REV. E
–5–
AD7814
CIRCUIT INFORMATION
The AD7814 is a 10-bit digital temperature sensor. The part
houses an on-chip temperature sensor, a 10-bit A/D converter,
a reference, and serial interface logic functions in a SOT-23
package. The A/D converter section consists of a conventional
successive-approximation converter based around a capacitor
DAC. The parts are capable of running on a 2.7 V to 5.5 V
power supply.
The on-chip temperature sensor allows an accurate measurement
of the ambient device temperature to be made. The working
measurement range of the AD7814 is –55°C to +125°C.
CONVERTER DETAILS
The conversion clock for the part is internally generated so no
external clock is required except when reading from and writing
to the serial port. In normal mode, an internal clock oscillator
runs the automatic conversion sequence. A conversion is initi-
ated approximately every 350 µs. At this time, the part wakes up
and performs a temperature conversion. This temperature con-
version typically takes 25 µs, at which time the part automati-
cally shuts down. The result of the most recent temperature
conversion is available in the serial output register at any time.
Once the conversion is finished, an internal oscillator starts
counting and is designed to time out every 350 µs. The AD7814
then powers up and does a conversion. Please note that if the
CS is brought low every 350 µs (± 30%), then the same tempera-
ture value will be output onto the DOUT line every time with-
out changing. It is recommended that the CS line is not brought
low every 350 µs (± 30%). The ± 30% covers process variation.
The CS should become active (high to low) outside this range.
The device is designed to autoconvert every 350 µs. If the
AD7814 is accessed during the conversion process, an internal
signal is generated to prevent any update of the temperature
value register during the conversion. Otherwise the user could
read back spurious data. The design of this feature results in this
internal lockout signal being reset only at the start of the next
autoconversion. Therefore, if the CS line goes active before the
internal lockout signal has been reset to its inactive mode, the
internal lockout signal will not be reset. To ensure that no lock-
out signal is set, bring the CS low at a greater or less time than
350 µs (± 30%). This will mean that the AD7814 is not inter-
rupted during a conversion process. The AD7814 can be placed
in a shutdown mode, via the control register, in which case the
on-chip oscillator is shut down and no further conversions are
initiated until the AD7814 is taken out of shutdown mode. The
conversion result from the last conversion prior to shutdown can
still be read from the AD7814 even when it is in shutdown mode.
In the automatic conversion mode, every time a read or write
operation takes place, the internal clock oscillator is restarted at
the end of the read or write operation. The result of the con-
version is typically available 25 µs later. Similarly, when the
part is taken out of shutdown mode, the internal clock oscil-
lator is restarted and the conversion result is typically available
25 µs later. Reading from the device before conversion is com-
plete will provide the same set of data.
TEMPERATURE VALUE REGISTER
The temperature value register is a read-only register that stores
the temperature reading from the ADC in 10-bit twos comple-
ment format. The temperature data format is shown in Table I.
This shows the full theoretical range of the ADC from –128°C
to +127°C, but in practice the temperature measurement range
is limited to the operating temperature range of the device (–55°C
to +125°C). A typical performance curve is shown in Figure 7.
Table I. Temperature Data Format
Digital Output
Temperature DB9 . . . DB0
–128°C 10 0000 0000
–125°C 10 0000 1100
–100°C 10 0111 0000
–75°C 10 1101 0100
–50°C 11 0011 1000
–25°C 11 1001 1100
–0.25°C 11 1111 1111
0°C 00 0000 0000
+0.25°C 00 0000 0001
+10°C 00 0010 1000
+25°C 00 0110 0100
+50°C 00 1100 1000
+75°C 01 0010 1100
+100°C 01 1001 0000
+125°C 01 1111 0100
+127°C 01 1111 1100
SERIAL INTERFACE
The serial interface on the AD7814 consists of four wires CS,
SCLK, DIN, and DOUT. The interface can be operated in
3-wire mode with DIN tied to Ground, in which case the inter-
face has read-only capability, with data being read from the data
register via the DOUT line. The DIN line is used to write the part
into standby mode, if required. The CS line is used to select the
device when more than one device is connected to the serial
clock and data lines.
The part operates in a slave mode and requires an externally
applied serial clock to the SCLK input to access data from the
data register. The serial interface on the AD7814 is designed to
allow the part to be interfaced to systems that provide a serial
clock that is synchronized to the serial data, such as the 80C51,
87C51, 68HC11, 68HC05, and PIC16Cxx microcontrollers, as
well as DSP processors.
A read operation from the AD7814 accesses data from the
temperature value register, while a write operation to the part
writes data to the control register.
