PRODUCT SPECIFICATION
RC4152
Applications
1
T = -------------
FOUT
Single Supply VFC
The stand-alone voltage-to-frequency converter is one of the
simplest applications for the RC4152. This application uses
only passive external components to create the least expen-
sive VFC circuit (see Figure 1).
TP
VIN
-----
T
------------ = I OUT
RB
where TP = 1.1 ROCO
VREF
IOUT = -------------
RS
The positive input voltage VIN is applied to the input com-
parator through a low pass filter. The one-shot will fire repet-
itively and the switched current source will pump out current
pulses of amplitude VREF/RS and duration 1.1 ROCO into the
integrator. Because the integrator is tied back to the inverting
comparator input, a feedback loop is created. The pulse repe-
tition rate will increase until the average voltage on the inte-
grator is equal to the DC input voltage at pin 7. The average
voltage at pin 6 is proportional to the output frequency
because the amount of charge in each current pulse is
precisely controlled.
By rearranging and substituting,
VIN RS
1
------------- ------ ----------------------
=
FOUT
VREF RB 1.1ROCO
Recommended component values for different operating
frequencies are shown in the table below.
Range
InputV
Output
Scale
Factor
F
R
C
C
R
B
IN
O
O
O
I
0 to -10V 0 to 1.0 kHz 0.1 KHz/V 6.8 kW 0.1 mF
0.05 mF 100 kW
Because the one-shot firing frequency is the same as the
open collector output frequency, the output frequency is
0 to -10V 0 to 10 kHz 1.0 KHz/V 6.8 kW 0.01 mF 0.005 mF 100 kW
0 to -10V 0 to 100 kHz 10 KHz/V 6.8 kW 0.001 mF 500 pF 100 kW
directly proportional to VIN
.
The graphs shown under Typical Performance Characteris-
tics show nonlinearity versus input voltage for the precision
current sourced VFC. The best linearity is achieved by using
an op amp having greater than 1.0 V/ms slew rate, but any op
amp can be used.
The external passive components set the scale factor. For
best linearity, RS should be limited to a range of 12 kW to
20 kW
The reference voltage is nominally 2.25V for the RC4152.
Recommended values for different operating frequencies are
shown in the table below.
Precision Voltage Sourced VFC
This circuit is identical to the current sourced VFC, except
that the current pulses into the integrator are derived directly
from the switched voltage reference. This improves tempera-
ture drift at the expense of high frequency linearity.
Operating
Range
R
C
R
C
B
O
O
B
DC to 1.0 kHz
DC to 10 kHz
DC to 100 kHz
6.8 kW
6.8 kW
6.8 kW
0.1 mF
100 kW
100 kW
100 kW
10 mF
10 mF
10 mF
0.01 mF
0.001 mF
The switched current source (pin 1) output has been tied to
ground, and RS has been put in series between the switched
voltage reference (pin 2) and the summing node of the op
amp. This eliminates temperature drift associated with the
switched current source. The graphs under the Typical
Performance Characteristics show that the nonlinearity error
is worse at high frequency, when compared with the current
sourced circuit.
The single supply VFC is recommended for uses where
dynamic range of the input is limited, and the input does not
reach 0V. With 10 kHz values, nonlinearity will be less than
1.0% for a 10 mV to 10V input range, and response time will
be about 135 ms.
Precision Current Sourced VFC
Single Supply FVC
This circuit operates similarly to the single supply VFC,
except that the passive R-C integrator has been replaced by
an active op amp integrator. This increases the dynamic
range down to 0V, improves the response time, and
eliminates the nonlinearity error introduced by the limited
compliance of the switched current source output.
A frequency-to-voltage converter performs the exact oppo-
site of the VFCs function; it converts an input pulse train into
an average output voltage. Incoming pulses trigger the input
comparator and fire the one-shot. The one-shot then dumps a
charge into the output integrator. The voltage on the integra-
tor becomes a varying DC voltage proportional to the
frequency of the input signal. Figure 4 shows a complete
single supply FVC.
The integrator algebraically sums the positive current pulses
from the switched current source with the current VIN/RB.
To operate correctly, the input voltage must be negative, so
that when the circuit is balanced, the two currents cancel.
6
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