Sensor circuit description

AN2679

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2

Sensor circuit description

The sensor HF oscillator circuit is based on the Colpitts oscillator type which has a simple

circuit configuration, produces a very clean sinusoidal wave signal and is capable of

oscillating in a wide frequency range. The resonant circuit consisting from the inductor L1

and capacitors C12, C9 and C8 determine the frequency of the oscillations according to the

formula in Equation 1. Actually the circuit will oscillate at a slightly lower frequency due to

coupling capacitor C10, junction capacitances of transistor Q1 and other stray capacitances.

Equation 1

The oscillator employs a transistor, Q1, operating in a common base configuration that

derives its feedback from the capacitor divider C9 andC8. Resistors R3 and R6 set its bias

point and the temperature of diode D2 stabilizes it.

The oscillator signal amplitude is further detected by diodes D4 and D5, and filtered by

capacitor C13. Together with C10 this circuit acts as a charge pump, thus the full range of

the ST7 ADC converter (0 V - 5 V) is used.

The other function of diode D2 is temperature sensing. The voltage across a diode operated

at constant current is linear in a very large range of temperature and reduces with

increasing temperature by approximately -2 mV/ k (see Figure 9). With the ST7 10-bit ADC

converter, the temperature can be measured with an accuracy of approximately 2.5 °C

which is enough for overall correction of the sensor temperature variations (see Figure 8).

2.1

Initial configuration and jumper settings

Table 1 and Figure 3 represent the initial (high side) output driver configuration and jumper

settings. The diagnostic LED D3 shares the MCU pin 2 with the ICC connector. In case of

programming/debugging problems, it can be disconnected by removing the jumper from

pins P14 and P15.

f

1

L

1

C

12

C

8

C

9

C

8

C

9

+

----------------------

+

⎝⎠

⎜⎟

⎛⎞

2

π

--------------------------------------------------------------

=

Table 1.

Initial configuration and jumper settings

Pins

Setting

P10 - P11

A - HS

P4 - P5

LED - C

P3 - P7

+Vs - LS

P14 - P15

+5V - A