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handbook of modern sensors

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180 5 Interface Electronic Circuits<br />

(A)<br />

(B)<br />

Fig. 5.25. Integrate and deintegrate (reset) phases in a charge-balance converter.<br />

Fig. 5.26. Integrator output in a charge-balance converter.<br />

capacitor C in . This causes the integrator to ramp down from its positive voltage with<br />

the rate proportional to V in . The amount <strong>of</strong> time required to reach the comparator’s<br />

threshold is<br />

T 1 =<br />

V<br />

dV /dt = t i − I in 1 i − I in<br />

os<br />

= t os . (5.28)<br />

C in I in /C in I in<br />

It is seen that the capacitor value does not affect the duration <strong>of</strong> the integration period.<br />

The output frequency is determined by<br />

f out =<br />

1<br />

t os + T 1<br />

= I in<br />

t os i = V in<br />

R in<br />

1<br />

t os i . (5.29)<br />

Therefore, the frequency <strong>of</strong> one-shot pulses is proportional to the input voltage. It<br />

depends also on the quality <strong>of</strong> the integrating resistor, stability <strong>of</strong> the current generator,<br />

and a one-shot circuit. With a careful design, this type <strong>of</strong> V/F converter may reach a<br />

nonlinearity error below 100 ppm and can generate frequencies from 1 Hz to 1 MHz.

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