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-

VFC logic output) which is usually left unused in F-to-V oper-

ation

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

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

10000V average DC value

but the output will be at

10065V 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

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

01% but only from 10 kHz down to 500 Hz Between

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 201 frequency range is adequate

TLH8494 – 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 A critical component is any component of a life

systems which (a) are intended for surgical implant

support device or system whose failure to perform can

into the body or (b) support or sustain life and whose

be reasonably expected to cause the failure of the life

failure to perform when properly used in accordance

support device or system or to affect its safety or

with instructions for use provided in the labeling can

effectiveness

be reasonably expected to result in a significant injury

to the user

National Semiconductor

National Semiconductor

National Semiconductor

National Semiconductor

Corporation

Europe

Hong Kong Ltd

Japan Ltd

1111 West Bardin Road

Fax (a49) 0-180-530 85 86

13th Floor Straight Block

Tel 81-043-299-2309

Arlington TX 76017

Email cnjwge tevm2nsccom

Ocean Centre 5 Canton Rd

Fax 81-043-299-2408

Tel 1(800) 272-9959

Deutsch Tel (a49) 0-180-530 85 85

Tsimshatsui Kowloon

Fax 1(800) 737-7018

English

Tel (a49) 0-180-532 78 32

Hong Kong

Fran ais Tel (a49) 0-180-532 93 58

Tel (852) 2737-1600

Italiano

Tel (a49) 0-180-534 16 80

Fax (852) 2736-9960

National does not assume any responsibility for use of any circuitry described no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications