TC9400_06 Datasheet, PDF (11/24 Pages) Microchip Technology – Voltage-to-Frequency/Frequency-to-Voltage Converters

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/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 Figure ).
500
V_DD = +5V
V_SS = -5V
400
R_IN = 1MW
V_IN = +10V
T_A = +25°C
300
10 kHz
200
100
100 kHz
0 _-1 -2 -3 -4 -5 -6 -7
V_REF (V)
FIGURE 5-1:
V_REF
Recommended C_REF vs.
5.2.4
V_DD, V_SS
Power supplies of ±5V are recommended. For high
accuracy requirements, 0.05% line and load regulation
and 0.1 μF disc decoupling capacitors, located near the
pins, are recommended.
TC9400/9401/9402
5.3 Adjustment Procedure
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 10 kHz full
scale frequency is as follows:
1. Set V_IN to 10 mV and trim the zero adjust circuit
to obtain a 10 Hz output frequency.
2. Set V_IN to 10V and trim either R_IN, V_REF, or C_REF
to obtain a 10 kHz 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 recommended
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”, 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.
© 2006 Microchip Technology Inc.
DS21483C-page 11