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AD7796 Datasheet(PDF) 21 Page - Analog Devices |
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AD7796 Datasheet(HTML) 21 Page - Analog Devices |
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21 / 24 page  AD7796/AD7797 Rev. A | Page 21 of 24 RESET The circuitry and serial interface of the AD7796/AD7797 can be reset by writing 32 consecutive 1s to the device. This resets the logic, the digital filter, and the analog modulator, while all on-chip registers are reset to their default values. A reset is automatically performed on power-up. When a reset is initiated, the user must allow a period of 500 μs before accessing any of the on-chip registers. A reset is useful if the serial interface becomes asynchronous because of noise on the SCLK line. BURNOUT CURRENTS The AD7796/AD7797 contain two 100 nA constant current generators, one sourcing current from AVDD to AIN(+) and one sinking current from AIN(–) to GND. Both currents are either on or off, depending on the burnout current enable (BO) bit in the configuration register. These currents can be used to verify that an external transducer is still operational before attempting to take measurements. When the burnout currents are turned on, they flow in the external transducer circuit, and a measure- ment of the input voltage on the analog input channel can be taken. If the resulting voltage is full scale, the user needs to verify why this is the case. A full-scale reading could mean that the front-end sensor is open circuit. It could also mean that the front-end sensor is overloaded and is justified in outputting full scale, or that the reference could be absent, thus clamping the data to all 1s. When reading all 1s from the output, the user needs to check these three cases before making a judgment. If the voltage measured is 0 V, it could indicate that the transducer has short circuited. For normal operation, these burnout currents are turned off by writing a 0 to the BO bit in the configuration register. AVDD MONITOR Along with converting external voltages, the ADC can be used to monitor the voltage on the AVDD pin. When Bit CH2 to Bit CH0 equal 1, the voltage on the AVDD pin is internally attenuated by 6. The resulting voltage is applied to the Σ-Δ modulator using an internal 1.17 V reference for analog-to- digital conversion. This is useful because variations in the power supply voltage can be monitored. TEMPERATURE MONITOR The AD7796/AD7797 have an embedded temperature sensor that is accessed when Bit CH2 to Bit CH0 are equal to 1, 1, 0, respectively. When the internal temperature sensor is selected, the AD7796/AD7797 use an internal 1.17 V reference for the conversions. The temperature sensor has a sensitivity of 0.81 mV/°C. However, a two-point calibration is required to optimize the accuracy. The temperature sensor is not factory calibrated; a user calibration is required. Following a calibration, the accuracy is 2°C. CALIBRATION The AD7796/AD7797 provide three calibration modes that can be programmed via the mode bits in the mode register. These are internal zero-scale calibration, system zero-scale calibration, and system full-scale calibration, which effectively reduces the offset error and full-scale error to the order of the noise. After each conversion, the ADC conversion result is scaled using the ADC calibration registers before being written to the data register. The offset calibration coefficient is subtracted from the result prior to multiplication by the full-scale coefficient. To start a calibration, write the relevant value to the MD2 to MD0 bits in the mode register. DOUT/RDY goes high when the calibration is initiated. After the calibration is complete, the contents of the corresponding calibration registers are updated, the RDY bit in the status register is set, the DOUT/ RDY pin goes low (if CS is low), and the AD7796/AD7797 revert to idle mode. During an internal zero-scale calibration, the zero input is automatically connected internally to the ADC input pins. A system calibration, however, expects the system zero-scale and system full-scale voltages to be applied to the ADC pins before the calibration mode is initiated. In this way, external ADC errors are removed. From an operational point of view, a calibration should be treated like another ADC conversion. A zero-scale calibration (if required) should always be performed before a full-scale calibration. System software should monitor the RDY bit in the status register or the DOUT/RDY pin to determine the end of calibration via a polling sequence or an interrupt-driven routine. Both an internal offset calibration and system offset calibration takes two conversion cycles. An internal offset calibration is not needed because the ADC itself removes the offset continuously. A system full-scale calibration takes two conversion cycles to complete. The measured full-scale coefficient is placed in the full-scale register. If system offset calibrations are being performed along with system full-scale calibrations, the offset calibration should be performed before the system full-scale calibration is initiated. |
Similar Part No. - AD7796_15 |
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Similar Description - AD7796_15 |
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