LTC2401 Linear Technology, LTC2401 Datasheet - Page 10

• Manufacturer Part Number: LTC2401
• Description: 1-/2-Channel 24-Bit uPower No Latency ADC in MSOP-10
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
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
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 (15,600Hz) 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
10
U
U
100
P
t
W
V
Figure 8. RTD Remote Temperature Measurement
RTD
+
–
I
I
EXCITATION
EXCITATION
U
I
DC
R1
R2
= 0
= 200 A
= 200 A
th
2
4
3
5
the noise level of the device (3 V
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 4.2 V(= 3 V • 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 8).
The excitation current (typically 200 A) 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 (100 to
400 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
FS
CH0
ZS
CH1
SET
SET
LTC2402
GND
V
5V
CC
1
SDO
SCK
CS
F
O
9
8
7
10
3-WIRE
SPI INTERFACE
24012 F08
RMS
) divided by square