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AD637JQ Datenblatt(PDF) 5 Page - Analog Devices |
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AD637JQ Datenblatt(HTML) 5 Page - Analog Devices |
5 / 10 page REV. E –5– AD637 OPTIONAL TRIMS FOR HIGH ACCURACY The AD637 includes provisions to allow the user to trim out both output offset and scale factor errors. These trims will result in significant reduction in the maximum total error as shown in Figure 4. This remaining error is due to a nontrimmable input offset in the absolute value circuit and the irreducible non- linearity of the device. The trimming procedure on the AD637 is as follows: l. Ground the input signal, VIN and adjust R1 to give 0 V out- put from Pin 9. Alternatively R1 can be adjusted to give the correct output with the lowest expected value of VIN. 2. Connect the desired full scale input to VIN, using either a dc or a calibrated ac signal, trim R3 to give the correct output at Pin 9, i.e., 1 V dc should give l.000 V dc output. Of course, a 2 V peak-to-peak sine wave should give 0.707 V dc output. Remaining errors are due to the nonlinearity. INPUT LEVEL – Volts 5.0 2.5 5.0 0 2.0 0.5 1.0 0 2.5 1.5 AD637K MAX INTERNAL TRIM AD637K EXTERNAL TRIM AD637K: 0.5mV 0.2% 0.25mV 0.05% EXTERNAL Figure 4. Max Total Error vs. Input Level AD637K Internal and External Trims 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 V rms OUT R4 147 + R3 1k SCALE FACTOR ADJUST, 2% R2 1M R1 50k –VS +VS VIN OUTPUT OFFSET ADJUST ABSOLUTE VALUE Figure 5. Optional External Gain and Offset Trims CHOOSING THE AVERAGING TIME CONSTANT The AD637 will compute the true rms value of both dc and ac input signals. At dc the output will track the absolute value of the input exactly; with ac signals the AD637’s output will ap- proach the true rms value of the input. The deviation from the ideal rms value is due to an averaging error. The averaging error is comprised of an ac and dc component. Both components are functions of input signal frequency f, and the averaging time constant τ (τ: 25 ms/µF of averaging capacitance). As shown in Figure 6, the averaging error is defined as the peak value of the ac component, ripple, plus the value of the dc error. The peak value of the ac ripple component of the averaging er- ror is defined approximately by the relationship: 50 6.3 τf in % of reading where (t > 1/f) DC ERROR = AVERAGE OF OUTPUT–IDEAL DOUBLE-FREQUENCY RIPPLE EO TIME AVERAGE ERROR IDEAL EO Figure 6. 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 post filtering net- work 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 equation: 1 0.16 + 6.4τ2 f 2 in % of reading Since 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 will not be affected by post filtering. SINEWAVE INPUT FREQUENCY – Hz 100 0.1 1.0 10 10k 1k 100 10 DC ERROR PEAK RIPPLE Figure 7. Comparison of Percent DC Error to the Percent Peak Ripple over Frequency Using the AD637 in the Stan- dard RMS Connection with a 1 × µF C AV The ac ripple component of averaging error can be greatly reduced by increasing the value of the averaging capacitor. There are two major disadvantages to this: first, the value of the averaging capacitor will become extremely large and second, 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 through the use of the post filter network, shown in Figure 8. This network can be used in either a one or two pole configura- tion. For most applications the single pole filter will give the best overall compromise between ripple and settling time. |
Ähnliche Teilenummer - AD637JQ |
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Ähnliche Beschreibung - AD637JQ |
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