In other words, the reading is presented directly in decibels and is scaled to 1.06 V per decade, or 53 mV/dB. The power supply voltage and reference voltage for the AD7466 12-bit ADC are provided by the ADL5902 internal 2.3 V reference. Because the AD7466 consumes so little current (16 µA when sampling at 10 kSPS), the ADL5902’s reference voltage output can supply the ADC, as well as the temperature compensating and rms accuracy-scaling network consisting of R9, R10, R11, and R12. The ADC full-scale voltage is equal to 2.3 V. The maximum detector output voltage (when operating in its linear input range) is approximately 3.5 V (see ADL5902 data sheet figures 6, 7, 8, 12, 13, and 14) and must, therefore, be scaled down by a factor of 0.657 before driving the AD7466. This scaling is implemented using a simple resistor divider R10 and R11 (1.21 kΩ and 2.0 kΩ). These values provide an actual scaling factor of 0.623, which ensures that the ADL5902 RF detector does not overdrive the ADC by building in some room for resistor tolerance. A typical plot of detector output voltage vs. input power is shown in Figure 2 (without output scaling). 4.0 0 –70 10 PIN (dBm) 0.5 1.0 1.5 2.0 2.5 3.0 3.5 –60 –50 –40 –30 –20 –10 0 INTERCEPT Figure 2. ADL5902 RMS Detector, Output Voltage vs. Input Power @ 900 MHz The transfer function of the detector can be approximated by the equation VOUT = SLOPE_DETECTOR × (PIN − INTERCEPT) where SLOPE_DETECTOR is in mV/dB; INTERCEPT is the x-axis intercept with a unit of dBm; PIN is the input power in dBm. At the output of the ADC, VOUT is replaced by the ADC’s output code, and the equation can be rewritten as CODE = SLOPE × (PIN − INTERCEPT) where SLOPE is the combined slope of the detector, the scaling resistors, and the ADC, and has the unit of counts/dB; PIN and INTERCEPT still have the unit of dBm. Figure 3 shows a typical detector power sweep in terms of input power and observed ADC output codes for a 700 MHz input signal. 4096 3755 3413 3072 2731 2389 2048 1707 1365 1024 683 341 0 6 5 4 3 2 1 0 –1 –2 –3 –4 –5 –6 –70 –60 –50 –40 –30 –20 –10 0 10 PIN (dBm) +85°C CODE –40°C CODE +25°C CODE +25°C ERROR 4-POINT CAL @ 0dBm, –20dBm, –45dBm, AND, –58dBm +85°C ERROR 4-POINT CAL –40°C ERROR 4-POINT CAL Figure 3. ADC Output Code and Error vs. RF Input Power @ 700 MHz Overall SLOPE and INTERCEPT will vary from system to system. This variation is caused by part to part variations in the transfer function of the RF detector, the scaling resistors, and the ADC. As a result, a system level calibration is required to determine the complete system SLOPE and INTERCEPT. In this application, a 4-point calibration is used to correct for some nonlinearity in the RF detector’s transfer function, particularly at the low end. This 4-point calibration scheme yields three SLOPE and three INTERCEPT calibration coefficients, which should be stored in nonvolatile RAM (NVM) after calibration. |
Podobny numer części - ADL5902 |
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Podobny opis - ADL5902 |
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