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AD7877 Scheda tecnica(PDF) 19 Page - Analog Devices |
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AD7877 Scheda tecnica(HTML) 19 Page - Analog Devices |
19 / 46 page AD7877 Data Sheet Temperature Calculations If an explicit temperature reading in °C is required, then this is calculated as follows for the single measurement method: 1. Calculate the scale factor of the ADC in degrees per LSB: Degrees per LSB = ADC LSB size/−2.1 mV = (VREF/4096)/−2.1 mV 2. Save the ADC output, DCAL, at the calibration temperature, TCAL. 3. Take ADC reading, DAMB, at the temperature to be measured, TAMB. 4. Calculate the difference in degrees between TCAL and TAMB using ∆T = (DAMB − DCAL) × degrees per LSB 5. Add ∆T to TCAL Example: The internal 2.5 V reference is used. 1. Degrees per LSB = (2.5/4096)/−2.1 × 10−3 = −0.291 2. The ADC output is 983 decimal at 25°C, equivalent to a diode forward voltage of 0.6 V. 3. The ADC output at TAMB is 880. 4. ∆T = (880 − 983) × −0.291 = 30° 5. TAMB = 25 + 30 = 55°C To calculate the temperature explicitly using the differential method: 1. Calculate the LSB size of the ADC in V: LSB = VREF/4096 2. Subtract TEMP1 from TEMP2 and multiply by LSB size to get ∆VBE. 3. Multiply by 2490 and subtract 273 to obtain the temperature in °C. Example: The internal 2.5 V reference is used. 1. LSB size = 2.5 V/4096 = 6.1 × 10−4 V(610 µV) 2. TEMP1 = 880 and TEMP2 = 1103: ∆VBE = (1103 − 880) × 6.1× 10−4 = 0.136 V 3. T = 0.136 × 2490 − 273 = 65°C BATTERY MEASUREMENT The AD7877 can monitor battery voltages from 0.5 V to 5 V on two inputs, BAT1 and BAT2. Figure 34 shows a block diagram of a battery voltage monitored through the BAT1 pin. The voltage to the VCC pin of the AD7877 is maintained at the desired supply voltage via the dc/dc regulator while the input to the regulator is monitored. This voltage on BAT1 is divided down by 2 internally, so that a 5 V battery voltage is presented to the ADC as 2.5 V. To conserve power, the divider circuit is on only during the sampling of a voltage on BAT1. The BAT2 input circuitry is identical. The BAT1 input is ADC Channel 0110b and the result is stored in Register 10110b. The BAT2 input is ADC Channel 0111b and the result is stored in Register 10111b. ADC 0.25V–2.5V SW BAT1 VCC VREF 5k Ω 5k Ω DC-DC CONVERTER BATTERY 0.5V TO 5V Figure 34. Block Diagram of Battery Measurement Circuit Figure 34 shows the ADC using the internal reference of 2.5 V. The maximum battery voltage that the AD7877 can measure changes when a different reference voltage is used. The maxi- mum voltage that is measurable is VREF × 2, because this voltage gives a full-scale output from the ADC. If a smaller reference is used, such as 2 V, then the maximum measurable battery voltage is 4 V. If a larger reference is used, such as 3.5 V, then the maxi- mum measurable battery voltage is 7 V. The internal reference is particularly suited for use when measuring lithium-ion batteries, wherein the minimum voltage is about 2.7 V and the maximum voltage is about 4.2 V. A proper choice of external reference ensures that other voltage ranges can be accommodated. Rev. E | Page 18 of 45 |
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