LTC2415-1IGN datasheet(33/40 Pages) LINER

33 / 40 page

LTC2415/LTC2415-1

sn2415 24151fs

APPLICATIO S I FOR ATIO

Figure 36. Connecting the LTC2415/LTC2415-1 to a 68HC11 MCU Using the SPI Serial Interface

2415 F35

REF+

REF–

IN+

IN–

1, 7, 8, 9,

10, 15, 16

LTC2415/

LTC2415-1

VCC

GND

µF

TO OTHER

DEVICES

74HC4052

Figure 35. Use a Differential Multiplexer to Expand Channel Capability

LTC2415/

LTC2415-1

SCK

SDO

SCK (PD4)

MISO (PD2)

SS (PD5)

68HC11

2415 F36

Sample Driver for LTC2415/LTC2415-1 SPI Interface

Figure 35 shows the use of an LTC2415/LTC2415-1 with

a differential multiplexer. This is an inexpensive multi-

plexer that will contribute some error due to leakage if

used directly with the output from the bridge, or if

resistors are inserted as a protection mechanism from

overvoltage. Although the bridge output may be within the

input range of the A/D and multiplexer in normal opera-

tion, some thought should be given to fault conditions

that could result in full excitation voltage at the inputs to

the multiplexer or ADC. The use of amplification prior to

the multiplexer will largely eliminate errors associated

with channel leakage developing error voltages in the

source impedance.

The LTC2415/LTC2415-1 have a very simple serial inter-

face that makes interfacing to microprocessors and

microcontrollers very easy.

The listing in Figure 38 is a simple assembler routine for

the 68HC11 microcontroller. It uses PORT D, configuring

it for SPI data transfer between the controller and the

LTC2415/LTC2415-1. Figure 36 shows the simple 3-wire

SPI connection.

The code begins by declaring variables and allocating four

memory locations to store the 32-bit conversion result.

This is followed by initializing PORT D’s SPI configuration.

The program then enters the main sequence. It activates

the LTC2415/LTC2415-1 serial interface by setting the SS

output low, sending a logic low to CS. It next waits in a loop

for a logic low on the data line, signifying end-of-conver-

sion. After the loop is satisfied, four SPI transfers are

completed, retrieving the conversion. The main sequence

ends by setting SS high. This places the LTC2415/

LTC2415-1 serial interface in a high impedance state and

initiates another conversion.

The performance of the LTC2415/LTC2415-1 can be

verified using the demonstration board DC291A, see

Figure 40 for the schematic. This circuit uses the

computer’s serial port to generate power and the SPI

digital signals necessary for starting a conversion and

reading the result. It includes a Labview application

software program (see Figure 39) which graphically cap-

tures the conversion results. It can be used to determine

noise performance, stability and with an external source,

linearity. As exemplified in the schematic, the LTC2415/

LTC2415-1 are extremely easy to use. This demonstra-

tion board and associated software is available by con-

tacting Linear Technology.

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