TC9400/9401/9402
5.0
VOLTAGE-TO-FREQUENCY
(V/F) CONVERTER DESIGN
INFORMATION
Input/Output Relationships
C
REF
(pF) +12pF
FIGURE 5-1:
500
400
300
RECOMMENDED
C
REF
VS. V
REF
V
DD
= +5V
V
SS
= -5V
R
IN
= 1MΩ
V
IN
= +10V
T
A
= +25°C
10kHz
5.1
The output frequency (F
OUT
) is related to the analog
input voltage (V
IN
) by the transfer equation:
EQUATION 5-1:
Frequency Out =
V
IN
1
,x
(V
REF
)(V
REF
)
R
IN
200
100
100kHz
5.2
5.2.1
External Component Selection
R
IN
5.2.4
0
-1
-2
-3
-4
V
REF
(V)
-5
-6
-7
The value of this component is chosen to give a full
scale input current of approximately 10µA:
V
DD
, V
SS
EQUATION 5-2:
R
IN
≅
V
IN FULLSCALE
10µA
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
EQUATION 5-3:
R
IN
≅
10V
= 1MΩ
10µA
Figure 3-1 shows a circuit for trimming the zero loca-
tion. 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.
Set V
IN
to 10V and trim either R
IN
, V
REF
, or C
REF
to obtain a 10kHz output frequency.
Note that the value is an approximation and the exact
relationship is defined by the transfer equation. In prac-
tice, the value of R
IN
typically would be trimmed to
obtain full scale frequency at V
IN
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.
2.
If adjustments are performed in this order, there should
be no interaction and they should not have to be
repeated.
5.4
5.2.2
C
INT
Improved Single Supply V/F
Converter Operation
The exact value is not critical but is related to C
REF
by
the relationship:
3C
REF
≤
C
INT
≤
10C
REF
Improved stability and linearity are obtained when
C
INT
≤
4C
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.
5.2.3
C
REF
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).
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,
V
REF
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.
©
DS21483B-page 10
2002 Microchip Technology Inc.
MICROCHIP [ MICROCHIP TECHNOLOGY ]
