Frequency-to-Voltage Converter Uses Sample-and-Hold Improve Response R

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Frequency-to-Voltage Converter Uses Sample-and-Hold Improve Response R

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Frequency-to-Voltage Converter Yses Sample-and-Hold Improve Response Ripple
Frequency-to-Voltage Converter Uses Sample-and-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 such
National Semiconductor Linear Brief April 1979
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
00849401
FIGURE Basic Frequency-to-Voltage Converter
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2002 National Semiconductor Corporation
AN008494
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00849402
FIGURE Improved F-to-V Converter Using Sample-and-Hold
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 V/RS 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 fIN. average voltage will same full scale, according same formula Figure peak-to-peak ripple computed peak, mVp-p, using appropriate formula.
Now, input sample-and-hold have 10.000V average value, output will 10.065V, because sample occurs peak value Thus, output with offset, resistor used offset signal lower level. Trim offset adjust VOUT kHz, 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 0.1%, only from down Between VOUT very proportional fIN. output will indeterminate, because sample-and-hold will never sample! However, there many F-to-V applications where 20:1 frequency range adequate.
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Frequency-to-Voltage Converter Yses Sample-and-Hold Improve Response Ripple
Details Operation
(Continued)
00849403
FIGURE Waveforms, Improved F-to-V Converter
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Frequency-to-Voltage Converter Uses Sample-and-Hold Improve Response R

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