LTC1291 Linear Technology, LTC1291 Datasheet - Page 18

• Manufacturer Part Number: LTC1291
• Description: Single Chip 12-Bit Data Acquisition System
• Manufacturer: Linear Technology

LTC1291
A
Overvoltage Protection
Applying signals to the LTC1291’s analog inputs that
exceed the positive supply or that go below ground will
degrade the accuracy of the A/D and possibly damage the
device. For example this condition would occur if a signal
is applied to the analog inputs before power is applied to
the LTC1291. It can also happen if the input source is
operating from supplies of larger value than the LTC1291
supply. These conditions should be prevented either with
proper supply sequencing or by use of external circuitry to
clamp or current limit the input source.
There are two ways to protect the inputs. In Figure 13 diode
clamps from the inputs to V
second method is to put resistors in series with the analog
inputs for current limiting. Limit the current to 15mA per
channel. The +IN input can accept a resistor value of 1k but
the –IN input cannot accept more than 250 when clocked
at its maximum clock frequency of 1MHz. If the LTC1291
is clocked at the maximum clock frequency and 250 is
not enough to current limit the input source then the clamp
diodes are recommended (Figures 14 and 15). The reason
for the limit on the resistor value is the MSB bit test is
affected by the value of the resistor placed at the –IN input
(see discussion on Analog Inputs and the typical perfor-
mance characteristics Maximum CLK Frequency vs Source
Resistance).
Because a unique input protection structure is used on the
digital input pins, the signal levels on these pins can
exceed the device V
18
PPLICATI
O
U
CC
S
without damaging the device.
I FOR ATIO
U
CC
and GND are used. The
W
U
1N4148 DIODES
Figure 15. Overvoltage Protection for Inputs
Figure 13. Overvoltage Protection for Inputs
Figure 14. Overvoltage Protection for Inputs
1k
1N4148 DIODES
250
1k
CS
CH0
CH1
GND
CS
CH0
CH1
GND
CS
CH0
CH1
GND
LTC1291
LTC1291
V
V
CC
LTC1291
CC
V
(V
(V
CC
D
REF
CLK
D
OUT
D
REF
CLK
OUT
D
(V
IN
D
IN
)
REF
CLK
OUT
)
D
IN
)
5V
LTC1291 F14
LTC1291 F15
LTC1291 F13
5V
5V