TC9400_06 Datasheet, PDF (10/24 Pages) Microchip Technology – Voltage-to-Frequency/Frequency-to-Voltage Converters

TC9400/9401/9402
4.9 V_REF
A reference voltage from either a precision source, or
the V_SS supply is applied to this pin. Accuracy of the
TC9400 is dependent on the voltage regulation and
temperature characteristics of the reference circuitry.
Since the TC9400 is a charge balancing V/F converter,
the reference current will be equal to the input current.
For this reason, the DC impedance of the reference
voltage source must be kept low enough to prevent lin-
earity errors. For linearity of 0.01%, a reference imped-
ance of 200W or less is recommended. A 0.1 μF
bypass capacitor should be connected from V_REF to
ground.
4.10 V_REF Out
The charging current for C_REF is supplied through this
pin. When the op amp output reaches the threshold
level, this pin is internally connected to the reference
voltage and a charge, equal to V_REF x C_REF, is removed
from the integrator capacitor. After about 3μsec, this pin
is internally connected to the summing junction of the
op amp to discharge C_REF. Break-before-make switch-
ing ensures that the reference voltage is not directly
applied to the summing junction.
5.0 VOLTAGE-TO-FREQUENCY
(V/F) CONVERTER DESIGN
INFORMATION
5.1 Input/Output Relationships
The output frequency (F_OUT) is related to the analog
input voltage (V_IN) by the transfer equation:
EQUATION 5-1:
Frequency Out = ^VIN , x
1
R_IN (V_REF)(V_REF)
5.2 External Component Selection
5.2.1
R_IN
The value of this component is chosen to give a full
scale input current of approximately 10 μA:
EQUATION 5-2:
R_IN ≅
V_IN ^FULLSCALE
10 μA
EQUATION 5-3:
R_IN ≅
10V
10 μA
= 1 MΩ
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”, Adjustment
Procedure). Metal film resistors with 1% tolerance or
better are recommended for high accuracy applications
because of their thermal stability and low noise gener-
ation.
5.2.2
C_INT
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.
DS21483C-page 10
© 2006 Microchip Technology Inc.