LM331MWC【PDF 18/18 页】VOLTAGE-FREQUENCY CONVERTER, 0.1 MHz, UU

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Typical Applications (Continued)

DETAILS OF OPERATION, FREQUENCY-TO-

VOLTAGE CONVERTERS (

Figure 6 and Figure 7)

In these applications, a pulse input at f

IN is differentiated by

a C-R network and the negative-going edge at pin 6 causes

the input comparator to trigger the timer circuit. Just as with

a V-to-F converter, the average current flowing out of pin 1 is

AVERAGE = i x (1.1 RtCt)xf.

In the simple circuit of

Figure 6, this current is filtered in the

network R

L = 100 k and 1 F. The ripple will be less than 10

mV peak, but the response will be slow, with a 0.1 second

time constant, and settling of 0.7 second to 0.1% accuracy.

In the precision circuit, an operational amplifier provides a

buffered output and also acts as a 2-pole filter. The ripple will

be less than 5 mV peak for all frequencies above 1 kHz, and

the response time will be much quicker than in

Figure 6.

However, for input frequencies below 200 Hz, this circuit will

have worse ripple than

Figure 6. The engineering of the filter

time-constants to get adequate response and small enough

ripple simply requires a study of the compromises to be

made. Inherently, V-to-F converter response can be fast, but

F-to-V response can not.

DS005680-6

*Use stable components with low temperature coefficients.

See Typical Applications section.

**This resistor can be 5 k

or 10 k for VS=8V to 22V, but must be 10 k for VS=4.5V to 8V.

***Use low offset voltage and low offset current op amps for A1: recommended types LF411A or LF356.

FIGURE 5. Precision Voltage-to-Frequency Converter,

100 kHz Full-Scale, ±0.03% Non-Linearity

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