Datasheet

28
LTC2421/LTC2422
24212f
APPLICATIO S I FOR ATIO
WUUU
The selection between CH0 and CH1 is automatic. Initially,
after power-up, a conversion is performed on CH0. For
each subsequent conversion, the input channel selection
is alternated. Embedded within the serial data output is a
status bit indicating which channel corresponds to the
conversion result. If the conversion was performed on
CH0, this bit (Bit 22) is LOW and is HIGH if the conversion
was performed on CH1 (see Figure 33).
There are no extra control or status pins required to per-
form the alternating 2-channel measurements. The
LTC2422 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
the common mode input voltage. Many applications cur-
rently using fully differential analog-to-digital converters
for any of the above reasons may migrate to a pseudo
differential conversion using the LTC2422.
Direct Connection to a Full Bridge
The LTC2422 interfaces directly to a 4- or 6-wire bridge,
as shown in Figure 34. The LTC2422 includes a FS
SET
and
a ZS
SET
for sensing the excitation voltage directly across
the bridge. This eliminates errors due to excitation cur-
rents flowing through parasitic resistors. The LTC2422
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.
The LTC2422’s single ended rejection of line frequencies
(±2%) and harmonics is better than 110dB. Since the
device performs two independent single ended conver-
sions each with >110dB rejection, the overall common
mode and differential rejection is much better than the
80dB rejection typically found in other differential input
delta-sigma converters.
In addition to excellent rejection of line frequency noise,
the LTC2422 also exhibits excellent single ended noise
rejection over a wide range of frequencies due to its 4
th
order sinc filter. Each single ended conversion indepen-
dently rejects high frequency noise (>60Hz). Care must be
taken to insure noise at frequencies below 15Hz and at
multiples of the ADC sample rate (15,360Hz) are not
present. For this application, it is recommended the
LTC2422 is placed in close proximity to the bridge sensor
in order to reduce the noise injected into the ADC input. By
performing three successive conversions (CH0-CH1-CH0),
the drift and low frequency noise can be measured and
compensated for digitally.
Figure 33. Embedded Selected Channel Indicator
24212 F33
SCK
SDO
CH1
• • • • • •
CH1 DATA OUT
CH0 DATA OUT
EOC
CH0
EOC
Figure 34. Pseudo Differential Strain Guage Application
V
CC
LTC2422
FS
SET
ZS
SET
SCK
CH1
SDO
F
O
CS
CH0
GND
3-WIRE
SPI INTERFACE
1
5V
9
8
7
10
6
24212 F32
2
350Ω 350Ω
350Ω 350Ω
3
4
5
I
DC
= 0
I
EXCITATION
I
DC
= 0