LTC1291 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1291' PDF : 20 Pages View PDF
LTC1291
APPLICATI S I FOR ATIO
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 VCC and GND are used. The
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 250Ω when clocked
at its maximum clock frequency of 1MHz. If the LTC1291
is clocked at the maximum clock frequency and 250Ω 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 VCC without damaging the device.
1N4148 DIODES
CS VCC (VREF)
CH0
CLK
LTC1291
CH1
DOUT
GND
DIN
5V
LTC1291 F13
Figure 13. Overvoltage Protection for Inputs
1k
250Ω
CS VCC (VREF)
CH0
CLK
LTC1291
CH1
DOUT
GND
DIN
5V
LTC1291 F14
Figure 14. Overvoltage Protection for Inputs
1N4148 DIODES
CS VCC (VREF)
1k
CH0
CLK
LTC1291
CH1
DOUT
GND
DIN
5V
LTC1291 F15
Figure 15. Overvoltage Protection for Inputs
18
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