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12 page 
TC9400/9401/9402 DS21483B-page 12 © 2002 Microchip Technology Inc. 6.0 FREQUENCY-TO-VOLTAGE (F/V) CIRCUIT DESCRIPTION When used as an F/V converter, the TC9400 generates an output voltage linearly proportional to the input frequency waveform. Each zero crossing at the threshold detector's input causes a precise amount of charge (q = CREF ∞ VREF) to be dispensed into the Op Amp's summing junction. This charge, in turn, flows through the feedback resis- tor, generating voltage pulses at the output of the Op Amp. A capacitor (CINT)acrossRINT averages these pulses into a DC voltage, which is linearly proportional to the input frequency. 7.0 F/V CONVERTER DESIGN INFORMATION 7.1 Input/Output Relationships The output voltage is related to the input frequency (FIN) by the transfer equation: EQUATION 7-1: The response time to a change in FIN is equal to (RINT CINT). The amount of ripple on VOUT is inversely proportional to CINT and the input frequency. CINT can be increased to lower the ripple. Values of 1µF to 100 µF are perfectly acceptable for low frequencies. When the TC9400 is used in the Single Supply mode, VREF is defined as the voltage difference between Pin 7 and Pin 2. 7.2 Input Voltage Levels The input frequency is applied to the Threshold Detec- tor input (Pin 11). As discussed in the V/F circuit section of this data sheet, the threshold of Pin 11 is approxi- mately (VDD +VSS)/2 ±400mV. Pin 11's input voltage range extends from VDD to about 2.5V below the thresh- old. If the voltage on Pin 11 goes more than 2.5 volts below the threshold, the V/F mode start-up comparator will turn on and corrupt the output voltage. The Thresh- old Detector input has about 200mV of hysteresis. In ±5V applications, the input voltage levels for the TC9400 are ±400mV, minimum. If the frequency source being measured is unipolar, such as TTL or CMOS operating from a +5V source, then an AC cou- pled level shifter should be used. One such circuit is shownin Figure7-1(a). The level shifter circuit in Figure 7-1(b) can be used in single supply F/V applications. The resistor divider ensures that the input threshold will track the supply voltages. The diode clamp prevents the input from going far enough in the negative direction to turn on the start-up comparator. The diode's forward voltage decreases by 2.1mV/°C, so for high ambient tempera- ture operation, two diodes in series are recommended; also, see Figure 7-2. FIGURE 7-1: FREQUENCY INPUT LEVEL SHIFTER VOUT =[VREF CREF RINT]FIN +5V 14 64 +5V -5V VDD 1.0M 11 33k IN914 VSS DET TC9400 (a) ±5V Supply (b) Single Supply 0.01 µF Frequency Input 0V GND +8V to +5V 14 10k 4 +5V VDD 1.0M 11 33k IN914 VSS DET TC9400 0.01 µF Frequency Input 0V 0.1 µF 10k |
