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AD637BR 데이터시트(PDF) 8 Page - Analog Devices |
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AD637BR 데이터시트(HTML) 8 Page - Analog Devices |
8 / 10 page AD637 REV. E –8– VIN – V rms 2.0 1.8 0.0 2.0 0.5 1.0 1.5 1.2 0.6 0.4 0.2 1.6 1.4 0.8 1.0 CF = 10 CF = 7 CF = 3 Figure 13. Error vs. RMS Input Level for Three Common Crest Factors the energy is contained in the peaks). The duty cycle and peak amplitude were varied to produce crest factors from l to 10 while maintaining a constant 1 volt rms input amplitude. CONNECTION FOR dB OUTPUT Another feature of the AD637 is the logarithmic or decibel out- put. The internal circuit which computes dB works well over a 60 dB range. The connection for dB measurement is shown in Figure 14. The user selects the 0 dB level by setting R1 for the proper 0 dB reference current (which is set to exactly cancel the log output current from the squarer/divider circuit at the desired 0 dB point). The external op amp is used to provide a more convenient scale and to allow compensation of the +0.33%/ °C temperature drift of the dB circuit. The special T.C. resistor R3 is available from Tel Labs in Londenderry, New Hampshire (model Q-81) and from Precision Resistor Inc., Hillside, N.J. (model PT146). DB CALIBRATION 1. Set VIN = 1.00 V dc or 1.00 V rms 2. Adjust R1 for 0 dB out = 0.00 V 3. Set VIN = 0.1 V dc or 0.10 V rms 4. Adjust R2 for dB out = – 2.00 V Any other dB reference can be used by setting VIN and R1 accordingly. LOW FREQUENCY MEASUREMENTS If the frequencies of the signals to be measured are below 10 Hz, the value of the averaging capacitor required to deliver even 1% averaging error in the standard rms connection be- comes extremely large. The circuit shown in Figure 15 shows an alternative method of obtaining low frequency rms measure- ments. The averaging time constant is determined by the prod- uct of R and CAV1, in this circuit 0.5 s/ µF of C AV. This circuit permits a 20:1 reduction in the value of the averaging capacitor, permitting the use of high quality tantalum capacitors. It is suggested that the two pole Sallen-Key filter shown in the dia- gram be used to obtain a low ripple level and minimize the value of the averaging capacitor. If the frequency of interest is below 1 Hz, or if the value of the averaging capacitor is still too large, the 20:1 ratio can be increased. This is accomplished by increasing the value of R. If this is done it is suggested that a low input current, low offset voltage amplifier like the AD548 be used instead of the internal buffer amplifier. This is necessary to minimize the offset error introduced by the combination of amplifier input currents and the larger resistance. AD707JN –VS +VS dB SCALE FACTOR ADJUST R2 COMPENSATED dB OUTPUT + 100mV/dB 5k 33.2k R3 60.4 1k * BUFFER AD637 SQUARER/DIVIDER BIAS SECTION FILTER 25k 25k 1 2 3 4 5 6 7 14 13 12 11 10 9 8 CAV –VS +VS + ABSOLUTE VALUE BUFFER OUTPUT ANALOG COM OUTPUT OFFSET BUFFER INPUT NC CHIP SELECT DENOMINATOR INPUT dB SIGNAL INPUT NC 1 F SIGNAL INPUT RMS OUTPUT +VS AD508J +2.5 VOLTS R1 500k 0dB ADJUST 10k *1k + 3500ppm TC RESISTOR TEL LAB Q81 PRECISION RESISTOR PT146 OR EQUIVALENT Figure 14. dB Connection |
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