CS209A

http://onsemi.com

4

TYPICAL PERFORMANCE CHARACTERISTICS

0

Output Load (kΩ)

Temperature (°C)

Figure 2. Output Switching Delay vs.

Output Load

Figure 3. Output Switching Delay vs.

Temperature

Distance To Object (in.)

Figure 4. Demodulator Voltage vs. Distance for Different RF

6.5

−20

1.5

0

8

6

4

2

4

8

12

16

20

−40

0

20

40

60

80

100

120

5.5

4.5

3.5

2.5

Object

Detected

5.0 kΩ

2.5 kΩ

7.5 kΩ

12.5 kΩ

15 kΩ

17.5 kΩ

Object Not

Detected,

L Unloaded

0.100

0.200

0.300

0.400

1.75

1.25

1.0

0.75

0.1 kΩ

PRINCIPLE OF OPERATION

The CS209A is a metal detector circuit which operates on

the principle of detecting a reduction in Q of an inductor

when it is brought into close proximity of metal. The

CS209A contains an oscillator set up by an external parallel

resonant tank and a feedback resistor connected between

OSC and RF. (See Figure 5.) The impedance of a parallel

resonant tank is highest when the frequency of the source

driving it is equal to the tank’s resonant frequency. In the

CS209A the internal oscillator operates close to the resonant

frequency of the tank circuit selected. As a metal object is

brought close to the inductor, the amplitude of the voltage

across the tank gradually begins to drop. When the envelope

of the oscillation reaches a certain level, the IC causes the

output stages to switch states.

The detection is performed as follows: A capacitor

connected to DEMOD is charged via an internal 30 μA current

source. This current, however, is diverted away from the

capacitor in proportion to the negative bias generated by the

tank at TANK. Charge is therefore removed from the capacitor

tied to DEMOD on every negative half cycle of the resonant

voltage. (See Figure 6) The voltage on the capacitor at

DEMOD, a DC voltage with ripple, is then directly compared

to an internal 1.44 V reference. When the internal comparator

trips it turns on a transistor which places a 23.6 kΩ resistor in

parallel to the 4.8 kΩ. The resulting reference then becomes

approximately 1.2 V. This hysteresis is necessary for preventing

false triggering.