AD7298(RevPrA) Ver la hoja de datos (PDF) - Analog Devices
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AD7298 Datasheet PDF : 18 Pages
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AD7298
ADC Transfer Function
The output coding of the AD7298 is straight binary for the
analog input channel conversion results and twos complement,
for the temperature conversion result. The designed code
transitions occur at successive LSB values (that is, 1 LSB, 2 LSBs,
and so forth). The LSB size is VREF/4096 for the AD7298. The
ideal transfer characteristic for the AD7298 for straight binary
coding is shown in Figure 11.
111…111
111…110
•
•
111…000
•
011…111
•
•
000…010
000…001
000…000
0V 1LSB
1LSB = VREF/4096
+VREF – 1LSB
ANALOG INPUT
NOTES
VREF IS EITHER REFIN OR 2 × REFIN.
Figure 11. Straight Binary Transfer Characteristic
TEMPERATURE SENSOR OPERATION
The AD7298 contains one local temperature sensor. The on-
chip, band gap temperature sensor measures the temperature of
the AD7298 die. The temperature sensor module on the
AD7298 is based on the three current principle (see Figure 12),
where three currents are passed through a diode and the
forward voltage drop is measured, allowing the temperature to
be calculated free of errors caused by series resistance.
Figure 12. Top Level Structure of Internal Temperature Sensor
Preliminary Technical Data
The temperature conversion consists of two phases, the
integration followed by the conversion. The integration is
initiated on the CS falling edge. It takes a period of
approximately100μs to complete the integration and conversion
of the temperature result. When the integration is completed
the conversion is initiated automatically. Once the temperature
integration is initiated, the TSENSEBUSY signal goes high to
indicate that a temperature conversion is in progress and
remains high until the conversion is completed.
Theoretically, the temperature measuring circuit can measure
temperatures from –512°C to +511°C with a resolution of
0.25°C. However, temperatures outside TA (the specified
temperature range for the AD7298) are outside the guaranteed
operating temperature range of the device. The temperature
sensor is selected by setting the TSENSE bit in the control register.
Temperature Sensor Averaging
The AD7298 incorporates a temperature sensor averaging
feature to enhance the accuracy of the temperature
measurements. To enable the temperature sensor averaging
feature both the TSENSEAVG bit and the TSENSE bit must be
enabled in the control register. In this mode the temperature is
internally averaged to reduce the effect of noise on the
temperature result. The temperature is measured each time a
TSENSE conversion is performed and a moving average method is
used to determine the result in the TSENSE Average Result
Register. The average result is given by the following equation;
TSENSE _ AVG =
7 (Pr evious _ Re sult) + 1 (Current _ Re sult)
8
8
The TSENSE result read when averaging is enabled is TSENSEAVG
result, a moving average temperature measurement.
The first TSENSE conversion result given by the AD7298 after the
temperature sensor and averaging mode has been selected in
the control register (bit D1 & D5) is the actual first TSENSE
conversion result. If the control register is written to and the
content of the TSENSEAVG bit changed the averaging function is
reset and the next TSENSE average conversion result is the current
temperature conversion result. If the status of the TSENSEAVG bit
is not changed on successive writes to the control register, the
averaging function will not be reinitialized and will continue
calculating the cumulative average.
The user has the option of disabling the averaging by setting bit
TSENSEAVG to ‘0’ in the control register. The AD7298 defaults on
power-up with the averaging function disabled. The total time
to measure, a temperature channel is typically 100 μs.
Rev. PrA | Page 14 of 18
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