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HS574A Datasheet(PDF) 8 Page - Sipex Corporation |
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HS574A Datasheet(HTML) 8 Page - Sipex Corporation |
8 / 14 page 10 signals between ground traces and cross digital lines at right angles only. Grounding Considerations Any ground path from the analog and digital ground should be as low resistance as possible to accommodate the ground currents present with this device. The analog ground current is approximately 6mA DC while the digital ground is 3mA DC. The analog and digital common pins should be tied together as close to the package as possible to guarantee best performance. The code–depen- dent currents flow through the V LOGIC and VCC terminals and not through the analog and digital common pins. Power Supplies The supply voltages for the SPx74A must be kept as quiet as possible from noise pickup and also regulated from transients or drops. Because the part has 12–bit accuracy, voltage spikes on the supply lines can cause several LSB devia- tions on the output. Switching power supply noise can be a problem. Careful filtering and shielding should be employed to prevent the noise from being picked up by the converter. Capacitor bypass pairs are needed from each supply pin to its respective ground to filter noise and counter the problems caused by the varia- tions in supply current. A 10 µF tantalum and a 0.1 µF ceramic type in parallel between V LOGIC (pin 1) and digital common (pin15), and V CC (pin 7) and analog common (pin 9) is sufficient. V EE is generated internally so pin 11 may be grounded or connected to a negative supply if the SPx74A is being used to upgrade an already existing design. CALIBRATION AND CONNECTION PROCEDURES Unipolar The calibration procedure consists of adjusting the converter’s most negative output to its ideal value for offset adjustment, and then adjusting the most positive output to its ideal value for gain adjustment. Starting with offset adjustment and referring to Figure 4, the midpoint of the first LSB increment should be positioned at the origin to get an output code of all 0s. To do this, an input of +1⁄ 2 LSB or +1.22mV for the 10V range and +2.44mV for the 20V range should be applied to the SPx74A. Adjust the offset potentiometer R 1 for code tran- sition flickers between 0000 0000 0000 and 0000 0000 0001. The gain adjustment should be done at positive full scale. The ideal input corresponding to the last code change is applied. This is 11⁄ 2LSB below the nominal full scale which is +9.9963V for the 10V range and +19.9927V for the 20V range. Adjust the gain potentiometer R 2 for flicker between codes 1111 1111 1110 and 1111 1111 1111. If calibration is not necessary for the intended application, replace R 2 with a 50Ω, 1% metal film resistor and remove the network ana- Figure 6. SPx74A Control Logic STS DELAY R CK Q D H D CK Q Q EOC8 EOC12 CE R/C A0 CS 12/8 READ CONTROL A0 LATCH INPUT BUFFERS NIBBLE B ZERO OVERRIDE NIBBLE A, B NIBBLE C |
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