3-296

TELCOM SEMICONDUCTOR, INC.

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

dispensed into the op amp's summing junction. This charge

in turn flows through the feedback resistor, generating

is linearly proportional to the input frequency.

F/V CONVERTER DESIGN INFORMATION

Input/Output Relationships

by the transfer equation:

to 100

µF are perfectly acceptable for low frequencies.

When the TC9400 is used in the single-supply mode,

pin 2.

+5V

14

64

+5V

–5V

VDD

1.0M

11

33k

IN914

VSS

DET

TC9400

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

(B) Single Supply

(A)

Figure 6.

Frequency Input Level Shifter

Input Voltage Levels

The input frequency is applied to the Threshold Detector

input (Pin 11). As discussed in the V/F circuit section of this

11 goes more than 2.5 volts below the threshold, the V/F

mode startup comparator will turn on and corrupt the output

voltage. The Threshold Detector input has about 200 mV of

hysteresis.

In

±5 V applications, the input voltage levels for the

TC9400 are

±400mV, minimum. If the frequency source

being measured is unipolar, such as TTL or CMOS operat-

ing from a +5V source, then an AC coupled level shifter

should be used. One such circuit is shown in Figure 6a.

The level shifter circuit in Figure 6b 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 startup comparator. The

diode's forward voltage decreases by 2.1 mV/

°C, so for high

ambient temperature operation two diodes in series are

recommended.

VOLTAGE-TO-FREQUENCY/

FREQUENCY-TO-VOLTAGE CONVERTERS

TC9400

TC9401

TC9402