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AD673JP Folha de dados(PDF) 5 Page - Analog Devices |
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AD673JP Folha de dados(HTML) 5 Page - Analog Devices |
5 / 9 page AD673 REV. A –4– Full-Scale Calibration The 5 k Ω thin-film input resistor is laser trimmed to produce a current which matches the full-scale current of the internal DAC-plus about 0.3%—when an analog input voltage of 9.961 volts (10 volts – 1 LSB) is applied at the input. The input resis- tor is trimmed in this way so that if a fine trimming potentio- meter is inserted in series with the input signal, the input current at the full scale input voltage can be trimmed down to match the DAC full-scale current as precisely as desired. How- ever, for many applications the nominal 9.961 volt full scale can be achieved to sufficient accuracy by simply inserting a 15 Ω re- sistor in series with the analog input to Pin 14. Typical full-scale calibration error will then be within ±2 LSB or ±0.8%. If more precise calibration is desired, a 200 Ω trimmer should be used instead. Set the analog input at 9.961 volts, and set the trimmer so that the output code is just at the transition between 111111 10 and 11111111. Each LSB will then have a weight of 39.06 mV. If a nominal full scale of 10.24 volts is desired (which makes the LSB have a weight of exactly 40.0 mV), a 100 Ω resistor and a 100 Ω trimmer (or a 200 Ω trimmer with good resolution) should be used. Of course, larger full-scale ranges can be arranged by using a larger input resistor, but lin- earity and full-scale temperature coefficient may be compro- mised if the external resistor becomes a sizeable percentage of 5 k Ω Figure 3 illustrates the connections required for full-scale calibration. Figure 3. Standard AD673 Connections Unipolar Offset Calibration Since the Unipolar Offset is less than ±1/2 LSB for all versions of the AD673, most applications will not require trimming. Fig- ure 4 illustrates two trimming methods which can be used if greater accuracy is necessary. Figure 4a shows how the converter zero may be offset to correct for initial offset and/or input signal offsets. As shown, the circuit gives approximately symmetrical adjustment in unipolar mode. Figure 5 shows the nominal transfer curve near zero for an AD673 in unipolar mode. The code transitions are at the edges of the nominal bit weights. In some applications it will be prefer- able to offset the code transitions so that they fall between the nominal bit weights, as shown in the offset characteristics. Figure 5. AD673 Transfer Curve—Unipolar Operation (Approximate Bit Weights Shown for Illustration, Nominal Bit Weights % 39.06 mV) This offset can easily be accomplished as shown in Figure 4b. At balance (after a conversion) approximately 2 mA flows into the Analog Common terminal. A 10 Ω resistor in series with this terminal will result in approximately the desired l/2 bit offset of the transfer characteristics. The nominal 2 mA Analog Common current is not closely controlled in manufacture. If high accuracy is required, a 20 Ω potentiometer (connected as a rheostat) can be used as R1. Additional negative offset range may be obtained by using larger values of R1. Of course, if the zero transition point is changed, the full-scale transition point will also move. Thus, if an offset of 1/2 LSB is introduced, full scale trimming as described on the previous page should be done with an analog input of 9.941 volts. NOTE: During a conversion, transient currents from the Analog Common terminal will disturb the offset voltage. Capacitive decoupling should not be used around the offset network. These transients will settle appropriately during a conversion. Capaci- tive decoupling will “pump up” and fail to settle resulting in conversion errors. Power supply decoupling, which returns to analog signal common, should go to the signal input side of the resistive offset network. Figure 4. Unipolar Offset Trimming Figure 4a. Figure 4b. |
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