29
LTC2401/LTC2402
The LTC2402’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 LTC2402 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
LTC2402 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.
The absolute accuracy (less than 10 ppm total error) of the
LTC2402 enables extremely accurate measurement of
small signals sitting on large voltages. Each of the two
pseudo differential measurements performed by the
LTC2402 is absolutely accurate independent of the com-
mon mode voltage output from the bridge. The pseudo
differential result obtained from digitally subtracting the
two single ended conversion results is accurate to within
the noise level of the device (3µV
RMS
) times the square
root of 2, independent of the common mode input voltage.
Typically, a bridge sensor outputs 2mV/V full scale. With
a 5V excitation, this translates to a full-scale output of
10mV. Divided by the RMS noise of 4.2µV(= 3µV • 1.414),
this circuit yields 2,300 counts with no averaging or
amplification. If more counts are required, several conver-
sions may be averaged (the number of effective counts is
increased by a factor of square root of 2 for each doubling
of averages).
An RTD Temperature Digitizer
RTDs used in remote temperature measurements often
have long lead lengths between the ADC and RTD sensor.
These long lead lengths lead to voltage drops due to
excitation current in the interconnect to the RTD. This
voltage drop can be measured and digitally removed using
the LTC2402 (see Figure 33).
The excitation current (typically 200µA) flows from the
ADC through a long lead length to the remote temperature
sensor (RTD). This current is applied to the RTD, whose
resistance changes as a function of temperature (100Ω to
400Ω for 0°C to 800°C). The same excitation current flows
back to the ADC ground and generates another voltage
drop across the return leads. In order to get an accurate
measurement of the temperature, these voltage drops
must be measured and removed from the conversion
result. Assuming the resistance is approximately the same
Figure 33. RTD Remote Temperature Measurement
APPLICATIO S I FOR ATIO
WUUU
V
CC
LTC2402
FS
SET
ZS
SET
SCK
CH0
SDO
F
O
CS
CH1
GND
3-WIRE
SPI INTERFACE
1
5V
9
8
7
10
24012 F33
2
4
3
+
–
V
RTD
P
t
100Ω
5
I
DC
= 0
I
EXCITATION
= 200µA
I
EXCITATION
= 200µA
R2
R1
5k
25Ω
1000pF
5k25Ω
0.1µF
