LTC2636 datasheet(21/28 Pages) LINER

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LTC2668

2668fa

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operation

The LTC2668 supports three types of toggle operations:

a first in which all selected channels are toggled together

using the SPI port; a second in which all selected channels

aretoggledtogetherusinganexternalclockorlogicsignal;

and a third in which any combination of channels can be

instructed to update from either input register A or B.

The internal toggle-update circuit is edge triggered, so

only transitions (of TGB or TGP) trigger an update from

the respective input register.

To toggle all selected channels together using the SPI port,

ensure the TGP pin is high and that the bits in the toggle

select register corresponding to the desired channels are

also high. Use the global toggle command (1101b) to

alternate codes, sequentially changing the global toggle

bit TGB (see Figure 7). Changing TGB from 1 to 0 updates

the DAC registers from their respective input registers

A. Changing TGB from 0 to 1 updates the DAC registers

from their respective input registers B. Note that in this

way up to 16 channels may be toggled with just one se-

rial command.

To toggle all selected channels using an external logic

signal, ensure that the TGB bit in the global toggle register

is high and that in the toggle select register, the bits cor-

responding to the desired channels are also high. Apply

a clock or logic signal to the TGP pin to alternate codes.

TGP falling edges update the DAC registers from their

associated input registers A. TGP rising edges update

the DAC registers from their associated input registers B.

Note that once the input registers are set up, all toggling

is triggered by the signal applied to the TGP pin, with no

further SPI instructions needed.

To cause any combination of channels to update from

either input register A or B, ensure the TGP pin is high

and that the TGB bit in the global toggle register is also

high. Using the toggle select command, set the toggle

select bits as needed to select the input register (A or B)

with which each channel is to be updated. Then update all

channels, either by using the serial command (1001b) or

by applying a negative pulse to the LDAC pin. Any channels

whose toggle select bits are 0 update from input register

Figure 8. Simplified Toggle Block Diagram. Conceptual Only, Actual Circuit May Differ

2668 F08

TOGGLE SELECT BIT T15

UPD

CS/LD

SCK

SDI

LDAC 15

TGP

TGB

32-BIT SHIFT REGISTER

LOGIC

16-BIT TOGGLE SELECT REGISTER

ONE Tx BIT PER CHANNEL

GLOBAL TOGGLE

BIT (TGB)

INPUT REGISTER B

(16 BIT)

INPUT REGISTER A

(16 BIT)

A/B

MUX

DAC REGISTER

16-BIT

SOFTSPAN DAC

LTC2668

CHANNEL 15

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