4

where R is the effective resistance of the tank. The resis-

tance component of the inductor consists primarily of core

losses and Òskin effectÓ or AC resistance.

The resonant capacitor should be selected to resonate with

the inductor within the frequency range recommended in

order to yield the highest Q. The capacitor type should be

selected to have low ESR: multilayer ceramic for example.

Detection distances vary for different metals. Following

are different detection distances for some selected metals

and metal objects relative to one particular circuit set-up:

Commonly encountered metals:

¥ Stainless steel

0.101"

¥ Carbon steel

0.125"

¥ Copper

0.044"

¥ Aluminum

0.053"

¥ Brass

0.052"

Coins:

¥ US Quarter

0.055"

¥ Canadian Quarter

0.113"

¥ 1 German Mark

0.090"

¥ 1 Pound Sterling

0.080"

¥ 100 Japanese Yen

0.093"

¥ 100 Italian Lira

0.133"

12 oz. soda can:

0.087"

Principle of Operation: continued

Note that the above is only a comparison among different

metals and no attempt was made to achieve the greatest

detection distance.

A different type of application involves, for example,

detecting the teeth of a rotating gear. For these applica-

tions the capacitor on DEMOD should not be selected too

small (not below 1000pF) where the ripple becomes too

large and not too large (not greater than 0.01µF) that the

response time is too slow. Figure 1 for example shows the

capacitor ripple only and Figure 2 shows the entire capaci-

tor voltage and the output pulses for an 8-tooth gear rotat-

ing at about 2400 rpm using a 2200pF capacitor on the

DEMOD pin.

Because the output stages go into hard saturation, a time

interval is required to remove the stored base charge

resulting in both outputs being low for approximately 3µs

(see Output Switching Delay vs. Temperature graph). If

more information is required about output switching

characteristics please consult the factory.

Figure 1. Capacitor ripple.

Figure 2. Output pulse for an 8 tooth gear.

1