Datasheet

28
LTC2401/LTC2402
There are no extra control or status pins required to
perform the alternating 2-channel measurements. The
LTC2402 only requires two digital signals (SCK and SDO).
This simplification is ideal for isolated temperature mea-
surements or systems where minimal control signals are
available.
Pseudo Differential Applications
Generally, designers choose fully differential topologies
for several reasons. First, the interface to a 4- or 6-wire
bridge is simple (it is a differential output). Second, they
require good rejection of line frequency noise. Third, they
typically look at a small differential signal sitting on a
large common mode voltage; they need accurate
measurements of the differential signal independent of
Figure 31. Embedded Selected Channel Indicator
Figure 32. Pseudo Differential Strain Guage Application
APPLICATIO S I FOR ATIO
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the common mode input voltage. Many applications
currently using fully differential analog-to-digital con-
verters for any of the above reasons may migrate to a
pseudo differential conversion using the LTC2402.
Direct Connection to a Full Bridge
The LTC2402 interfaces directly to a 4- or 6-wire bridge, as
shown in Figure 32. The LTC2402 includes a FS
SET
and a
ZS
SET
for sensing the excitation voltage directly across the
bridge. This eliminates errors due to excitation currents
flowing through parasitic resistors. The LTC2402 also
includes two single ended input channels which can tie
directly to the differential output of the bridge. The two
conversion results may be digitally subtracted yielding the
differential result.
24012 F31
SCK
SDO
CH1
• • • • • •
CH1 DATA OUT
CH0 DATA OUT
EOC
CH0
EOC
V
CC
LTC2402
FS
SET
ZS
SET
SCK
CH1
SDO
F
O
CS
CH0
GND
3-WIRE
SPI INTERFACE
1
5V
9
8
7
10
24012 F32
2
350Ω 350Ω
350Ω 350Ω
3
4
5
I
DC
= 0
I
EXCITATION
I
DC
= 0