參數(shù)資料
型號(hào)： AD7877ACP-REEL
廠商： ANALOG DEVICES INC
元件分類： 消費(fèi)家電
英文描述： Touch Screen Controller
中文描述： SPECIALTY CONSUMER CIRCUIT, QCC32
封裝： 5 X 5 MM, MO-220VHHD-2, LFCSP-32
文件頁數(shù)： 18/44頁
文件大?。?/td> 1002K
代理商： AD7877ACP-REEL
AD7877
Rev. A | Page 18 of 44
0
TEMP1
TEMP2
MUX
ADC
I
V
BE
105
×
I
Figure 32. Block Diagram of Temperature Measurement Circuit
Temperature Calculations
If an explicit temperature reading in °C is required, then this
can be 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 =
V
REF
/4096)/
2.1 mV
2.
Save the ADC output
D
CAL
at the calibration temperature
T
CAL
.
3.
Take ADC reading
D
AMB
at temperature to be measured
T
AMB
.
4.
Calculate the difference in degrees between
T
CAL
and
T
AMB
using
T
= (
D
AMB
D
CAL
) ×
degrees per LSB
5.
Add
T
to
T
CAL
.
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
T
AMB
is 880.
4.
T
= (880
983) ×
0.291 = 30°.
5.
T
AMB
= 25 + 30 = 55°C.
To calculate the temperature explicitly using the differential
method:
1.
Calculate the LSB size of the ADC in V:
LSB
=
V
REF
/4096
2.
Subtract
TEMP1
from
TEMP2
and multiply by LSB size to
get
V
BE
.
3.
Multiply by 2490 and subtract 273 to get 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:
V
BE
= (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 33 shows a block diagram
of a battery voltage monitored through the BAT1 pin. The
voltage to the V
CC
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.
0
ADC
0.25V–2.5V
SW
BAT1
V
CC
V
REF
5k
5k
DC-DC
CONVERTER
BATTERY
0.5V TO 5V
Figure 33. Block Diagram of Battery Measurement Circuit
Figure 33 shows the ADC using the internal reference of 2.5 V.
If a different reference voltage is used, then the maximum
battery voltage that the AD7877 can measure changes. The
maximum voltage measurable is V
REF
× 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 battery voltage measurable
is 4 V. If a larger reference is used, such as 3.5 V, then the
maximum battery voltage measurable is 7 V. The internal
reference is particularly suited for use when measuring Li-Ion
batteries, where the minimum voltage is about 2.7 V and the
maximum is about 4.2 V. A proper choice of external reference
ensures that other voltage ranges can be accommodated.
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