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| 型號： | AD7877ACPZ-REEL7 |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 11/45頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC ADC 12BIT TOUCHSCREEN 32LFCSP |
| 標準包裝： | 1,500 |
| 類型： | 電阻 |
| 觸摸面板接口： | 4 線 |
| 輸入數(shù)/鍵： | 1 TSC |
| 分辨率（位）： | 12 b |
| 評估套件： | 可供 |
| 數(shù)據(jù)接口： | 串行，SPI? |
| 數(shù)據(jù)速率/采樣率 (SPS，BPS)： | 125k |
| 電壓基準： | 外部，內(nèi)部 |
| 電源電壓： | 2.7 V ~ 5.25 V |
| 電流 - 電源： | 1µA |
| 工作溫度： | -40°C ~ 85°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 32-VFQFN 裸露焊盤，CSP |
| 供應(yīng)商設(shè)備封裝： | 32-LFCSP-VQ |
| 包裝： | 帶卷 (TR) |
| 配用： | EVAL-AD7877EBZ-ND - BOARD EVALUATION FOR AD7877 |
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AD7877
Data Sheet
Rev. D | Page 18 of 44
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 × 103 = 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 × 104 V(610 V)
2. TEMP1 = 880 and TEMP2 = 1103:
VBE = (1103 880) × 6.1× 104 = 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.
03796-012
ADC
0.25V–2.5V
SW
BAT1
VCC
VREF
5k
5k
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.
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