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TC9400

Voltage-to-Frequency/Frequency-to-Voltage Converters

Microchip Technology 

TC9400/9401/9402

5.0 VOLTAGE-TO-FREQUENCY

(V/F) CONVERTER DESIGN

INFORMATION

5.1 Input/Output Relationships

The output frequency (FOUT) is related to the analog

input voltage (VIN) by the transfer equation:

EQUATION 5-1:

Frequency Out =

VIN

RIN

,x

1

(VREF)(VREF)

5.2 External Component Selection

5.2.1

RIN

The value of this component is chosen to give a full

scale input current of approximately 10µA:

EQUATION 5-2:

RIN ≅

VIN FULLSCALE

10µA

EQUATION 5-3:

RIN ≅

10V

10µA

= 1MΩ

Note that the value is an approximation and the exact

relationship is defined by the transfer equation. In prac-

tice, the value of RIN typically would be trimmed to

obtain full scale frequency at VIN full scale (see

Section 5.3, Adjustment Procedure). Metal film resis-

tors with 1% tolerance or better are recommended for

high accuracy applications because of their thermal

stability and low noise generation.

5.2.2

CINT

The exact value is not critical but is related to CREF by

the relationship:

3CREF ≤ CINT ≤ 10CREF

Improved stability and linearity are obtained when

CINT ≤ 4CREF. Low leakage types are recommended,

although mica and ceramic devices can be used in

applications where their temperature limits are not

exceeded. Locate as close as possible to Pins 12

and 13.

5.2.3

CREF

The exact value is not critical and may be used to trim

the full scale frequency (see Section 7.1, Input/Output

Relationships). Glass film or air trimmer capacitors are

recommended because of their stability and low leak-

age. Locate as close as possible to Pins 5 and 3 (see

Figure 5-1).

DS21483B-page 10

FIGURE 5-1:

500

400

300

200

RECOMMENDED

CREF VS. VREF

10kHz

VDD = +5V

VSS = -5V

RIN = 1MΩ

VIN = +10V

TA = +25°C

100

100kHz

0 -1 -2 -3 -4 -5 -6 -7

VREF (V)

5.2.4

VDD, VSS

Power supplies of ±5V are recommended. For high

accuracy requirements, 0.05% line and load regulation

and 0.1µF disc decoupling capacitors, located near the

pins, are recommended.

5.3 Adjustment Procedure

Figure 3-1 shows a circuit for trimming the zero loca-

tion. Full scale may be trimmed by adjusting RIN, VREF,

or CREF. Recommended procedure for a 10kHz full

scale frequency is as follows:

1. Set VIN to 10mV and trim the zero adjust circuit

to obtain a 10Hz output frequency.

2. Set VIN to 10V and trim either RIN, VREF, or CREF

to obtain a 10kHz output frequency.

If adjustments are performed in this order, there should

be no interaction and they should not have to be

repeated.

5.4 Improved Single Supply V/F

Converter Operation

A TC9400, which operates from a single 12 to 15V vari-

able power source, is shown in Figure 5-2. This circuit

uses two Zener diodes to set stable biasing levels for

the TC9400. The Zener diodes also provide the refer-

ence voltage, so the output impedance and tempera-

ture coefficient of the Zeners will directly affect power

supply rejection and temperature performance. Full

scale adjustment is accomplished by trimming the input

current. Trimming the reference voltage is not recom-

mended for high accuracy applications unless an

Op Amp is used as a buffer, because the TC9400

requires a low impedance reference (see Section 4.9,

VREF pin description, for more information).

The circuit of Figure 5-2 will directly interface with

CMOS logic operating at 12V to 15V. TTL or 5V CMOS

logic can be accommodated by connecting the output

pull-up resistors to the +5V supply. An optoisolator can

also be used if an isolated output is required; also, see

Figure 5-3.

© 2002 Microchip Technology Inc.

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