LTC2686 datasheet(24/45 Pages) AD

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Data Sheet

LTC2686

THEORY OF OPERATION

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Rev. A | 24 of 45

SPAN SELECTION AND CHANNEL SETTINGS

Figure 51. DAC Transfer Functions for LTC2686-16, All Voltage Spans and Bit

Resolutions, Internal Reference Mode

Figure 52. DAC Transfer Functions for LTC2686-12, All Voltage Spans and Bit

Resolutions, Internal Reference Mode

The LTC2686 allows individual programming of the voltage span

(output voltage range) for each DAC channel (SoftSpan). The

device also supports 5% overrange spans. The span selection bits,

S[3:0], encode span settings as summarized in Table 14 and Table

15, and the corresponding DAC transfer functions are depicted in

Figure 51.

Table 14. Span Selection Bits S[3:0] Settings

Output Range

Internal Reference External Reference

0 V to 5 V

0 V to 5 × (VREF/4.096)

0 V to 10 V

0 V to 10 × (VREF/4.096)

±5 V

±5 × (VREF/4.096)

±10 V

±10 × (VREF/4.096)

±15 V

±15 × (VREF/4.096)

Table 15. Span Selection Bits S[3:0] Settings, 5% Overrange

Output Range

Internal Reference External Reference

0 V to 5.25 V

0 V to 5 × (VREF/4.096)

0 V to 10.5 V

0 V to 10 × (VREF/4.096)

±5.25 V

±5 × (VREF/4.096)

±10.5 V

±10 × (VREF/4.096)

±15.75 V

±15 × (VREF/4.096)

Use Command 1, detailed in Table 13, to program DAC channel

settings and span ranges. Span settings are contained in Bits S[3:0]

and the default value is 0x0. Note that X represents the don’t care

bits and C represents the CRC code bits. If CRC error checking is

not used, the third data byte can be omitted (24-bit word length).

See the Toggle and Dither Operation section for a detailed descrip-

tion of related registers and functions.

Command 1 writes the settings of DAC Channel x, with DAC

Channel Address x encoded in A[3:0]. Command 16 writes settings

to all DAC channels, whereas Command 17 also updates all DAC

channels.

READ COMMANDS

Read operations are initiated with read commands as shown in

Table 16. The command byte is followed by X (don’t care bits),

and an optional CRC code. For example, when reading the config-

uration register, the user must send Command 24 (see Table 16)

to the device. The requested data appears at the SDO pin in the

next command cycle (see Table 17) and mirrors the syntax of the

corresponding write command. Note that the device outputs trailing

zeros for the third byte. For a list of supported read commands,

refer to Table 9.

Table 16. Command Word for Reading Configuration Register, Command 24

Command Code

Byte 1

Byte 2

Byte 3

1111 0000

C/X

Table 17. SDO Pin Data-Words After the Read Configuration Register Command

Command Code

Byte 1

Byte 2

Byte 3

1111 0000

RST

TSD_

DIS

EXT_

REF

SPI_

CRC

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