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LTC2411IMS Datasheet(PDF) 11 Page - Linear Technology |
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LTC2411IMS Datasheet(HTML) 11 Page - Linear Technology |
11 / 40 page LTC2411/LTC2411-1 11 CONVERTER OPERATION Converter Operation Cycle The LTC2411/LTC2411-1 are low power, delta-sigma ana- log-to-digital converters with an easy-to-use 3-wire serial interface (see Figure 1). Their operation is made up of three states. The converter operating cycle begins with the con- version, followed by the low power sleep state and ends with the data output (see Figure 2). The 3-wire interface consists of serial data output (SDO), serial clock (SCK) and chip select (CS). Initially, the LTC2411/LTC2411-1 perform a conversion. Once the conversion is complete, the devices enter the sleep state. While in this sleep state, power consumption is reduced by an order of magnitude. The parts remain in the sleep state as long as CS is HIGH. The conversion result is held indefinitely in a static shift register while the converter is in the sleep state. Once CS is pulled LOW, the devices begin outputting the conversion result. There is no latency in the conversion result. The data output corresponds to the conversion just performed. This result is shifted out on the serial data out pin (SDO) under the control of the serial clock (SCK). Data is updated on the falling edge of SCK allowing the user to reliably latch data on the rising edge of SCK (see Figure 3). The data output state is concluded once 32 bits are read out of the ADC or when CS is brought HIGH. The devices automatically initiate a new conversion and the cycle repeats. Through timing control of the CS and SCK pins, the LTC2411/LTC2411-1 offer several flexible modes of op- eration (internal or external SCK and free-running conver- sion modes). These various modes do not require programming configuration registers; moreover, they do not disturb the cyclic operation described above. These modes of operation are described in detail in the Serial Interface Timing Modes section. Conversion Clock A major advantage the delta-sigma converter offers over conventional type converters is an on-chip digital filter (commonly implemented as a Sinc or Comb filter). For high resolution, low frequency applications, this filter is typically designed to reject line frequencies of 50 or 60Hz plus their harmonics. The filter rejection performance is directly related to the accuracy of the converter system clock. The LTC2411/LTC2411-1 incorporate a highly ac- curate on-chip oscillator. This eliminates the need for external frequency setting components such as crystals or oscillators. Clocked by the on-chip oscillator, the LTC2411 achieves a minimum of 110dB rejection at the line fre- quency (50Hz or 60Hz ±2%) and the LTC2411-1 achieves a minimum of 87dB rejection over 49Hz to 61.2Hz. Ease of Use The LTC2411/LTC2411-1 data output has no latency, filter settling delay or redundant data associated with the conversion cycle. There is a one-to-one correspondence between the conversion and the output data. Therefore, multiplexing multiple analog voltages is easy. The LTC2411/LTC2411-1 perform offset and full-scale calibrations in every conversion cycle. This calibration is transparent to the user and has no effect on the cyclic operation described above. The advantage of continuous calibration is extreme stability of offset and full-scale read- ings with respect to time, supply voltage change and tem- perature drift. Power-Up Sequence The LTC2411/LTC2411-1 automatically enter an internal reset state when the power supply voltage VCC drops below approximately 1.9V. This feature guarantees the Figure 2. LTC2411/LTC2411-1 State Transition Diagram CONVERT SLEEP DATA OUTPUT 2411 F02 TRUE FALSE CS = LOW AND SCK APPLICATIO S I FOR ATIO |
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