LTC2654 Datasheet, PDF(19/24 Page) Linear Technology – Quad 16-/12-Bit Rail-to-Rail DACs with 10ppm/°C Max Reference

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LTC2654 Datasheet, PDF (19/24 Pages) Linear Technology – Quad 16-/12-Bit Rail-to-Rail DACs with 10ppm/°C Max Reference

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LTC2654

OPERATION

Any channel or combination of DAC channels can be put

into power-down mode by using command 0100b in

combination with the appropriate DAC address, (n). The

integrated reference is automatically powered down when

external reference is selected using command 0111b. In

addition, all the DAC channels and the integrated refer-

ence together can be put into power-down mode using

Power-Down Chip command 0101b. For all power-down

commands the 16-bit data word is ignored, but still needs

to be clocked in.

Normal operation resumes by executing any command

which includes a DAC update, in software as shown in

Table 1 or by taking the asynchronous LDAC pin low.

The selected DAC is powered up as its voltage output is

updated. When a DAC which is in a powered-down state is

powered up and updated, normal settling is delayed. If less

than four DACs are in a powered-down state prior to the

update command, the power-up delay time is 12Î¼s. If on

the other hand, all four DACs and the integrated reference

are powered down, then the main bias generation circuit

block has been automatically shut down in addition to the

individual DAC ampliï¬ers and integrated reference. In this

case, the power up delay time is 14Î¼s. The power-up of

integrated reference depends on the command that pow-

ered it down. If the reference is powered down using the

Select External Reference Command (0111b) then it can

only be powered back-up using Select Internal Reference

Command (0110b). However if the reference was powered

down using Power-Down Chip Command (0101b) then in

addition to Select Internal Reference Command (0110b),

any command that powers up the DACs will also power-up

the integrated reference.

Asynchronous DAC Update Using LDAC

In addition to the update commands shown in Table 1, the

LDAC pin asynchronously updates all the DAC registers

with the contents of the input registers.

If CS/LD is high, a low on the LDAC pin causes all the

DAC registers to be updated with the contents of the in-

put registers.

If CS/LD is low, a low going pulse on the LDAC pin before

the rising edge of CS/LD powers up all the DAC outputs but

does not cause the output to be updated. If LDAC remains

low after the rising edge of CS/LD, then LDAC is recognized,

the command speciï¬ed in the 24-bit word just transferred

is executed and the DAC outputs are updated.

The DAC outputs are powered up when LDAC is taken

low, independent of the state of CS/LD. The integrated

reference is also powered up if it was powered down us-

ing Power-Down Chip (0101b) command. The integrated

reference will not power up when LDAC is taken low, if

it was powered down using Select External Reference

(0111b) Command.

If LDAC is low at the time CS/LD goes high, it inhibits any

software power-down command (Power-Down n, Power-

Down Chip, Select External Reference) that was speciï¬ed

in the input word.

Reference Modes

For applications where an accurate external reference is

not available, the LTC2654 has a user-selectable, inte-

grated reference. The LTC2654-L has a 1.25V reference

that provides a full-scale output of 2.5V. The LTC2654-H

has a 2.048V reference that provides a full-scale output

of 4.096V. Both references exhibit a typical temperature

drift of 2ppm/Â°C. Internal reference mode can be selected

by using command 0110b, and is the power-on default.

A buffer is needed if the internal reference is required to

drive external circuitry. For reference stability and low

noise, connect a 0.1Î¼F capacitor between REFCOMP and

GND. In this conï¬guration, the internal reference can drive

up to 0.1Î¼F capacitive load without any stability problems.

In order to ensure stable operation, the capacitive load on

REFIN/OUT pin should not exceed the capacitive load on

the REFCOMP pin.

The DAC can also operate in external reference mode using

command 0111b. In this mode, REFIN/OUT pin acts as an

input that sets the DACâs reference voltage. The input is

high impedance and does not load the external reference

source. The acceptable voltage range at this pin is 0.5V â¤

REFIN/OUT â¤ VCC/2. The resulting full-scale output voltage

is 2â¢VREFIN/OUT. For using External Reference at start-up,

see the Power Supply Sequencing and Start-Up Section.

2654f

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