Voltage to Freq. & Freq. to Voltage Converter

V - F & F - V Converter

Voltage to Frequency Converter : In this application the XR-4151 functions as a stand-alone voltage-to-frequency converter operating of a single positive power supply. Refer to the functional block diagram, the circuit connection for XR-4151 contains a voltage comparator, a one-shot, and a precision switched current source. The voltage comparator compares a positive input voltage applied at pin 7 to the voltage at pin 6. If the input voltage is higher, the comparator will fire the one-shot. The output of the one-shot Is connected to both the logic output and the precision switched current source. During the one-shot period, T, the logic output will go low and the current will turn on with current I. At the end of the one-shot period the logic output will go high and the current injected an amount of charge Q = IOT into the network RB=CB. If this charge has not increased the voltage VB such that VB>VI, the comparator again fires the one-shot and the current source injects another lump of charge, Q, into the RB-CB network. This process continues until VB>VI. When this condition is achieved. The current source remains off and the voltage VB decays until VB is again equal to VI. This completes one cycle. The VFC will now run in a steady state mode. The current source dumps lumps of charge in to the capacitor CB at a rate fast enough to keep VB > VI. Since the discharge rate of capacitor CB is proportional to VB / RB, the freqneucy at which the system runs will be proportional to the input voltage.

Frequency to Voltage Conversion : The XR-4151 can be used as a frequency-to-voltage converter. Figure shows the single - supply FVC configuration. With  no signal applied, the resistor bias net-works tied to pins 6 and 7 hold the input comparator in the off state. A negative going pulse applied to pin 6 (or positive pulse to pin 7) will cause the comparator to fire the one-shot. For proper operation, the pulse width must be  less than period of the one-shot. T = 1.1 RO CO. For  a 5V  p-p square - wave input the differentiator network formed by the input coupling capacitor and the resistor bias network will provide pulses which correctly trigger  the one-shot. An external voltage comparator  can be used to “ square - up” sinusoidal input signals before they are applied to the XR-4151. Also the component values for the input signal differentiator and bias network can be altered to accommodate square waves with different amplitudes and frequencies. The passive integrator network RB CB filters the current pulses from the pin 1 output. For less output ripple, increase the value of CB.
For increased accuracy and linearity, use an operational amplifier integrator as shown in fig. 6 the precision FVC configuration. Trim the offset to give -10mV out with 10Hz in and trim the full scale adjust for 10V out with 10KHz in.  Input signal conditioning for this circuit is necessary just as for the single supply mode and the scale factor can be programmed by the choice of component values. A trade off exists between the amount of output ripple and the response time, through the choice or integration capacitor C1. If C1 = 0.1µF, the ripple will be about 100mV. Response time constant TR = RB = 100 k? and C1 =0.1µF tR = 10msec.



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