LB-45 Datasheet, PDF (2/2 Pages) National Semiconductor (TI) – Frequency-to-Voltage Converter uses Sampleand- Hold to Improve Response and Ripple

DETAILS OF OPERATION (Refer to Figure 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 R_t and C_t 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 R_t c C_t e 75 ms for the
example shown) a precision current source i e 1 9 V R_S
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 into hold This entire
operation is repeated at the same frequency as f_IN 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 V_OUT e 1V at 1 kHz and trim the
gain adjust pot to get V_OUT 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 V_OUT is not very proportional to f_IN 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
LIFE SUPPORT POLICY
FIGURE 3 Waveforms Improved F-to-V Converter
TL H 8494 – 3
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be reasonably expected to cause the failure of the life
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