TC9400/9401/9402
DS21483D-page 10
?2007 Microchip Technology Inc.
4.0 VOLTAGE-TO-FREQUENCY
(V/F) CONVERTER DESIGN
INFORMATION
4.1 Input/Output Relationships
The output frequency (F
OUT
) is related to the analog
input voltage (V
IN
) by the transfer equation:
EQUATION 4-1:
4.2 External Component Selection
4.2.1 R
IN
The value of this component is chosen to give a full
scale input current of approximately 10礎:
EQUATION 4-2:
EQUATION 4-3:
Note that the value is an approximation and the exact
relationship is defined by the transfer equation. In
practice, the value of R
IN
typically would be trimmed to
obtain full scale frequency at V
IN
full scale (see
Section4.3 Adjustment Procedure , Adjustment
Procedure). Metal film resistors with 1% tolerance or
better are recommended for high accuracy applications
because of their thermal stability and low noise
generation.
4.2.2 C
INT
The exact value is not critical but is related to C
REF
by
the relationship:
3C
REF
d C
INT
d 10C
REF
Improved stability and linearity are obtained when
C
INT
d4C
REF
. 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.
4.2.3 C
REF
The exact value is not critical and may be used to trim
the full scale frequency (see Section6.1 Input/Out-
put Relationships , Input/Output Relationships).
Glass film or air trimmer capacitors are recommended
because of their stability and low leakage. Locate as
close as possible to Pins 5 and 3 (see Figure4-1).
FIGURE 4-1: Recommended C
REF
vs.
V
REF
.
4.2.4 V
DD
, V
SS
Power supplies of ?V are recommended. For high
accuracy requirements, 0.05% line and load regulation
and 0.1礔 disc decoupling capacitors, located near the
pins, are recommended.
4.3 Adjustment Procedure
Figure3-1 shows a circuit for trimming the zero
location. Full scale may be trimmed by adjusting R
IN
,
V
REF
, or C
REF
. Recommended procedure for a 10kHz
full scale frequency is as follows:
1. Set V
IN
to 10mV and trim the zero adjust circuit
to obtain a 10Hz output frequency.
2. Set V
IN
to 10V and trim either R
IN
, V
REF
, or C
REF
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.
4.4 Improved Single Supply V/F
Converter Operation
A TC9400, which operates from a single 12 to 15V
variable power source, is shown in Figure4-2. This
circuit uses two Zener diodes to set stable biasing
levels for the TC9400. The Zener diodes also provide
the reference voltage, so the output impedance and
temperature coefficient of the Zeners will directly affect
power supply rejection and temperature performance.
Full scale adjustment is accomplished by trimming the
input current.
Frequency Out
V
IN
R
IN
--------
1
V
REF
( ) C
REF
( )
------------------------------------
"
=
V
IN
FULL SCALE
10 ?/SPAN>A
R
IN
E
10V
10 ?/SPAN>A
R
IN
E
= 1 M?/SPAN>
500
400
300
200
100
0
-1 -2 -3 -4 -5 -6 -7
V
REF
(V)
10 kHz
100 kHz
V
DD
= +5V
V
SS
= -5V
R
IN
= 1MW
V
IN
= +10V
T
A
= +25癈
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