Lm331VfSample&Hold

TL/H/8494 F re q u e n c y -to -V o lta g e C o n v e rte r u s e s S a m p le -a n d -H o ld to Im p ro v e R e s p o n s e a n d R ip p le L B -4 5 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 0.6 second settling time, when CFILTER is 1 mF. If you want less ripple than that, the response time will be even slower. If you want quicker response, it is easy to decrease CFILTER, but the ripple will 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, or ten times quicker than the ‘‘basic’’ FVC with CFILTER e 1 mF. VOUT e fIN c # RLRS J c (1.9V) c (1.1RtCt)# outputripplep-p J e # 1CFILTER J c (1.9V) c (1.1RtCt)RS TL/H/8494–1 FIGURE 1. Basic Frequency-to-Voltage Converter TL/H/8494–2 FIGURE 2. Improved F-to-V Converter Using Sample-and-Hold C1995 National Semiconductor Corporation RRD-B30M115/Printed in U. S. A. F re q u e n c y -t o -V o lt a g e C o n v e rt e r u s e s S a m p le -a n d -H o ld L B -4 5 to Im p ro v e R e s p o n s e a n d R ip p le DETAILS OF OPERATION (Refer toFigure 3, Waveforms) When the input frequency waveform has a negative-going transition, pin 6 of the LM331 is driven momentarily lower than the 13V threshold voltage at pin 7. This initiates a tim- ing cycle controlled by the Rt and Ct at pin 5, and also causes a transition from a5V to 0V at pin 3, (the normal VFC logic output) which is usually left unused in F-to-V oper- ation. During the timing cycle (t e 1.1 c Rt c Ct e 75 ms, for the example shown) a precision current source i e 1.9 V/RS flows out of pin 1 of the LM331, and charges V1 up to a value slightly higher than the average DC value of V1. At the end of the timing cycle, V1 stops charging up, and also V2 rises. The 10 kX pull-up resistor is coupled (through the 200 pF capacitor) to V3, and causes the LF398 to sample for about 5 ms. Then the LF398 goes back intohold . This entire operation is repeated at the same frequency as fIN. The average voltage at V1 will be the same 10V full scale, ac- cording to the same formula of Figure 1. And the peak-to- peak ripple can be computed as 65 mV peak, 130 mVp-p, using the appropriate formula. Now, the input to the sample-and-hold at pin 3 may have a 10.000V average DC value, but the output will be at 10.065V, because the sample occurs at the peak value of V1. Thus, to get an output with low offset, a 15 MX resistor is used to offset the V1 signal to a lower level. Trim the offset adjust pot to get VOUT e 1V at 1 kHz, and trim the gain adjust pot to get VOUT e 10V at 10 kHz (the interac- tion is minor), as measured at V4, V5, or V6. The rms value of the ripple at V4 is rather small, but the peak-to-peak rip- ple (spikes and glitches) may be excessive. A simple R-C filter can provide a filtered output at V5; or a simple active filter using an inexpensive LF351, will give sub-millivolt (peak) ripple at V6, with improved settling time and low out- put impedance. This F-to-V converter will have a good linearity, better than 0.1%, but only from 10 kHz down to 500 Hz. Between 200 Hz and 20 Hz, VOUT is not very proportional to fIN. And at 0 Hz, the output will be indeterminate, because the sam- ple-and-hold will never sample! However, there are many F- to-V applications where a 20:1 frequency range is adequate. TL/H/8494–3 FIGURE 3. Waveforms, Improved F-to-V Converter LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or 2. 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