AN929 Datasheet, PDF (17/22 Pages) Microchip Technology – Temperature Measurement Circuits for Embedded Applications

An accurate thermistor is required for the PGA circuit to
provide an advantage with the variable gain. A BC
Components™ #2322 640 5103 thermistor was chosen
that has a resistance tolerance of 1% at 25°C and a ∆R
accuracy of 0.75%, which corresponds to a tempera-
ture accuracy of approximately 0.2°C at 25°C and 1°C
at 100°C. The accuracy of a standard thermistor is
typically ±2°C to ±5°C and the magnitude of the sensor
error is too large to achieve the improved resolution
benefit of an adjustable gain circuit.
FIGURE 26:
PGA V_OUT vs. Temperature.
AN929
Silicon IC Sensors
SERIAL OUTPUT
Figure 28 shows a schematic of the TC77 serial-output
sensor which integrates the temperature sensor, ADC
and digital registers on a single chip that is connected
to the processor through the SPI serial bus. The SPI
bus uses SCK, SI/O and CS pins to transmit and
receive data. Temperature is measured by monitoring
the voltage of a diode with a 13-bit ADC. The tempera-
ture data is stored in the Temperature register. If a
Temperature register read operation occurs while an
ADC conversion is in progress, the previous completed
conversion will be outputted. The Configuration
register is used to select either the continuous
temperature conversion or shut-down operating
modes. The Shutdown mode disables the temperature-
conversion circuitry to minimize the power consump-
tion. However, the serial I/O communication port
remains active. The test registers are used for offset
and gain calibration by the vendor and are not available
to the user.
FIGURE 27:
PGA ∆V_OUT vs. Temperature.
V_DD
0.1 µF
V_DD
TC77
PICmicro^®
MCU
CS
SCK
SI/O
V_SS
AN0
SCK
SDI
SDO
V_DD
Internal
Diode
Temperature
Sensor
13-Bit
Delta-Sigma
ADC
Temperature
Register
V_SS
Calibration
Registers
TC77
Manufacturer
ID Register
Serial
Port
Interface
(SPI™)
Configuration
Register
CS
SI/O
SCK
FIGURE 28:
Interfacing a TC77 Silicon IC Sensor to a Microcontroller.
 2004 Microchip Technology Inc.
DS00929A-page 17