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LTC2656

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OPERATION

The LTC2656 is a family of octal voltage output DACs in

20-lead 4mm × 5mm QFN and in 20-lead thermally en-

hancedTSSOPpackages.EachDACcanoperaterail-to-rail

in external reference mode, or with its full-scale voltage

set by an integrated reference. Four combinations of ac-

curacy (16-bit and 12-bit), and full-scale voltage (2.5V or

4.096V) are available. The LTC2656 is controlled using a

4-wire SPI/MICROWIRE compatible interface.

Power-On Reset

TheLTC2656-L/LTC2656-Hcleartheoutputtozero-scaleif

the PORSEL pin is tied to GND, when power is first applied,

makingsysteminitializationconsistentandrepeatable.For

some applications, downstream circuits are active during

DAC power-up and may be sensitive to nonzero outputs

from the DAC during this time. The LTC2656 contains

circuitry to reduce the power-on glitch. The analog outputs

typicallyriselessthan10mVabovezero-scaleduringpower

on if the power supply is ramped to 5V in 1ms or more.

In general, the glitch amplitude decreases as the power

supply ramp time is increased. See Power-On Reset Glitch

in the Typical Performance Characteristics.

Alternatively, if the PORSEL pin is tied to VCC, the

LTC2656-L/ LTC2656-H sets the output to mid-scale when

power is first applied.

Power Supply Sequencing and Start-Up

For the LTC2656 family of parts, the internal reference is

powered up at start-up by default. If an external reference

is to be used, the REFCOMP pin must be hardwired to

GND. Having REFCOMP hardwired to GND at power up

will cause the REFIN/OUT pin to become high impedance

and will allow for the use of an external reference at start-

up. However in this configuration, the internal reference

will still be on even though it is disconnected from the

REFIN/OUT pin and will draw supply current. In order

to use external reference after power-up, the command

Select External Reference (0111b) should be used to turn

the internal reference off (see Table 1.)

The voltage at REFIN/OUT should be kept within the range

– 0.3V ≤ REFIN/OUT ≤ VCC + 0.3V if the external reference

is to be used (see Absolute Maximum Ratings). Particular

care should be taken to observe these limits during power

supply turn-on and turn-off sequences, when the voltage

at VCC is in transition.

Transfer Function

The digital-to-analog transfer function is:

VOUT(IDEAL) =









• 2 • VREF – VREFLO

(

) + VREFLO

where k is the decimal equivalent of the binary DAC input

code, N is the resolution of the DAC, and VREF is the volt-

age at the REFIN/OUT pin. The resulting DAC output span

is0Vto2•VREF, as it is necessary to tie REFLO to GND.

VREF is nominally 1.25V for LTC2656-L and 2.048V for

LTC2656-H, in internal reference mode.

Table 1. Command and Address Codes

COMMAND*

C3 C2 C1 C0

0 Write to Input Register n

1 Update (Power Up) DAC Register n

0 Write to Input Register n, Update (Power Up) All

1 Write to and Update (Power Up) n

0 Power Down n

1 Power Down Chip (All DACs and Reference)

0 Select Internal Reference (Power-Up Reference)

1 Select External Reference (Power-Down

Reference)

1 No Operation

ADDRESS (n)*

A3 A2 A1 A0

0 DAC A

1 DAC B

0 DAC C

1 DAC D

0 DAC E

1 DAC F

0 DAC G

1 DAC H

1 All DACs

*Command and address codes not shown are reserved and should not

be used.

Serial Interface

The CS/LD input is level triggered. When this input is taken

low, it acts as a chip-select signal, powering on the SDI

and SCK buffers and enabling the input shift register. Data

(SDI input) is transferred at the next 24 rising SCK edges.

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