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ADS1210 Datasheet(PDF) 16 Page - Texas Instruments |
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ADS1210 Datasheet(HTML) 16 Page - Texas Instruments |
16 / 50 page ![]() ADS1210, ADS1211 16 SBAS034B www.ti.com Also, during this cycle, the sampling capacitor is discon- nected from the converter’s analog input and is connected across REFIN. A gain calibration is initiated and proceeds over the next three conversions. After this, the input capaci- tor is once again connected to the analog input. Conversions proceed as usual over the next three cycles in order to fill the digital filter. DRDY remains HIGH during this time. On the next cycle, the DRDY signal goes LOW indicating valid data, the input to the sampling capacitor is shorted, and an offset calibration is initiated. At this point, the Background Calibration sequence repeats. In essence, the Background Calibration Mode performs continuous self-calibration where the offset and gain cali- brations are interleaved with regular conversions. Thus, the data rate is reduced by a factor of 6. The advantage is that the converter is continuously adjusting to environmental changes such as ambient or component temperature (due to airflow variations). The ADS1210/11 will remain in the Background Calibra- tion Mode indefinitely. To move to any other mode, the Command Register Operation Mode bits (MD2 through MD0) must be set to the appropriate values. System Calibration Offset and Full-Scale Calibration Limits The System Offset and Full-Scale Calibration range of the ADS1210/11 is limited and is listed in the Specifications Table. The range is specified as: (VFS – | VOS |) < 1.3 • (2 • REFIN)/GAIN (VFS – | VOS |) > 0.7 • (2 • REFIN)/GAIN where VFS is the system full-scale voltage and | VOS | is the absolute value of the system offset voltage. In the following discussion, keep in mind that these voltages are differential voltages. For example, with the internal reference (2.5V) and a gain of two, the previous equations become (after some manipulation): VFS – 3.25 < VOS < VFS – 1.75 If VFS is perfect at 2.5V (positive full-scale), then VOS must be greater than –0.75V and less than 0.75V. Thus, when offset calibration is performed, the positive input can be no more than 0.75V below or above the negative input. If this range is exceeded, the ADS1210/11 may not calibrate properly. This calculation method works for all gains other than one. For a gain of one and the internal reference (2.5V), the equation becomes: VFS – 6.5 < VOS < VFS – 3.5 With a 5V positive full-scale input, VOS must be greater than –1.5V and less than 1.5V. Since the offset represents a common-mode voltage and the input voltage range in a gain of one is 0V to 5V, a common-mode voltage will cause the actual input voltage to possibly go below 0V or above 5V. The specifications also show that for the specifications to be valid, the input voltage must not go below AGND by more than 30mV or above AVDD by more than 30mV. This will be an important consideration in many systems which use a 2.5V or greater reference, as the input range is constrained by the expected power supply variations. In addition, the expected full-scale voltage will impact the allowable offset voltage (and vice-versa) as the combination of the two must remain within the power supply and ground potentials, regardless of the results obtained via the range calculation shown previously. There are only two solutions to this constraint: either the system design must ensure that the full-scale and offset voltage variations will remain within the power supply and ground potentials, or the part must be used in a gain of 2 or greater. SLEEP MODE The Sleep Mode is entered after the bits 110 have been written to the Command Register Operation Mode bits (MD2 through MD0). This mode is exited by entering a new mode into the MD2-MD0 bits. The Sleep Mode causes the analog section and a good deal of the digital section to power down. For full analog power down, the VBIAS generator and the internal reference must also be powered down by setting the BIAS and REFO bits in the Command Register accordingly. The power dissipa- tion shown in the Specifications Table is with the internal reference and the VBIAS generator disabled. To initiate serial communication with the converter while it is in Sleep Mode, one of the following procedures must be used: If CS is being used, simply taking CS LOW will enable serial communication to proceed normally. If CS is not being used (tied LOW) and the ADS1210/11 is in the Master Mode, then a falling edge must be produced on the SDIO line. If SDIO is LOW, the SDIO line must be taken HIGH for 2 • tXIN periods (minimum) and then taken LOW. Alternatively, SDIO can be forced HIGH after putting the ADS1210/11 to “sleep” and then taken LOW when the Sleep Mode is to be exited. Finally, if CS is not being used (tied LOW) and the ADS1210/11 is in the Slave Mode, then simply sending a normal Instruction Register command will re-establish communication. Once serial communication is resumed, the Sleep Mode is exited by changing the MD2-MD0 bits to any other mode. When a new mode (other than Sleep) has been entered, the ADS1210/11 will execute a very brief internal power-up sequence of the analog and digital circuitry. Once this has been done, one normal conversion cycle is performed before the new mode is actually entered. At the end of this conversion cycle, the new mode takes effect and the converter will respond accordingly. The DRDY signal will remain HIGH through the first conversion cycle. It will also remain HIGH through the second, even if the new mode is the Normal Mode. If the VBIAS generator and/or the internal reference have been disabled, then they must be manually re-enabled via the appropriate bits in the Command Register. In addition, the internal reference will have to charge the external bypass capacitor(s) and possibly other circuitry. There may also be |
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