AD7877 Analog Devices, AD7877 Datasheet - Page 18

• Manufacturer Part Number: AD7877
• Description: Touch Screen Controller
• Manufacturer: Analog Devices

Specifications of AD7877

• Resolution (bits): 12bit
• # Chan: 9
• Sample Rate: 125kSPS
• Interface: Ser
• Analog Input Type: SE-Uni
• Ain Range: Uni (Vref),Uni 2.5V
• Adc Architecture: SAR
• Pkg Type: CSP

AD7877
Temperature Calculations
If an explicit temperature reading in °C is required, then this is
calculated as follows for the single measurement method:
1.
2.
3.
4.
5.
Example:
The internal 2.5 V reference is used.
1.
2.
3.
4.
5.
To calculate the temperature explicitly using the differential
method:
1.
2.
3.
Example:
The internal 2.5 V reference is used.
1.
2.
3.
Calculate the scale factor of the ADC in degrees per LSB:
Degrees per LSB = ADC LSB size/−2.1 mV =
(V
Save the ADC output, D
temperature, T
Take ADC reading, D
measured, T
Calculate the difference in degrees between T
using
∆T = (D
Add ∆T to T
Degrees per LSB = (2.5/4096)/−2.1 × 10
The ADC output is 983 decimal at 25°C, equivalent to a
diode forward voltage of 0.6 V.
The ADC output at T
∆T = (880 − 983) × −0.291 = 30°
T
Calculate the LSB size of the ADC in V:
LSB = V
Subtract TEMP1 from TEMP2 and multiply by LSB size to
get ∆V
Multiply by 2490 and subtract 273 to obtain the
temperature in °C.
LSB size = 2.5 V/4096 = 6.1 × 10
TEMP1 = 880 and TEMP2 = 1103:
∆V
T = 0.136 × 2490 − 273 = 65°C
AMB
REF
BE
/4096)/−2.1 mV
= 25 + 30 = 55°C
= (1103 − 880) × 6.1× 10
BE
REF
AMB
.
/4096
− D
AMB
CAL
CAL
.
CAL
.
) × degrees per LSB
AMB
AMB
CAL
, at the temperature to be
is 880.
, at the calibration
−4
−4
= 0.136 V
V(610 µV)
−3
= −0.291
CAL
and T
AMB
Rev. D | Page 18 of 44
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 V
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.
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 V
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.
0.5V TO 5V
BATTERY
Figure 34. Block Diagram of Battery Measurement Circuit
CC
pin of the AD7877 is maintained at the
CONVERTER
BAT1
DC-DC
5kΩ
5kΩ
SW
0.25V–2.5V
REF
V
CC
× 2, because this voltage
Data Sheet
V
ADC
REF