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參數(shù)資料
| 型號： | AD7877ACPZ-500RL7 |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 消費家電 |
| 英文描述： | Touch Screen Controller |
| 中文描述： | SPECIALTY CONSUMER CIRCUIT, QCC32 |
| 封裝： | 5 X 5 MM, ROHS COMPLIANT, MO-220VHHD-2, LFCSP-32 |
| 文件頁數(shù)： | 17/44頁 |
| 文件大?。?/td> | 1002K |
| 代理商： | AD7877ACPZ-500RL7 |
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AD7877
It is only during the sample or acquisition phase of the
AD7877’s ADC operation that noise from the LCD screen has
an effect on the ADC’s measurements. During the hold or
conversion phase, the noise has no effect, because the voltage at
the input of the ADC has already been acquired. Therefore, to
minimize the effect of noise on the touch screen measurements,
the ADC acquisition phase should be halted.
Rev. A | Page 17 of 44
The LCD control signal should be applied to the STOPACQ pin.
To ensure that acquisition never takes place during the noisy
period when the LCD signal is active, the AD7877 monitors this
signal. No acquisitions take place when the control signal is
active. Any acquisition that is in progress when the signal
becomes active is aborted and restarts when the signal becomes
inactive again.
To accommodate signals of different polarities on the
STOPACQ pin, a user-programmable register bit is used to
indicate whether the signal is active high or low. The POL bit is
Bit 3 in Control Register 2, Address 02h. Setting POL to 1
indicates that the signal on STOPACQ is active high; setting
POL to 0 indicates that it is active low. POL defaults to 0 on
power-up. To disable monitoring of STOPACQ, the pin should
be tied low if POL = 1, or tied high if POL = 0. Under no
circumstances should the pin be left floating.
The signal on STOPACQ has no effect while the ADC is in
conversion mode, or during the first conversion delay time. (See
the Control Registers section for details on first conversion
delay.)
When enabled, the STOPACQ monitoring function is imple-
mented on all input channels to the ADC: AUX1, AUX2, BAT1,
BAT2, TEMP1, and TEMP2, as well as on the touch screen input
channels.
TEMPERATURE MEASUREMENT
Two temperature measurement options are available on the
AD7877: the single conversion method and the differential
conversion method. The single conversion method requires
only a single measurement on ADC Channel 1000b. Differential
conversion requires two measurements, one on ADC Channel
1000b and a second on ADC Channel 1001b. The results are
stored in the results registers with Addresses 11000b (TEMP1)
and 11001b (TEMP2). The AD7877 does not provide an explicit
output of the temperature reading. Some external calculations
must be performed by the system. Both methods are based on
an on-chip diode measurement.
Single Conversion Method
The single conversion method makes use of the fact that the
temperature coefficient of a silicon diode is approximately
2.1 mV/°C. However, this small change is superimposed on the
diode forward voltage, which can have a wide tolerance. It is,
therefore, necessary to calibrate by measuring the diode voltage
at a known temperature to provide a baseline from which the
change in forward voltage with temperature can be measured.
This method provides a resolution of approximately 0.3°C and a
predicted accuracy of ±2.5°C.
The temperature limit comparison is performed on the result in
the TEMP1 results register, which is simply the measurement of
the diode forward voltage. The values programmed into the
high and low limits should be referenced to the calibrated diode
forward voltage to make accurate limit comparisons. An
example is shown in the Limit Comparison section.
Differential Conversion Method
The differential conversion method is a 2-point measurement.
The first measurement is performed with a fixed bias current
into a diode (when the TEMP1 channel is selected), and the
second measurement is performed with a fixed multiple of the
bias current into the same diode (when the TEMP2 channel is
selected). The voltage difference in the diode readings is
proportional to absolute temperature and is given by the
following formula:
V
BE
= (
KT
/
q
) × (1
n N
)
(4)
where:
V
BE
represents the diode voltage.
N
is the bias current multiple (typical value for AD7877 =120).
k
is Boltzmann’s constant.
q
is the electron charge.
This method provides a resolution of approximately 1.6°C, and
a guaranteed accuracy of ±4°C without calibration. Determina-
tion of the N value on a part-by-part basis improves accuracy.
Assuming a current multiple of 120, which is a typical value for
the AD7877, taking Boltzmann’s constant,
k
= 1.38054 ×
10
23
electrons V/°K, the electron charge
q
= 1.602189 × 10
19
,
then
T
, the ambient temperature in Kelvin, would be calculated
as follows:
V
BE
= (
KT
/
q
) × (1
n N
)
T
°K = (
V
BE
× q)/(k × 1n N)
=
V
BE
× 1.602189 × 10
19
)/(1.38054 × 10
23
× 4.65)
T
°C = 2.49
× 103 ×
V
BE
273
V
BE
is calculated from the difference in readings from the first
conversion and second conversion. The user must perform the
calculations to get
V
BE
, and then calculate the temperature
value in degrees.
Figure 32 shows a block diagram of the temperature
measurement circuit.
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