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參數(shù)資料
| 型號： | LB-45 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Frequency-to-Voltage Converter uses Sampleand- Hold to Improve Response and Ripple |
| 中文描述： | 頻率至電壓轉(zhuǎn)換器使用Sampleand持有的提高反應(yīng)和紋波 |
| 文件頁數(shù)： | 2/2頁 |
| 文件大?。?/td> | 82K |
| 代理商： | LB-45 |
F
L
t
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 R
t
and C
t
at pin 5, and also
causes a transition from
a
5V 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
m
s, 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 k
X
pull-up resistor is coupled (through the 200
pF capacitor) to V3, and causes the LF398 to sample for
about 5
m
s. Then the LF398 goes back intohold. 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 M
X
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.
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
systems which, (a) are intended for surgical implant
into the body, or (b) support or sustain life, and whose
failure to perform, when properly used in accordance
with instructions for use provided in the labeling, can
be reasonably expected to result in a significant injury
to the user.
2. A critical component is any component of a life
support device or system whose failure to perform can
be reasonably expected to cause the failure of the life
support device or system, or to affect its safety or
effectiveness.
National Semiconductor
Corporation
1111 West Bardin Road
Arlington, TX 76017
Tel: 1(800) 272-9959
Fax: 1(800) 737-7018
National Semiconductor
Europe
Fax: (
a
49) 0-180-530 85 86
Email: cnjwge
@
tevm2.nsc.com
Deutsch Tel: (
a
49) 0-180-530 85 85
English
Tel: (
a
49) 0-180-532 78 32
Fran
3
ais Tel: (
a
49) 0-180-532 93 58
Italiano
Tel: (
a
49) 0-180-534 16 80
National Semiconductor
Hong Kong Ltd.
13th Floor, Straight Block,
Ocean Centre, 5 Canton Rd.
Tsimshatsui, Kowloon
Hong Kong
Tel: (852) 2737-1600
Fax: (852) 2736-9960
National Semiconductor
Japan Ltd.
Tel: 81-043-299-2309
Fax: 81-043-299-2408
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.
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