Adc Input Buffer And Protection Techniques

Cirrus Logic, Inc.

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AN20
Application Note
ADC INPUT Buffer AND PROTECTION TECHNIQUES
By Steven Green
+15V
CR5
5.6V
2k
Left
Analog
Input
2k
CR1
51
R1
2k
Right
Analog
Input
2k
51
R2
10 nF
VA+
0.1 uF
+
1 uF
VA-
5
27
1
CR4
10 nF
CR2
51
+ 1 uF
0.1 uF
4
VA+
25
VL+
0.1 uF
BAT-85
HP5082-2810
2 AINL
CS5336
CS5338
CS5339
AINR
AGND
CR3
-15V
CR6
5.6V
Figure 1. ADC Input Protection ±15 V Op-Amp
INTRODUCTION
The design of input Buffer and protection circuits
for analog-to-digital-converters (ADC) is critical
to an optimized and reliable Data Acquisition sys-
tem. The Crystal Semiconductor application note
"ADC Input Buffers" covered this area well and the
system designer should review this information.
Since the publication of "ADC Input Buffers" there
have been many requests for additional informa-
tion and circuits relating to ADC input protection.
This application note describes suitable Buffer pro-
tection circuits for the CS5336 family of convert-
ers. The techniques described are equally
applicable to the other families of Crystal analog-
to-digital converters.
Cirrus Logic, Inc.
Crystal Semiconductor Products Division
P.O. Box 17847, Austin, Texas 78760
(512) 445 7222 FAX: (512) 445 7581
http://www.crystal.com
SCR LATCH-UP
SCR latch-up has been defined as "the creation of a
low impedance path between the power supply
rails by the triggering of parasitic, four- layer bipo-
lar structures (SCR’s) inherent in CMOS input and
output circuitry." This is a self-sustaining condition
and once latched, a CMOS device will remain so
regardless of the I/O pin voltages until the power
supply voltages are removed. The excessive power
dissipation during latch-up may also damage the
device. Latch-up is most often caused by forcing
current into the inputs or outputs of a CMOS device
by applying voltages greater than the power supply
rails. When powered, Crystal Semiconductor
ADC’s are extremely immune to latch-up because
Copyright
©
Cirrus Logic, Inc. 1998
(All Rights Reserved)
FEB ‘98
AN20REV2
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