CS209A

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6

For this application it is recommended to use a core which

concentrates the magnetic field in only one direction. This

is accomplished very well with a pot core half. The next step

is to select a core material with low loss factor (inverse of Q).

The loss factor can be represented by a resistance in series

with the inductor which arises from core losses and is a

function of frequency.

The final step in obtaining a high Q inductor is the selection

of wire size. The higher the frequency the faster the decrease

in current density towards the center of the wire. Thus most

of the current flow is concentrated on the surface of the wire

resulting in a high AC resistance. LITZ wire is recommended

for this application. Considering the many factors involved,

it is also recommended to operate at a resonant frequency

between 200 and 700 kHz. The formula commonly used to

determine the Q for parallel resonant circuits is:

QP ^ R

2pfRL

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

component of the inductor consists primarily of core losses

and “skin effect” 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″

. . . . . . . . . . . . . . . . . . . . . . . . . . .

Other

12 oz. soda can

0.087″

. . . . . . . . . . . . . . . . . . . . . . . . . . .

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 applications

the capacitor on DEMOD should not be selected too small

(not below 1000 pF) where the ripple becomes too large and

not too large (not greater than 0.01 μF) that the response time

is too slow. Figure 6 for example shows the capacitor ripple

only and Figure 7 shows the entire capacitor voltage and the

output pulses for an 8−tooth gear rotating at about 2400 rpm

using a 2200 pF 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 3.0 μs.

(See Figure 3.)