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부품번호 LF198 기능
기능 Monolithic Sample-and-Hold Circuits
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LF198 데이터시트, 핀배열, 회로
July 2000
LF198/LF298/LF398, LF198A/LF398A
Monolithic Sample-and-Hold Circuits
General Description
The LF198/LF298/LF398 are monolithic sample-and-hold
circuits which utilize BI-FET technology to obtain ultra-high
dc accuracy with fast acquisition of signal and low droop
rate. Operating as a unity gain follower, dc gain accuracy is
0.002% typical and acquisition time is as low as 6 µs to
0.01%. A bipolar input stage is used to achieve low offset
voltage and wide bandwidth. Input offset adjust is accom-
plished with a single pin, and does not degrade input offset
drift. The wide bandwidth allows the LF198 to be included in-
side the feedback loop of 1 MHz op amps without having sta-
bility problems. Input impedance of 1010allows high
source impedances to be used without degrading accuracy.
P-channel junction FET’s are combined with bipolar devices
in the output amplifier to give droop rates as low as 5 mV/min
with a 1 µF hold capacitor. The JFET’s have much lower
noise than MOS devices used in previous designs and do
not exhibit high temperature instabilities. The overall design
guarantees no feed-through from input to output in the hold
mode, even for input signals equal to the supply voltages.
Features
n Operates from ±5V to ±18V supplies
n Less than 10 µs acquisition time
n TTL, PMOS, CMOS compatible logic input
n 0.5 mV typical hold step at Ch = 0.01 µF
n Low input offset
n 0.002% gain accuracy
n Low output noise in hold mode
n Input characteristics do not change during hold mode
n High supply rejection ratio in sample or hold
n Wide bandwidth
n Space qualified, JM38510
Logic inputs on the LF198 are fully differential with low input
current, allowing direct connection to TTL, PMOS, and
CMOS. Differential threshold is 1.4V. The LF198 will operate
from ±5V to ±18V supplies.
An “A” version is available with tightened electrical
specifications.
Typical Connection and Performance Curve
Acquisition Time
DS005692-32
Functional Diagram
DS005692-16
© 2000 National Semiconductor Corporation DS005692
DS005692-1
www.national.com
This datasheet has been downloaded from http://www.digchip.com at this page




LF198 pdf, 반도체, 판매, 대치품
Typical Performance Characteristics (Continued)
Output Droop Rate
Hold Step
“Hold” Settling Time
(Note 10)
DS005692-20
DS005692-21
Leakage Current into Hold
Capacitor
Phase and Gain (Input to
Output, Small Signal)
Gain Error
DS005692-22
DS005692-23
Power Supply Rejection
DS005692-24
Output Short Circuit Current
Output Noise
DS005692-25
Note 10: See Definition
DS005692-26
DS005692-27
DS005692-28
www.national.com
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LF198 전자부품, 판매, 대치품
Application Hints (Continued)
logic input for signal delay, calculate the slope of the wave-
form at the threshold point to ensure that it is at least
1.0 V/µs.
Sampling Dynamic Signals
Sample error to moving input signals probably causes more
confusion among sample-and-hold users than any other pa-
rameter. The primary reason for this is that many users make
the assumption that the sample and hold amplifier is truly
locked on to the input signal while in the sample mode. In ac-
tuality, there are finite phase delays through the circuit creat-
ing an input-output differential for fast moving signals. In ad-
dition, although the output may have settled, the hold
capacitor has an additional lag due to the 300series resis-
tor on the chip. This means that at the moment the “hold”
command arrives, the hold capacitor voltage may be some-
what different than the actual analog input. The effect of
these delays is opposite to the effect created by delays in the
logic which switches the circuit from sample to hold. For ex-
ample, consider an analog input of 20 Vp-p at 10 kHz. Maxi-
mum dV/dt is 0.6 V/µs. With no analog phase delay and 100
ns logic delay, one could expect up to (0.1 µs) (0.6V/µs)
= 60 mVerror if the “hold” signal arrived near maximum dV/dt
of the input. A positive-going input would give a +60 mV er-
ror. Now assume a 1 MHz (3 dB) bandwidth for the overall
analog loop. This generates a phase delay of 160 ns. If the
hold capacitor sees this exact delay, then error due to analog
delay will be (0.16 µs) (0.6 V/µs) = −96 mV. Total output error
is +60 mV (digital) −96 mV (analog) for a total of −36 mV. To
add to the confusion, analog delay is proportioned to hold
capacitor value while digital delay remains constant. A family
of curves (dynamic sampling error) is included to help esti-
mate errors.
A curve labeled Aperture Time has been included for sam-
pling conditions where the input is steady during the sam-
pling period, but may experience a sudden change nearly
coincident with the “hold” command. This curve is based on
a 1 mV error fed into the output.
A second curve, Hold Settling Time indicates the time re-
quired for the output to settle to 1 mV after the “hold” com-
mand.
Digital Feedthrough
Fast rise time logic signals can cause hold errors by feeding
externally into the analog input at the same time the amplifier
is put into the hold mode. To minimize this problem, board
layout should keep logic lines as far as possible from the
analog input and the Ch pin. Grounded guarding traces may
also be used around the input line, especially if it is driven
from a high impedance source. Reducing high amplitude
logic signals to 2.5V will also help.
Guarding Technique
DS005692-5
Use 10-pin layout. Guard around Chis tied to output.
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