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ADS1240 Arkusz danych(PDF) 11 Page - Texas Instruments |
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ADS1240 Arkusz danych(HTML) 11 Page - Texas Instruments |
11 / 31 page ADS1240, 1241 11 SBAS173F www.ti.com Figure 4 shows a short-circuited sensor. Since the inputs are shorted and at the same potential, the ADS1240/41 signal outputs are approximately zero. (Note that the code for shorted inputs is not exactly zero due to internal series resistance, low-level noise and other error sources.) The buffer draws additional current when activated. The current required by the buffer depends on the PGA setting. When the PGA is set to 1, the buffer uses approximately 50 µA; when the PGA is set to 128, the buffer uses approxi- mately 500 µA. PGA The Programmable Gain Amplifier (PGA) can be set to gains of 1, 2, 4, 8, 16, 32, 64, or 128. Using the PGA can improve the effective resolution of the A/D converter. For instance, with a PGA of 1 on a 5V full-scale signal, the A/D converter can resolve down to 1 µV. With a PGA of 128 and a full-scale signal of 39mV, the A/D converter can resolve down to 75nV. AVDD current increases with PGA settings higher than 4. OFFSET DAC The input to the PGA can be shifted by half the full-scale input range of the PGA using the Offset DAC (ODAC) register. The ODAC register is an 8-bit value; the MSB is the sign and the seven LSBs provide the magnitude of the offset. Using the offset DAC does not reduce the performance of the A/D converter. For more details on the ODAC, please refer to TI application report SBAA077. MODULATOR The modulator is a single-loop second-order system. The modulator runs at a clock speed (fMOD) that is derived from the external clock (fOSC). The frequency division is deter- mined by the SPEED bit in the SETUP register, as shown in Table I. OPEN CIRCUIT AV DD AV DD 0V 2 µA 2 µA CODE = 0x7FFFFF H ADC FIGURE 3. Burnout detection while sensor is open-circuited. SHORT CIRCUIT AV DD AV DD/2 AV DD/2 2 µA 2 µA CODE ≅ 0 ADC FIGURE 4. Burnout detection while sensor is short-circuited. INPUT BUFFER The input impedance of the ADS1240/41 without the buffer enabled is approximately 5M Ω/PGA. For systems requiring very high input impedance, the ADS1240/41 provides a chopper-stabilized differential FET-input voltage buffer. When activated, the buffer raises the ADS1240/41 input impedance to approximately 5G Ω. The buffer’s input range is approximately 50mV to AVDD – 1.5V. The buffer’s linearity will degrade beyond this range. Differential signals should be adjusted so that both signals are within the buffer’s input range. The buffer can be enabled using the BUFEN pin or the BUFEN bit in the ACR register. The buffer is on when the BUFEN pin is high and the BUFEN bit is set to one. If the BUFEN pin is low, the buffer is disabled. If the BUFEN bit is set to zero, the buffer is also disabled. SPEED DR BITS 1st NOTCH fOSC BIT fMOD 00 01 10 FREQ. 2.4576MHz 0 19,200Hz 15Hz 7.5Hz 3.75Hz 50/60Hz 1 9,600Hz 7.5Hz 3.75Hz 1.875Hz 25/30Hz 4.9152MHz 0 38,400Hz 30Hz 15Hz 7.5Hz 100/120Hz 1 19,200Hz 15Hz 7.5Hz 3.75Hz 50/60Hz TABLE I. Output Configuration. CALIBRATION The offset and gain errors can be minimized with calibration. The ADS1240 and ADS1241 support both self and system calibration. Self-calibration of the ADS1240 and ADS1241 corrects inter- nal offset and gain errors and is handled by three commands: SELFCAL, SELFGAL, and SLEFOCAL. The SELFCAL com- mand performs both an offset and gain calibration. SELFGCAL performs a gain calibration and SELFOCAL performs an offset calibration, each of which takes two tDATA periods to complete. During self-calibration, the ADC inputs are discon- nected internally from the input pins. The PGA must be set to 1 prior to issuing a SELFCAL or SELFGCAL command. Any PGA is allowed when issuing a SELFOCAL command. For example, if using PGA = 64, first set PGA = 1 and issue |
Podobny numer części - ADS1240_14 |
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Podobny opis - ADS1240_14 |
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