RC4153 Ver la hoja de datos (PDF) - Fairchild Semiconductor

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RC4153

Voltage-to-Frequency Converter

Fairchild Semiconductor 

PRODUCT SPECIFICATION

RC4153

For this reason, the scale factor you choose should be below

1 KHz/V or as low as the acquisition time of your system

will allow.

Nonlinearity is also affected by the rate of CI to CO. Less

error can be achieved by increasing the value of CI, but this

affects response time and temperature drift. Optimum value

for CI and CO are shown In the tables in Figures 1, 2, and 3.

These values represent the best compromise of nonlinearity

and temperature drift. Polypropylene, mylar or polystyrene

capacitors should be used for CI.

The accuracy at low input voltages is limited by the offset

and VOS drift of the op amp. To improve this condition, an

offset adjust is provided.

Once your system is running, it may be calibrated as follows:

apply a measured full scale input voltage and adjust RIN until

the scale factor is correct. For precise applications, trimming

by soldering metal film resistors in parallel is recommended

instead of trimpots, which have bad tempco’s and are easily

taken out of adjustment by mechanical shock. After the scale

factor is calibrated, apply a known small input voltage

(approximately 10 mV) and adjust the op amp offset until the

output frequency equals the input multiplied by the scale

factor.

The output E consists of a series of negative going pulses

with a pulse width equal to the one-shot time. The open

collector pull-up resistor may be connected to a different

supply (such as 5V for TTL) as long a it does not exceed the

value of +VS applied to pin 10. The load current should be

kept below 10 mA in order to minimize strain on the device.

Pins 2 and 8 must be grounded in all applications, even if the

open collector transistor is not used.

Figure 6 shows the complete circuit for a precision

frequency-to-voltage converter. The circuit converts an

input frequency to a proportional voltage by integrating the

switched current source output. As the input frequency

increases, the number of IOUT pulses delivered to the integra-

tor increases, thus increasing the average output voltage.

Depending on the time constant of the integrator, there will

be some ripple on the output. The output may be further

filtered, but this will reduce the response time. A second

order filter will decrease ripple and improve response time.

The input waveform must meet three conditions for proper

frequency-to-voltage operation. First, it must have sufficient

amplitude and offset to swing above and below the 1.3V

trigger threshold (See Figure 6 for an example of AC

coupling and offset bias.) Second, it must be a fast slewing

waveform having a quick rise time. A comparator may be

used to square it up. Finally, the input pulse width must not

exceed the one-shot time, in order to avoid retriggering the

one-shot (AC couple the Input).

Capacitive coupling between the trigger input and the timing

capacitor pin may occur if the input waveform is a square-

wave or the input has a short period. This can cause gross

nonlinearity due to changes in the one-shot timing waveform

(See Figure 7). This problem can be avoided by keeping the

value of CO small, and thereby keeping the timing period

less than the input waveform period.

-15V +15V

+VS

RA

F IN

CA

RB

100 W

100K

Comparator with

Hysteresis

Input

Coupling

1

13

-VS

VOS2

10

+VS

VOUT

4

14 VOS1

7

Trig

3

VREF

5 IIN

4153

RI

-In

12

+In 11

RI

GND2 GND1 FOUT CO

2

8 96

CO

Figure 6. Frequency-to-Voltage Precision Converter

VOUT+

CI

4153-09

10

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