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AD7266 Datasheet(PDF) 4 Page - Analog Devices |
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AD7266 Datasheet(HTML) 4 Page - Analog Devices |
4 / 7 page CN-0187 Circuit Note Rev. 0| Page 4 of 7 1ms/DIV 70mV rms 160mV rms 250mV rms 400mV rms RF INPUT VRMS PULSED RFIN Figure 7. Output Response to Various RF Input Pulse Levels, Supply 3 V, 900 MHz Frequency, Square-Domain Filter Open, Output Filter 0.1 μF with Parallel 1 kΩ The RMS and PEAK outputs of the ADL5502 pass through unity gain buffers that drive cross-coupled stages for converting the single-ended outputs to differential signals. The internal +2.5 V reference of the AD7266 (via the DCAPA and DCAPB pins) passes through another unity gain buffer and a voltage divider. This sets the common-mode voltage of the network to +1.25 V. The AD7266 achieves simultaneous samples of the RMS and PEAK outputs and transfers the data within a 1 µs response time. The data is provided on a single serial data line. Because slope and intercept vary from device to device, board-level calibration must be performed to achieve high accuracy. In general, calibration is performed by applying two input power levels to the ADL5502 and measuring the corresponding output voltages. The calibration points are generally chosen to be within the linear operating range of the device. The best-fit line is characterized by calculating the conversion gain (or slope) and intercept using the following equations: Gain = (VVRMS2 − VVRMS1)/(VIN2 − VIN1) (1) Intercept = VVRMS1 − (Gain × VIN1) (2) where: VIN is the rms input voltage to RFIN. VVRMS is the voltage output at VRMS. Once gain and intercept are calculated, an equation can be written that allows calculation of an (unknown) input power based on the measured output voltage. VIN = (VVRMS − Intercept)/Gain (3) For an ideal (known) input power, the law conformance error of the measured data can be calculated as × – log × 20 (dB) = IN, IDEAL URED VRMS, MEAS V Gain Intercept V ERROR (4) Figure 8 and Figure 9 show plots of the VRMS and PEAK error at 25°C, the temperature at which the ADL5502 is calibrated. Note that the error is not zero; this is because the ADL5502 does not perfectly follow the ideal linear equation, even within its operating region. The error at the calibration points is, however, equal to zero by definition. –3 –2 –1 0 1 2 3 INPUT (dBm) –25 –20 –15 –10 –5 0 5 10 15 450MHz 900MHz 1900MHz 2350MHz 2600MHz Figure 8. Measured VRMS Linearity Error vs. Input Level, 450 MHz, 900 MHz, 1900 MHz, 2350 MHz, 2600 MHz, Supply +3.3 V –3 –2 –1 0 1 2 3 INPUT (dBm) –25 –20 –15 –10 –5 0 5 10 15 450MHz 900MHz 1900MHz 2350MHz 2600MHz Figure 9. Measured PEAK Linearity Error vs. Input Level, 450 MHz, 900 MHz, 1900 MHz, 2350 MHz, 2600 MHz, Supply +3.3 V When the characteristics (slope and intercept) of the VRMS and PEAK outputs are known, the calibration for the CF calculation is complete. A three-stage process must be taken to measure and calculate the crest factor of any waveform. First, the unknown signal must be applied to the RF input, and the corresponding VRMS level is measured. This level is indicated in Figure 10 as VVRMS-UNKNOWN. The RF input, VIN, is calculated using VVRMS-UNKNOWN and Equation 3. |
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