10

LTC2401/LTC2402

APPLICATIO S I FOR ATIO

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

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

the noise level of the device (3

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

µV(= 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 8).

The excitation current (typically 200

µA) 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

Ω to

400

Ωfor0°Cto800°C).Thesameexcitationcurrentflows

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

LTC2402

SCK

CH0

SDO

CS

CH1

GND

3-WIRE

SPI INTERFACE

1

5V

9

8

7

10

24012 F08

2

4

3

+

–

100

Ω

5

R2

R1

Figure 8. RTD Remote Temperature Measurement