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LTC2421/LTC2422

24212f

APPLICATIO S I FOR ATIO

The absolute accuracy (less than 10 ppm total error) of the

LTC2422 enables extremely accurate measurement of

small signals sitting on large voltages. Each of the two

pseudo differential measurements performed by the

LTC2422 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

µVRMS) 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 8.4

µV(= 6µV • 1.414),

this circuit yields 1190 counts with no averaging or ampli-

fication. If more counts are required, several conversions

may be averaged (the number of effective counts is in-

creased 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 exci-

tation current in the interconnect to the RTD. This voltage

drop can be measured and digitally removed using the

LTC2422 (see Figure 35).

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

Ω 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 for the

forward and return paths (R1 = R2), the auxiliary channel

on the LTC2422 can measure this drop. These errors are

then removed with simple digital correction.

The result of the first conversion on CH0 corresponds to an

input voltage of VRTD + R1 • IEXCITATION. The result of the

second conversion (CH1) is – R1 • IEXCITATION. Note, the

LTC2422’s input range is not limited to the supply rails, it

has underrange capabilities. The device’s input range is

– 300mV to VREF + 300mV. Adding the two conversion

results together, the voltage drop across the RTD’s leads

are cancelled and the final result is VRTD.

An Isolated, 20-Bit Data Acquisition System

The LTC1535 is useful for signal isolation. Figure 36 shows

a fully isolated, 20-bit differential input A/D converter imple-

mented with the LTC1535 and LTC2422. Power on the

isolated side is regulated by an LT1761-5.0 low noise, low

dropout micropower regulator. Its output is suitable for

driving bridge circuits and for ratiometric applications.

During power-up, the LTC2422 becomes active at VCC =

2.3V, while the isolated side of the LTC1535 must wait for

VCC2 to reach its undervoltage lockout threshold of 4.2V.

Figure 35. RTD Remote Temperature Measurement

VCC

LTC2422

FSSET

ZSSET

SCK

CH0

SDO

CH1

GND

3-WIRE

SPI INTERFACE

24212 F35

–

VRTD

100

Ω

IDC = 0

IEXCITATION = 200µA

Ω

1000pF

Ω

0.1

µF

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