ATA8741-PXQW【PDF 165/238 页】MCU W/TRANSMITTER ASK/FSK 24QFN

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Sheet目录52

161页 162页 163页 164页 [165页]166页 167页 168页 169页

165

9140CINDCO02/10

ATA8741

than the voltage of the negative pin or otherwise the voltage difference is saturated to zero. The

result is presented in one-sided form, from 0x000 (0d) through 0x3FF (+1023d). The GAIN is

either 1x or 20x.

25.8.3

Bipolar Differential Conversion

If differential channels and a bipolar input mode are used, the result is

where VPOS is the voltage on the positive input pin, VNEG the voltage on the negative input pin,

and VREF the selected voltage reference. The result is presented in twos complement form, from

0x200 (-512d) through 0x1FF (+511d). The GAIN is either 1x or 20x. Note that if the user wants

to perform a quick polarity check of the result, it is sufficient to read the MSB of the result (ADC9

in ADCH). If the bit is one, the result is negative, and if this bit is zero, the result is positive.

As default the ADC converter operates in the unipolar input mode, but the bipolar input mode

can be selected by writting the BIN bit in the ADCSRB to one. In the bipolar input mode

two-sided voltage differences are allowed and thus the voltage on the negative input pin can

also be larger than the voltage on the positive input pin.

25.9 Temperature Measurement

The temperature measurement is based on an on-chip temperature sensor that is coupled to a

single ended ADC8 channel. Selecting the ADC8 channel by writing the MUX5:0 bits in ADMUX

selected for the ADC reference source in the temperature sensor measurement. When the tem-

perature sensor is enabled, the ADC converter can be used in single conversion mode to

measure the voltage over the temperature sensor. The measured voltage has a linear relation-

ship to the temperature as described in Table 51. The voltage sensitivity is approximately 1 mV /

?SPAN class="pst ATA8741-PXQW_2172485_3">C and the accuracy of the temperature measurement is +/- 10?SPAN class="pst ATA8741-PXQW_2172485_3">C after offset calibration. Band-

gap is always calibrated and its accuracy is only guaranteed between 1.0V and 1.2V

The values described in Table 25-2 on page 165 are typical values. However, due to the process

variation the temperature sensor output voltage varies from one chip to another. To be capable

of achieving more accurate results the temperature measurement can be calibrated in the appli-

cation software. The software calibration requires that a calibration value is measured and

stored in a register or EEPROM for each chip, as a part of the production test. The sofware cali-

bration can be done utilizing the formula:

T = {[(ADCH << 8) | ADCL] - TOS} / k

where ADCn are the ADC data registers, k is a fixed coefficient and T

is the temperature sen-

sor offset value determined and stored into EEPROM as a part of the production test.To obtain

best accuracy the coefficient k should be measured using two temperature calibrations. Using

offset calibration, set k = 1.0, where k = (1024*1.07mV/癈)/1.1V~1.0 [1/癈].

ADC

POS

NEG

(

) 512

?/DIV>

REF

------------------------------------------------------- GAIN

?/DIV>

Table 25-2. Temperature vs. Sensor Output Voltage (Typical Case)

Temperature / 癈

-40癈

+25癈

+85癈

+125癈

Voltage / mV

243 mV

314 mv

380 mV

424 mV

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