LTC1293_15 Datasheet, PDF (24/28 Pages) Linear Technology – Single Chip 12-Bit Data Acquisition System

LTC1293/LTC1294/LTC1296
APPLICATI S I FOR ATIO
This means four channels can handle 7mA of input current
each. Reducing CLK frequency from a maximum of 1MHz
(See typical performance characteristics curves Maxi-
mum CLK Frequency vs Source Resistance and Sample
and Hold Acquisition Time vs Source Resistance) allows
the use of larger current limiting resistors. The “+” input
can accept a resistor value of 1kΩ but the “–” input cannot
accept more than 250Ω when the maximum clock fre-
quency of 1MHz is used. If the LTC1293/4/6 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 20a and 20b). 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 “–” input (see
discussion on Analog Inputs and the typical performance
characteristics curve Maximum CLK Frequency vs Source
Resistance).
If V_CC and V_REF are not tied together, then V_CC should be
turned on first, then V_REF. If this sequence cannot be met
connecting a diode from V_REF to V_CC is recommended (see
Figure 21).
For dual supplies (bipolar mode) placing two Schottky
diodes from V_CC and V^– to ground (Figure 22) will prevent
1N4148 DIODES
V_CC
+5V
1k
+
LTC1293/4/6
–
AGND
DGND
V^–
–5V
LTC1293 F20b
Figure 20b. Overvoltage Protection for Inputs
power supply reversal from occuring when an input source
is applied to the analog MUX before power is applied to the
device. Power supply reversal occurs, for example, if the
input is pulled below V^–. V_CC will then pull a diode drop
below ground which could cause the device not to power
up properly. Likewise, if the input is pulled above V_CC, V^–
will be pulled a diode drop above ground. If no inputs are
present on the MUX, the Schottky diodes are not required
if V^– is applied first then V_CC.
Because a unique input protection structure is used on the
digital input pins, the signal levels on these pins can
exceed the device V_CC without damaging the device.
1N4148 DIODES
V_CC
+5V
LTC1293/4/6
AGND
DGND
V^–
–5V
LTC1293 F19
Figure 19. Overvoltage Protection for Inputs
V_CC
+5V
1k
+
250Ω
LTC1293/4/6
–
AGND
DGND
V^–
–5V
LTC1293 F20a
Figure 20a. Overvoltage Protection for Inputs
24
V_CC
LTC1293/4/6
REF⁺
AGND
DGND
Figure 21
+5V
1N4148
+5V
LTC1293 F21
V_CC
LTC1293/4/6
+5V
1N5817
DGND
AGND
V^–
–5V
1N5817
LTC1293 F22
Figure 22. Power Supply Reversal
129346fs