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AD637ARZ Datenblatt(PDF) 9 Page - Analog Devices |
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AD637ARZ Datenblatt(HTML) 9 Page - Analog Devices |
9 / 20 page AD637 Rev. K | Page 9 of 20 5.0 2.5 –5.0 0 0.5 1.0 0 –2.5 1.5 2.0 AD637K MAX INTERNAL TRIM AD637K EXTERNAL TRIM AD637K: 0.5mV ± 0.2% 0.25mV ± 0.05% EXTERNAL INPUT LEVEL (V) Figure 7. Maximum Total Error vs. Input Level AD637K Internal and External Trims 1 25kΩ DEN INPUT BUFF IN BUFF OUT SQUARER/ DIVIDER –VS CS dB OUTPUT 3 COMMON BIAS 2NC 4 OUTPUT OFFSET R2 1MΩ 5 25kΩ 6 7 8 9 10 +VS 11 12 NC 13 14 NC VIN VOUT =VIN2 VIN CAV + AD637 ABSOLUTE VALUE –VS +VS CAV R4 147Ω SCALE FACTOR TRIM R1 50kΩ OUTPUT OFFSET TRIM 4.7kΩ –VS +VS +VS R3 1kΩ Figure 8. Optional External Gain and Offset Trims CHOOSING THE AVERAGING TIME CONSTANT The AD637 computes the true rms value of both dc and ac input signals. At dc, the output tracks the absolute value of the input exactly; with ac signals, the AD637 output approaches the true rms value of the input. The deviation from the ideal rms value is due to an averaging error. The averaging error comprises an ac component and a dc component. Both components are functions of input signal frequency f and the averaging time constant τ (τ: 25 ms/μF of averaging capacitance). Figure 9 shows that the averaging error is defined as the peak value of the ac component (ripple) and the value of the dc error. The peak value of the ac ripple component of the averaging error is defined approximately by the relationship ()f where reading of % in f 1 τ τ 6.3 50 > DOUBLE-FREQUENCY RIPPLE EO TIME AVERAGE ERROR IDEAL EO DC ERROR = AVERAGE OF OUTPUT – IDEAL Figure 9. Typical Output Waveform for a Sinusoidal Input This ripple can add a significant amount of uncertainty to the accuracy of the measurement being made. The uncertainty can be significantly reduced through the use of a postfiltering network or by increasing the value of the averaging capacitor. The dc error appears as a frequency dependent offset at the output of the AD637 and follows the relationship reading of % in f 2 2 4 . 6 16 . 0 1 τ + Because the averaging time constant, set by CAV, directly sets the time that the rms converter holds the input signal during computation, the magnitude of the dc error is determined only by CAV and is not affected by postfiltering. SINE WAVE INPUT FREQUENCY (Hz) 100 0.1 1.0 10 10k 1k 100 10 DC ERROR PEAK RIPPLE Figure 10. Comparison of Percent DC Error to the Percent Peak Ripple over Frequency Using the AD637 in the Standard RMS Connection with a 1 × μF CAV The ac ripple component of averaging error is greatly reduced by increasing the value of the averaging capacitor. There are two major disadvantages to this: the value of the averaging capacitor becomes extremely large and the settling time of the AD637 increases in direct proportion to the value of the averaging capacitor (TS = 115 ms/μF of averaging capacitance). A preferable method of reducing the ripple is by using the postfilter network, as shown in Figure 11. This network can be used in either a 1- pole or 2-pole configuration. For most applications, the 1-pole filter gives the best overall compromise between ripple and settling time. |
Ähnliche Teilenummer - AD637ARZ |
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Ähnliche Beschreibung - AD637ARZ |
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