TLH8494

National Semiconductor

Linear Brief 45

April 1979

Frequency-to-Voltage

Converter uses Sample-

and-Hold to Improve

Response and Ripple

Most frequency-to-voltage (F-to-V) converters suffer from

the classical tradeoff of ripple versus speed of response

For example the basic F-to-V converter shown below has

13 mVp-p of ripple and a rather slow 06 second settling

the response time will be even slower If you want quicker

increase by the same factor

The improved circuit in

Figure 2 makes an end-run around

these compromises A low-cost sample-and-hold circuit

such as LF398 can sample the F-to-V’s output at the peak

of its ripple and hold it until the next cycle The LF398 has

fairly low output ripple (rms) but it does have some short

duration noise spikes and glitches which can be removed

easily with a simple output filter The ripple at the output of

the active filter V6 is smaller than 1 mV peak but the set-

tling time for a step change of input frequency is only 60 ms

1 mF

S

ripple

p-p

TLH8494 – 1

FIGURE 1 Basic Frequency-to-Voltage Converter

TLH8494 – 2

FIGURE 2 Improved F-to-V Converter Using Sample-and-Hold

C1995 National Semiconductor Corporation

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