AN929 Datasheet, PDF (19/22 Pages) Microchip Technology – Temperature Measurement Circuits for Embedded Applications

AN929
Analog Output
Silicon Sensor
R_OUT
+ ^VOUT
C_FILTER
R_SWITCH
Sample
Hold
C_SAMPLE
ADC’s Input Stage
(R_OUT + R_SWITCH) × C_SAMPLE ≤ 0.1 × T_SAMPLE
FIGURE 30:
Interfacing an Analog Output Silicon Sensor to an ADC.
The sensor’s output impedance can affect the ADC.
The temperature sensor’s output pin is typically driven
by a buffer op amp, while the input of a typical ADC
consists of a sample and hold circuit with a switch that
connects V_OUT to C_SAMPLE. R_OUT, R_SWITCH and
C_SAMPLE form a time constant which must be less than
the sampling rate (T_SAMPLE) of the ADC, as illustrated
by the equation in Figure 30. An external capacitor
C_FILTER can be added to the output pin to provide
additional filtering. However, this may impact the time
response of the sensor. Enough time must be provided
to allow C_FILTER to charge sufficiently between ADC
conversions.
The magnitude of the C_FILTER capacitor should be in
the range of 1 nF to 100 nF to prevent the sensor
amplifier from oscillating. A small resistor of
approximately 10Ω to 100Ω can be added between the
output pin of the sensor and C_FILTER to isolate the
sensor’s amplifier from the capacitive load. The output
impedance of the sensor R_OUT varies as a function of
frequency. Thus, a series resistor should be added to
the effective R_OUT resistance, if C_FILTER is intended to
serve as the ADC’s anti-aliasing filter.
 2004 Microchip Technology Inc.
DS00929A-page 19