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TC9401CPD Datasheet(PDF) 12 Page - Microchip Technology

Part No. TC9401CPD
Description  Voltage-to-Frequency/Frequency-to-Voltage Converters
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Maker  MICROCHIP [Microchip Technology]
Homepage  http://www.microchip.com
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TC9400/9401/9402
DS21483B-page 12
© 2002 Microchip Technology Inc.
6.0
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 = CREF ∞ VREF)
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 (CINT)acrossRINT averages these
pulses into a DC voltage, which is linearly proportional
to the input frequency.
7.0
F/V CONVERTER DESIGN
INFORMATION
7.1
Input/Output Relationships
The output voltage is related to the input frequency
(FIN) by the transfer equation:
EQUATION 7-1:
The response time to a change in FIN is equal to (RINT
CINT). The amount of ripple on VOUT is inversely
proportional to CINT and the input frequency.
CINT can be increased to lower the ripple. Values of 1µF
to 100
µF are perfectly acceptable for low frequencies.
When the TC9400 is used in the Single Supply mode,
VREF is defined as the voltage difference between Pin 7
and Pin 2.
7.2
Input Voltage Levels
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 (VDD +VSS)/2 ±400mV. Pin 11's input voltage
range extends from VDD 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
shownin Figure7-1(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.
FIGURE 7-1:
FREQUENCY INPUT LEVEL SHIFTER
VOUT =[VREF CREF RINT]FIN
+5V
14
64
+5V
-5V
VDD
1.0M
11
33k
IN914
VSS
DET
TC9400
(a) ±5V Supply
(b) Single Supply
0.01
µF
Frequency
Input
0V
GND
+8V to +5V
14
10k
4
+5V
VDD
1.0M
11
33k
IN914
VSS
DET
TC9400
0.01
µF
Frequency
Input
0V
0.1
µF
10k



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