9

LTC2401/LTC2402

APPLICATIO S I FOR ATIO

Figure 6. Embedded Selected Channel Indicator

24012 F06

SCK

SDO

CH1

• • •

• • •

CH1 DATA OUT

CH0 DATA OUT

EOC

CH0

EOC

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 30) is LOW and is HIGH if the conversion

was performed on CH1 (see Figure 6).

There are no extra control or status pins required to

perform the alternating 2-channel measurements. The

LTC2402 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

currently using fully differential analog-to-digital con-

verters for any of the above reasons may migrate to a

pseudo differential conversion using the LTC2402.

Direct Connection to a Full Bridge

The LTC2402 interfaces directly to a 4- or 6-wire bridge, as

shown in Figure 7. Like the LTC2401, the LTC2402 in-

voltage directly across the bridge. This eliminates errors

due to excitation currents flowing through parasitic resis-

tors. The LTC2402 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.

LTC2402

SCK

CH1

SDO

CS

CH0

GND

3-WIRE

SPI INTERFACE

1

5V

9

8

7

10

24012 F07

2

350

Ω

350

Ω

350

Ω

350

Ω

3

4

5

Figure 7. Pseudo Differential Strain Guage Application