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AD7575BQ 데이터시트(PDF) 9 Page - Analog Devices |
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AD7575BQ 데이터시트(HTML) 9 Page - Analog Devices |
9 / 12 page AD7575 –9– REV. B UNIPOLAR OPERATION The basic operation for the AD7575 is in the unipolar single supply mode. Figure 15 shows the circuit connections to achieve this, while the nominal transfer characteristic for unipolar opera- tion is given in Figure 16. Since the offset and full-scale errors on the AD7575 are very small, in many cases it will not be nec- essary to adjust out these errors. If calibration is required, the procedure is as follows: Offset Adjust Offset error adjustment in single-supply systems is easily achiev- able by means of the offset null facility of an op amp when used as a voltage follower for the analog input signal, AIN. The op amp chosen should be able to operate from a single supply and allow a common-mode input voltage range that includes 0 V (e.g., TLC271). To adjust for zero offset, the input signal source is set to +4.8 mV (i.e., 1/2 LSB) while the op amp offset is varied until the ADC output code flickers between 000 . . . 00 and 000 . . . 01. Full-Scale Adjust The full scale or gain adjustment is made by forcing the analog input AIN to +2.445 V (i.e., Full-Scale Voltage –3/2 LSB). The magnitude of the reference voltage is then adjusted until the ADC output code flickers between 111 . . . 10 and 111. . . 11. BIPOLAR OPERATION The circuit of Figure 17 shows how the AD7575 can be config- ured for bipolar operation. The output code provided by the AD7575 is offset binary. The analog input voltage range is ±5 V, although the voltage appearing at the AIN pin of the AD7575 is in the range 0 V to +2.46 V. Figure 18 shows the transfer function for bipolar operation. The LSB size is now 39.06 mV. Calibration of the bipolar operation is outlined be- low. Once again, because the errors are small, it may not be necessary to adjust them. To maintain specified performance without the calibration, all resistors should be 0.1% tolerance with R4 and R5 replaced by one 3.3 k Ω resistor and R2 and R3 replaced by one 2.5 k Ω resistor. Offset Adjust Offset error adjustment is achieved by applying an analog input voltage of –4.9805 V (–FS +1/2 LSB). Resistor R3 is then adjusted until the output code flickers between 000 . . . 00 and 000 . . . 01. Full-Scale Adjust Full-scale or gain adjustment is made by applying an analog input voltage of +4.9414 V (+FS –3/2 LSB). Resistor R4 is then adjusted until the output code flickers between 111 . . . 10 and 111. . . 11. A A 47 F 0.1 F +5V +2.46V MAX A A 47 F 0.1 F A A + – AD589 3.3k +5V +1.23V CONTROL INPUTS +5V AIN VREF AGND CLK BUSY CS RD TP VDD D DB7–DB0 DATA OUT AD7575 DGND D +5V RCLK 100k , 1% CCLK 100pF, 2% Figure 15. Unipolar Configuration OUTPUT CODE FULL SCALE TRANSITION 11111111 11111110 11111101 00000011 00000010 00000001 00000000 FS = 2VREF 1LSB = FS 256 1LSB 3LSBs 2LSBs FS FS –1LSB AIN, INPUT VOLTAGE (IN TERMS OF LSBs) Figure 16. Nominal Transfer Characteristic for Unipolar Operation A A 47 F 0.1 F +5V A A 47 F 0.1 F A + – AD589 +5V +5V AIN VREF CLK BUSY CS RD TP VDD D DB7–DB0 DATA OUT AD7575 DGND D +5V RCLK 100k , 1% CCLK 100pF, 2% + A AGND R2 2.2k R3 500 INPUT VOLTAGE R4 500 R5 3k A A +5V R7 2.5k R6 2.5k A TLC271 R8 3.3k R1 10k Figure 17. Bipolar Configuration –1/2LSB +1/2LSB –FS +FS –1LSB AIN FS = 5V 1LSB = FS 256 OUTPUT CODE 111...111 111...110 100...010 100...001 100...000 011...111 011...110 000...001 000...000 Figure 18. Nominal Transfer Characteristic for Bipolar Operation |
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