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ADS947 Datasheet(PDF) 4 Page - List of Unclassifed Manufacturers |
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ADS947 Datasheet(HTML) 4 Page - List of Unclassifed Manufacturers |
4 / 8 page ADS-947 4 ®® CALIBRATION PROCEDURE Any offset and/or gain calibration procedures should not be implemented until devices are fully warmed up. To avoid interaction, offset must be adjusted before gain. The ranges of adjustment for the circuits in Figures 2 and 3 are guaranteed to compensate for the ADS-947’s initial accuracy errors and may not be able to compensate for additional system errors. A/D converters are calibrated by positioning their digital outputs exactly on the transition point between two adjacent digital output codes. This can be accomplished by connecting LED’s to the digital outputs and adjusting until certain LED’s “flicker” equally between on and off. Other approaches employ digital comparators or microcontrollers to detect when the outputs change from one code to the next. Offset adjusting for the ADS-947 is normally accomplished at the point where the MSB is a 1 and all other output bits are 0’s and the LSB just changes from a 0 to a 1. This digital output transition ideally occurs when the applied analog input is +½ LSB (+122µV). Gain adjusting is accomplished when all bits are 1’s and the LSB just changes from a 1 to a 0. This transition ideally occurs when the analog input is at +full scale minus 1½ LSB's (+1.99963V). Zero/Offset Adjust Procedure 1. Apply a train of pulses to the START CONVERT input (pin 18) so the converter is continuously converting. 2. Apply +122µV to the ANALOG INPUT (pin 21). 3. Adjust the offset potentiometer until the output bits are 10 0000 0000 0000 and the LSB flickers between 0 and 1. Gain Adjust Procedure 1. Apply +1.99963V to the ANALOG INPUT (pin 21). 2. Adjust the gain potentiometer until all output bits are 1’s and the LSB flickers between 1 and 0. 3. To confirm proper operation of the device, vary the input signal to obtain the output coding listed in Table 2. BIPOLAR INPUT VOLTAGE OFFSET BINARY SCALE (±2V RANGE) MSB LSB +FS –1 LSB +1.99976 11 1111 1111 1111 +3/4 FS +1.50000 11 1000 0000 0000 +1/2 FS +1.00000 11 0000 0000 0000 0 0.00000 10 0000 0000 0000 –1/2 FS –1.00000 01 0000 0000 0000 –3/4 FS –1.50000 00 1000 0000 0000 –FS +1 LSB –1.99976 00 0000 0000 0001 –FS –2.00000 00 0000 0000 0000 Table 2. Output Coding for Bipolar Operation INPUT VOLTAGE ZERO ADJUST GAIN ADJUST RANGE +½ LSB +FS –1½ LSB ±2V +122µV +1.99963V Table 1. Gain and Zero Adjust Figure 3. Typical ADS-947 Connection Diagram 0.1µF 0.1µF 4.7µF 4.7µF 22, 13 24 20 19 ADS-947 –5.2V 20k W 0.1µF 4.7µF +5V 14 –5V +5V 21 23 18 1 2 3 4 5 6 7 8 9 10 11 12 15 16 17 BIT 1 (MSB) BIT 2 BIT 3 BIT 4 BIT 5 BIT 6 BIT 7 BIT 8 BIT 9 BIT 10 BIT 11 BIT 12 BIT 13 BIT 14 (LSB) DATA VALID ANALOG INPUT START CONVERT A single +5V supply should be used for both the +5V analog and +5V digital. If separate supplies are used, the difference between the two cannot exceed 100mV. + + + ZERO/ OFFSET ADJUST |
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