VOLTAGE-TO-FREQUENCY/
FREQUENCY-TO-VOLTAGE CONVERTERS
TC9400
TC9401
TC9402
FREQUENCY-TO-VOLTAGE (F/V)
CIRCUIT DESCRIPTION
When used as an F/V converter, the TC9400 generates
an output voltage linearly proportional to the input frequency
waveform.
Each zero crossing at the threshold detector's input
causes a precise amount of charge (q = C
REF
×
V
REF
) to be
dispensed into the op amp's summing junction. This charge
in turn flows through the feedback resistor, generating
voltage pulses at the output of the op amp. A capacitor (C
INT
)
across R
INT
averages these pulses into a DC voltage which
is linearly proportional to the input frequency.
Input Voltage Levels
The input frequency is applied to the Threshold Detector
input (Pin 11). As discussed in the V/F circuit section of this
data sheet, the threshold of pin 11 is approximately (V
DD
+
V
SS
) /2
±400mV.
Pin 11's input voltage range extends from
V
DD
to about 2.5 V below the threshold. If the voltage on pin
11 goes more than 2.5 volts below the threshold, the V/F
mode startup comparator will turn on and corrupt the output
voltage. The Threshold Detector input has about 200 mV of
hysteresis.
In
±5
V applications, the input voltage levels for the
TC9400 are
±400mV,
minimum. If the frequency source
being measured is unipolar, such as TTL or CMOS operat-
ing from a +5V source, then an AC coupled level shifter
should be used. One such circuit is shown in Figure 6a.
The level shifter circuit in Figure 6b can be used in single
supply F/V applications. The resistor divider ensures that
the input threshold will track the supply voltages. The diode
clamp prevents the input from going far enough in the
negative direction to turn on the startup comparator. The
diode's forward voltage decreases by 2.1 mV/°C, so for high
ambient temperature operation two diodes in series are
recommended.
F/V CONVERTER DESIGN INFORMATION
Input/Output Relationships
The output voltage is related to the input frequency (f
IN
)
by the transfer equation:
V
OUT
= [V
REF
C
REF
R
INT
] f
IN
.
The response time to a change in f
IN
is equal to (R
INT
C
INT
). The amount of ripple on V
OUT
is inversely proportional
to C
INT
and the input frequency.
C
INT
can be increased to lower the ripple. Values of 1µF
to 100µF are perfectly acceptable for low frequencies.
When the TC9400 is used in the single-supply mode,
V
REF
is defined as the voltage difference between pin 7 and
pin 2.
+5V
14
VDD
+8V to +5V
14
VDD
10k
Frequency
Input
+5V
0V
33k
0.01µF
TC9400
11
IN914
1.0M
DET
Frequency
Input
+5V
0V
33k
0.01µF
TC9400
11
IN914
1.0M
DET
GND
6
VSS
4
–5V
0.1µF
10k
VSS
4
(A)
±
5V Supply
Figure 6.
3-296
Frequency Input Level Shifter
(B) Single Supply
TELCOM SEMICONDUCTOR, INC.
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
