AD7873
Data Sheet
the AD7873 is approximately 70 Ω. In this mode of operation,
therefore, some voltage is likely to be lost across the internal
switches, and it is unlikely that the internal switch resistance
will track the resistance of the touch screen over temperature
and supply, providing an additional source of error.
In the single conversion method, a diode voltage is digitized
and recorded at a fixed calibration temperature. Any subsequent
polling of the diode provides an estimate of the ambient tempera-
ture through extrapolation from the calibration temperature
diode result. This assumes a diode temperature drift of
approximately –2.1 mV/°C. This method provides a resolution
+V
CC
of approximately 0.3°C and a predicted accuracy of 3°C.
The differential conversion method is a two-point measurement.
The first measurement is performed with a fixed bias current
into a diode, and the second measurement is performed with a
fixed multiple of the bias current into the same diode. The
voltage difference in the diode readings is proportional to
absolute temperature and is given by the following formula:
Y+
X+
V
REF
REF+
IN+
IN+
ADC CORE
REF–
IN–
Y–
∆VBE
where:
BE represents the diode voltage.
=
(
kT / q
)
×
ln N
GND
DFR
Figure 28. Single-Ended Reference Mode (SER/
= 1)
V
The alternative to this situation is to set the SER/
DFR
bit low.
N is the bias current multiple.
k is Boltzmann’s constant.
q is the electron charge.
Again, making a Y coordinate measurement is considered, but
now the +REF and –REF nodes of the ADC are connected
directly to the Y+ and Y– pins. This means the analog-to-digital
conversion is ratiometric. The result of the conversion is always
a percentage of the external resistance, independent of how it
could change with respect to the on resistance of the internal
switches. Figure 29 shows the configuration for a ratiometric
Y coordinate measurement.
This method provides more accurate absolute temperature
measurement of 2°C. However, the resolution is reduced to
approximately 1.6°C. Assuming a current multiple of 105
(typical for the AD7873) taking Boltzmann’s constant,
k = 1.38054 ×10–23 electrons volts/degrees Kelvin, the electron
charge q = 1.602189 × 10–19, then T, the ambient temperature in
degrees centigrade, can be calculated as follows:
+V
CC
Y+
∆VBE
T =
=
(
kT / q
)
×
(
ln N
)
(
∆VBE × q
T(°C) = 2.49 × 103 × ∆VBE / 273 Κ
)
/
(
k × ln N
)
REF+
X+
Y–
IN+
IN+
ADC CORE
REF–
IN–
where ∆VBE is calculated from the difference in readings from
the first conversion and second conversion.
Figure 30 shows a block diagram of the temperature
measurement mode.
GND
DFR
Figure 29. Differential Reference Mode (SER/
= 0)
TEMP0 TEMP1
The disadvantage of this mode of operation is that during both
the acquisition phase and conversion process, the external
touch screen must remain powered. This results in additional
supply current for the duration of the conversion.
105 × I
I
MUX
ADC
MEASUREMENTS
Temperature Measurement
Figure 30. Block Diagram of Temperature Measurement Circuit
Two temperature measurement options are available on the
AD7873, the single conversion method and the differential
conversion method. Both methods are based on an on-chip
diode measurement.
Rev. F | Page 1± of 28
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