Frequency-to-Voltage Converter uses Sampleand-Hold Improve Response Ri

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Frequency-to-Voltage Converter uses Sampleand-Hold Improve Response Ri

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Frequency-to-Voltage Converter uses Sample-and-Hold Improve Response Ripple
Frequency-to-Voltage Converter uses Sampleand-Hold Improve Response Ripple
Most frequency-to-voltage (F-to-V) converters suffer from classical tradeoff ripple versus speed response example basic F-to-V converter shown below mVp-p ripple rather slow second settling time when CFILTER want less ripple than that response time will even slower want quicker response easy decrease CFILTER ripple will increase same factor improved circuit Figure makes end-run around these compromises low-cost sample-and-hold circuit
National Semiconductor Linear Brief April 1979
such LF398 sample F-to-V's output peak ripple hold until next cycle LF398 fairly output ripple (rms) does have some short duration noise spikes glitches which removed easily with simple output filter ripple output active filter smaller than peak settling time step change input frequency only times quicker than ``basic'' with CFILTER
VOUT
output ripple
FILTER
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FIGURE Basic Frequency-to-Voltage Converter
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FIGURE Improved F-to-V Converter Using Sample-and-Hold
C1995 National Semiconductor Corporation
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RRD-B30M115 Printed
Frequency-to-Voltage Converter uses Sample-and-Hold Improve Response Ripple
DETAILS OPERATION (Refer Figure Waveforms) When input frequency waveform negative-going transition LM331 driven momentarily lower than threshold voltage This initiates timing cycle controlled also causes transition from (the normal logic output) which usually left unused F-to-V operation During timing cycle example shown) precision current source flows LM331 charges value slightly higher than average value timing cycle stops charging also rises pull-up resistor coupled (through capacitor) causes LF398 sample about Then LF398 goes back into hold This entire operation repeated same frequency average voltage will same full scale according same formula Figure peak-topeak ripple computed peak mVp-p using appropriate formula
input sample-and-hold have 000V average value output will 065V because sample occurs peak value Thus output with offset resistor used offset signal lower level Trim offset adjust VOUT trim gain adjust VOUT (the interaction minor) measured value ripple rather small peak-to-peak ripple (spikes glitches) excessive simple filter provide filtered output simple active filter using inexpensive LF351 will give sub-millivolt (peak) ripple with improved settling time output impedance This F-to-V converter will have good linearity better than only from down Between VOUT very proportional output will indeterminate because sample-and-hold will never sample However there many Fto-V applications where frequency range adequate
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FIGURE Waveforms Improved F-to-V Converter LIFE SUPPORT POLICY NATIONAL'S PRODUCTS AUTHORIZED CRITICAL COMPONENTS LIFE SUPPORT DEVICES SYSTEMS WITHOUT EXPRESS WRITTEN APPROVAL PRESIDENT NATIONAL SEMICONDUCTOR CORPORATION used herein Life support devices systems devices systems which intended surgical implant into body support sustain life whose failure perform when properly used accordance with instructions provided labeling reasonably expected result significant injury user
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critical component component life support device system whose failure perform reasonably expected cause failure life support device system affect safety effectiveness
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Frequency-to-Voltage Converter uses Sampleand-Hold Improve Response Ri

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