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AD8362EVAL Datasheet(Fiches technique) 22 Page  Analog Devices 

22 page AD8362 Rev. B  Page 22 of 36 OPERATION IN MEASUREMENT MODES Figure 50 shows the general connections for operating the AD8362 as an RF power detector, more correctly viewed as an accurate measurement system. The full performance potential of this part, particularly at very high frequencies (above 500 MHz), is realized only when the input is presented to the AD8362 in differential (balanced) form. In this example, a fluxcoupled transformer is used at the input. Having a 1:4 impedance ratio (1:2 turns ratio), the 200 Ω differential input resistance of the AD8362 becomes 50 Ω at the input to the transformer, whose outputs can be connected directly to INHI and INLO. If a centertapped transformer is used, connect the tap to the DECL pins, which are biased to the same potential as the inputs (~3.6 V). Over the 0.9 GHz to 2.2 GHz range, a transmission line transformer (balun) may be used, as explained later. (The evaluation board is supplied with a M/ACOM ETC1.6423, 0.5 GHz to 2.5 GHz, 4:1 balun.) 16 15 14 13 12 11 10 9 1 2 3 4 5 6 7 8 COMM CHPF DECL INHI INLO DECL PWDN COMM ACOM VREF VTGT VPOS VOUT VSET ACOM CLPF 1nF VOUT RAILTORAIL CONTROL OUTPUT AD8362 SIGNAL INPUT Z = 50 Ω 1nF 300pF VS +5V nom, @ 24mA 0.1 µF 3.3 Ω 200 Ω 1nF NC 1:4 ZRATIO (1:2 TURNS RATIO) Figure 50. Connections for RF Power Measurement The output in this mode of use is a continuous, decibelscaled voltage ranging from about 0.5 V to 3.5 V. ( ) dB mV P P VOUT Z IN 50 × = (11) The equivalent input power, PIN, is expressed in dBm (decibels above 1 mW) in a particular system impedance, which in this case is 50 Ω. The intercept, PZ, is that input power for which the backextrapolated output crosses zero. Expressed as a voltage, it is 0.447 mV rms (−67 dBV, lasercalibrated at 100 MHz), corresponding to a PZ of −60 dBm in 200 Ω. However, the 1:2 turns ratio of the transformer halves the required input voltage, which moves the intercept down by 6 dB to 0.224 mV rms (−73 dBV) at the transformer’s input. Impedance mismatches and attenuation in the coupling elements significantly affect the intercept position. This error is stable over temperature and time, and thus can be removed during calibration in a specific system. The logarithmic slope of 50 mV/dB varies only slightly with frequency; corrected values for several common frequencies are provided in the Specifications section. LAW CONFORMANCE ERROR In practice, the response deviates slightly from the ideal straight line suggested by Equation 11. This deviation is called the law conformance error. In defining the performance of high accuracy measurement devices, it is customary to provide plots of this error. In general terms, it is computed by extracting the best straight line to the measured data using linear regression over a substantial region of the dynamic range and under clearly specified conditions. INPUT AMPLITUDE (dBm) 0.2 0.8 1.1 1.4 1.7 2.0 2.3 2.9 3.5 0.5 3.8 –3.0 –1.5 –1.0 –0.5 0.5 1.0 1.5 2.0 2.5 –2.0 –2.5 3.0 2.6 3.2 –60 –55 –50 –45 –40 –35 –30 –25 –20 –15 –5 0 5 10 15 –10 0 –40°C +25°C +85°C +25°C +85°C –40°C Figure 51. Output Voltage and Law Conformance Error, at TA = −40°C, +25°C, and +85°C Figure 51 shows the output of the circuit of Figure 50 over the full input range. The agreement with the ideal function (law conformance) is also shown. This was determined by linear regression on the data points over the central portion of the transfer function (35 mV to 250 mV rms) for the 25°C data. The error at +25°C, −40°C, and +85°C was then calculated by subtracting the ideal output voltage at each input signal level from the actual output and dividing this quantity by the mean slope of the regression equation to provide a measurement of the error in decibels (scaled on the righthand axis of Figure 51). The error curves generated in this way reveal not only the deviations from the ideal transfer function at a nominal temperature but also all of the additional errors caused by temperature changes. Notice there is a small temperature dependence in the intercept (the vertical position of the error plots); this variation is within 0.5 dB at high powers. Figure 51 further reveals that there is a periodic ripple in the conformance curves. This is due to the interpolation technique used to select the signals from the attenuator, not only at discrete tap points, but anywhere in between, thus providing continuous attenuation values. The selected signal is then applied to the 3.5 GHz, 40 dB fixed gain amplifier in the remaining stages of the AD8362’s VGA. 
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