AD7242 Просмотр технического описания (PDF) - Analog Devices

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AD7242

LC2MOS Dual, Complete, 12-Bit/14-Bit Serial DACs

Analog Devices 

AD7242/AD7244

TIMING AND CONTROL

Communication with the AD7242/AD7244 is via six serial logic

inputs. These consist of separate serial clocks, word framing and

data lines for each DAC. DAC updating is controlled by two

digital inputs: LDACA for updating VOUTA and LDACB for

updating VOUTB. These inputs can be asserted independently of

the microprocessor by an external timer when precise updating

intervals are required. Alternatively, the LDACA and LDACB

inputs can be driven from a decoded address bus allowing the

microprocessor control over DAC updating as well as data

communication to the AD7242/AD7244 input latches.

The AD7242/AD7244 contains two latches per DAC, an input

latch and a DAC latch. Data must be loaded to the input latch

under the control of TCLKA, TFSA and DTA for input Latch

A and TCLKB, TFSB and DTB for input Latch B. Data is then

transferred from input Latch A to DAC Latch A under the control

of the LDACA signal, while LDACB controls the loading of DAC

Latch B from input Latch B. Only the data held in the DAC

latches determines the analog outputs of the AD7242/AD7244.

Data is loaded to the input latches under control of the respec-

tive TCLK, TFS and DT signals. The AD7242/AD7244

expects a 16-bit stream of serial data on its DT inputs. Data

must be valid on the falling edge of TCLK. The TFS input

provides the frame synchronization signal that tells the AD7242/

AD7244 that valid serial data will be available on the DT input

for the next 16 falling edges of TCLK. Figure 6 shows the

timing diagram for operation of either of the two serial input

ports on the part.

Although 16 bits of data are clocked into the input latch, only

12 bits are transferred into the DAC latch for the AD7242 and

14 bits are transferred for the AD7244. Therefore, 4 bits in the

AD7242 data stream and 2 bits in the AD7244 data stream are

don’t cares since their value does not affect the DAC latch data.

The bit positions are the don’t cares followed by the DAC data

starting with the MSB (see Figure 6).

The respective LDAC signals control the transfer of data to the

respective DAC latches. Normally, data is loaded to the DAC

latch on the falling edge of LDAC. However, if LDAC is held

low, serial data is loaded to the DAC latch on the sixteenth

falling edge of TCLK. If LDAC goes low during the loading of

serial data to the input latch, no DAC latch update takes place

on the falling edge of LDAC. If LDAC stays low until the serial

transfer is completed, then the update takes place on the sixteenth

falling edge of TCLK. If LDAC returns high before the serial

data transfer is completed, no DAC latch update takes place.

If seventeen or more TCLK edges occur while TFS is low, the

seventeenth (and beyond) clock edges are ignored, i.e., no

further data is clocked into the input latch after the sixteenth

TCLK edge following a falling edge on TFS.

Figure 6. AD7242/AD7244 Timing Diagram

–8–

REV. A

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