TC9400/9401/9402
6.0
FREQUENCY-TO-VOLTAGE
(F/V) CIRCUIT DESCRIPTION
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.
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 resis-
tor, 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.
7.2
Input Voltage Levels
7.0
7.1
F/V CONVERTER DESIGN
INFORMATION
Input/Output Relationships
The input frequency is applied to the Threshold Detec-
tor input (Pin 11). As discussed in the V/F circuit section
of this data sheet, the threshold of Pin 11 is approxi-
mately (V
DD
+ V
SS
)/2 ±400mV. Pin 11's input voltage
range extends from V
DD
to about 2.5V below the thresh-
old. If the voltage on Pin 11 goes more than 2.5 volts
below the threshold, the V/F mode start-up comparator
will turn on and corrupt the output voltage. The Thresh-
old Detector input has about 200mV of hysteresis.
In ±5V applications, the input voltage levels for the
TC9400 are ±400mV, minimum. If the frequency
source being measured is unipolar, such as TTL or
CMOS operating from a +5V source, then an AC cou-
pled level shifter should be used. One such circuit is
shown in Figure 7-1(a).
The level shifter circuit in Figure 7-1(b) 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
start-up comparator. The diode's forward voltage
decreases by 2.1mV/°C, so for high ambient tempera-
ture operation, two diodes in series are recommended;
also, see Figure 7-2.
The output voltage is related to the input frequency
(F
IN
) by the transfer equation:
EQUATION 7-1:
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.
FIGURE 7-1:
FREQUENCY INPUT LEVEL SHIFTER
+8V to +5V
+5V
14
V
DD
10k
14
V
DD
TC9400
Frequency
Input
+5V
0V
GND
6
V
SS
4
-5V
33k
0.01µF
11
IN914
1.0M
DET
Frequency
Input
+5V
0V
0.1µF
10k
33k
0.01µF
11
IN914
1.0M
TC9400
DET
V
SS
4
(a) ±5V Supply
(b) Single Supply
©
DS21483B-page 12
2002 Microchip Technology Inc.
MICROCHIP [ MICROCHIP TECHNOLOGY ]
