TC9402CPD datasheet(12/20 Pages) MICROCHIP

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TC9400/9401/9402

DS21483B-page 12

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

-5V

VDD

1.0M

33k

IN914

VSS

DET

TC9400

(a) ±5V Supply

(b) Single Supply

0.01

µF

Frequency

Input

GND

+8V to +5V

10k

+5V

VDD

1.0M

33k

IN914

VSS

DET

TC9400

0.01

µF

Frequency

Input

0.1

µF

10k

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