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AD7490BRUZ-REEL Datasheet(PDF) 18 Page - Analog Devices |
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AD7490BRUZ-REEL Datasheet(HTML) 18 Page - Analog Devices |
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18 / 29 page  Data Sheet AD7490 Rev. D | Page 17 of 28 When no amplifier is used to drive the analog input, the source impedance should be limited to low values. The maximum source impedance depends on the amount of total harmonic distortion (THD) that can be tolerated. The THD increases as the source impedance increases, and performance degrades (see Figure 9). ADC TRANSFER FUNCTION The output coding of the AD7490 is either straight binary or twos complement depending on the status of the LSB (CODING bit) in the control register. The designed code transitions occur midway between successive LSB values (that is, 1 LSB, 2 LSBs, and so on). The LSB size is equal to REFIN/4096. The ideal transfer characteristic for the AD7490 when straight binary coding is selected is shown in Figure 18. 111...111 111...110 111...000 000...010 011...111 000...001 000...000 0V 1LSB +VREF – 1LSB 1LSB = VREF/4096 VREF IS EITHER REFIN OR 2 × REFIN ANALOG INPUT Figure 18. Straight Binary Transfer Characteristic Handling Bipolar Input Signals Figure 20 shows how useful the combination of the 2 × REFIN input range and the twos complement output coding scheme is for handling bipolar input signals. If the bipolar input signal is biased about REFIN and twos complement output coding is selected, REFIN becomes the zero code point, −REFIN is negative full scale, and +REFIN becomes positive full scale, with a dynamic range of 2 × REFIN. 011...111 011...110 000...001 000...000 111...111 100...010 100...001 100...000 +VREF – 1LSB –VREF + 1LSB 1LSB = 2 × VREF/4096 VREF – 1LSB ANALOG INPUT Figure 19. Twos Complement Transfer Characteristic with REFIN ± REFIN Input Range VREF REFIN VIN0 DOUT TWOS COMPLEMENT DSP/µP VDRIVE VDD VDD AD7490 VIN15 V 0V V 0.1µF R3 R1 = R2 = R3 = R4 R2 R1 R4 +REFIN (= 2 × REFIN) –REFIN (= 0V) REFIN 011...111 000...000 100...000 Figure 20. Handling Bipolar Signals |
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