RATIOMETRIC_20_kHz_V_F_CONVERTER

Published:2009/7/12 22:46:00 Author:May | From:SeekIC

This ratiometric 20-kHz voltage-to-frequency converter (VFC) provides superior perforrnance with strain gauges and other ratio-responding transducers-even with noisy, unregulated excitation voltages. Feedback isn't used to achieve the excellent 4-Hz linearity, so there is very-low-frequency jitter-period measurements can be used to get several digits of resolution- even when operating at a fraction of full scale. An optional synchronizing transistor starts the VFC with zero charge at the beginning of each count cycle, eliminating the characteristic digit-jumping often encountered with VFC designs. Grood linearity is attained by making the comparator's reference voltage vary with the input voltage, which precisely compensates for the finite capacitor reset time:where AVin△V/△t. If Vref is made to include the amount t1AVin, then the effect of t1 is elirninated:The MPSA18 is a remarkably high-gain transistor, even at low currents, producing good current-source linearity down to 0 Hz. In addition, bipolar transistors work well with the low collector voltages encountered in this single-supply 10-V design. Moreover, most single-supply op amps will work in place of the LM10. But the LM10 also has a reference amplifier that could be used to construct a 10-V excitation regulator. The LM311 propagation delay gives a reset pulse width near 400 ns, which gives the transistor time to discharge the capacitor. Also, the 311's bias current produces a small negative offset that ensures a 0-Hz output for Vin.



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