廬
APPLICATION BULLETIN
By Juergen Metzger, Burr-Brown International, Gmbh
Mailing Address: PO Box 11400 鈥?Tucson, AZ 85734 鈥?Street Address: 6730 S. Tucson Blvd. 鈥?Tucson, AZ 85706
Tel: (520) 746-1111 鈥?Twx: 910-952-111 鈥?Telex: 066-6491 鈥?FAX (520) 889-1510 鈥?Immediate Product Info: (800) 548-6132
MULTIPLEXER DATA ACQUISTION SYSTEM
MULTIPLEXERS GENERATE STEEP SIGNAL
EDGES AND TRANSPORT THE CHARGE
Multiplexers have been used for acquisition of quasi-static
signals for a long time. While these signals usually have a
low bandwidth of only a few hertz, applications generally
require that the multiplexer send each signal as fast as
possible to the converter and that the signals be processed
quickly. To ensure fast processing, such acquisition systems
usually include a sense amplifier behind the multiplexer to
adjust the signal level to the analog-to-digital converter and
to prevent current caused by the signals coming from the
multiplexer. But as common as this configuration is today,
it still produces unexpected errors again and again.
Unless the multiplexer is driven in low-impedance mode or
from a capacitive source, charge transmission from one
channel to another can be expected. Errors the size of several
LSBs of a 12-bit converter can arise, especially in passive
low-pass circuits in front of the input, such as those used for
EMC rejection, ESD, and overvoltage protection.
Besides leakage currents, which are very small in modern
designs, important factors are the switching of the multi-
plexer output capacitance and of the input current from the
succeeding sense amplifiers, which are usually dynamically
overloaded at the time of switching.
DYNAMIC OVERLOAD
Multiplexers switch from one channel to the next in a few
nanoseconds. This switching produces a correspondingly
steep signal pulse for the succeeding sense amplifier. This
steep edge, however, is only compensated by feedback
amplifiers after the settling time has finished. Using the
example of an op amp, this process is easy to explain.
Settling time with an op amp: there is +10V at channel 1 and
0V at channel 2 (see Figure 2). The switching produces a
voltage step from 10V to 0V. The output of the op amp also
goes from 10V to 0V in a few microseconds, depending
upon the slew rate. Meanwhile, a stress flow arises at the
input with a peak value of 10V since the feedback input of
the op amp is connected to the output.
MUX
10k鈩?/div>
10V
100nF
10k鈩?/div>
0V
100nF
10k鈩?/div>
100nF
10k鈩?/div>
100nF
10k鈩?/div>
FIGURE 1. Step Response of Single-Ended Multiplexer,
Switching Between 0V and +10V.
10k鈩?/div>
10V
100nF
10k鈩?/div>
0V
100nF
10k鈩?/div>
V
IN
V
OUT
V
IN
V
OUT
Slew Rate
The information provided herein is believed to be reliable; however, BURR-BROWN assumes
no responsibility for inaccuracies or omissions. BURR-BROWN assumes no responsibility for
the use of this information, and all use of such information shall be entirely at the user鈥檚 own
risk. Prices and specifications are subject to change without notice. No patent rights or licenses
to any of the circuits described herein are implied or granted to any third party. BURR-BROWN
does not authorize or warrant any BURR-BROWN product for use in life support devices and/
or systems.
漏
t
FIGURE 2. Overload of the Op Amp Caused by Fast
Switching.
Printed in U.S.A. October, 1995
1995 Burr-Brown Corporation
AB-100
AB-100 產(chǎn)品屬性
4現(xiàn)貨
1 : ￥257.04000散裝
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散裝
在售
清潔刷
連接器
AB-100相關(guān)型號PDF文件下載
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英文版
Application Bulletin AB-12
FAIRCHILD
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Application Bulletin AB-12
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