AD5744R datasheet(21/32 Pages) AD

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AD5744R

Rev. A | Page 21 of 32

THEORY OF OPERATION

The AD5744R is a quad, 14-bit, serial input, bipolar voltage output

DAC that operates from supply voltages of ±11.4 V to ±16.5 V and

has a buffered output voltage of up to ±10.5263 V. Data is written to

the AD5744R in a 24-bit word format via a 3-wire serial interface.

The AD5744R also offers an SDO pin that is available for daisy

chaining or readback.

The AD5744R incorporates a power-on reset circuit that ensures

that the DAC registers are loaded with 0x0000 at power-up. The

AD5744R features a digital I/O port that can be programmed via

the serial interface, an analog die temperature sensor, on-chip

10 ppm/°C voltage reference, on-chip reference buffers, and per

channel digital gain and offset registers.

DAC ARCHITECTURE

The DAC architecture of the AD5744R consists of a 14-bit current

mode segmented R-2R DAC. The simplified circuit diagram for

the DAC section is shown in Figure 39.

E15

VREF

E14

S11

S10

12-BIT, R-2R LADDER

4 MSBs DECODED INTO

15 EQUAL SEGMENTS

VOUTx

AGNDx

R/8

IOUT

Figure 39. DAC Ladder Structure

The four MSBs of the 14-bit data-word are decoded to drive

15 switches, E1 to E15. Each of these switches connects one of

the 15 matched resistors to either AGNDx or IOUT. The remaining

12 bits of the data-word drive Switch S0 to Switch S11 of the 12-bit

R-2R ladder network.

REFERENCE BUFFERS

The AD5744R can operate with either an external or an internal

reference. The reference inputs (REFAB and REFCD) have an

input range of up to 7 V. This input voltage is then used to provide

a buffered positive and negative reference for the DAC cores.

The positive reference is given by

+VREF = 2 × VREFIN

The negative reference to the DAC cores is given by

−VREF = −2 × VREFIN

These positive and negative reference voltages (along with the

gain register values) define the output ranges of the DACs.

SERIAL INTERFACE

The AD5744R is controlled over a versatile 3-wire serial interface

that operates at clock rates of up to 30 MHz and is compatible

with SPI, QSPI™, MICROWIRE™, and DSP standards.

Input Shift Register

The input shift register is 24 bits wide. Data is loaded into the

device, MSB first, as a 24-bit word under the control of a serial

clock input, SCLK. The input register consists of a read/write bit,

a reserved bit that must be set to 0, three register select bits, three

DAC address bits, and 16 data bits, as shown in Table 9. The timing

diagram for this operation is shown in Figure 2.

Upon power-up, the DAC registers are loaded with zero code

(0x0000), and the outputs are clamped to 0 V via a low impedance

path. The outputs can be updated with the zero code value by

asserting either LDAC or CLR. The corresponding output voltage

depends on the state of the BIN/2sCOMP pin. If the BIN/2sCOMP

pin is tied to DGND, the data coding is twos complement and the

outputs update to 0 V. If the BIN/2sCOMP pin is tied to DVCC,

the data coding is offset binary and the outputs update to negative

full scale. To have the outputs power-up with zero code loaded

to the outputs, the CLR pin should be held low during power-up.

Standalone Operation

The serial interface works with both a continuous and noncon-

tinuous serial clock. A continuous SCLK source can be used only

if SYNC is held low for the correct number of clock cycles. In gated

clock mode, a burst clock containing the exact number of clock

cycles must be used, and SYNC must be taken high after the final

clock to latch the data. The first falling edge of SYNC starts the

write cycle. Exactly 24 falling clock edges must be applied to SCLK

before SYNC is brought high again. If SYNC is brought high before

the 24th falling SCLK edge, the data written is invalid. If more

than 24 falling SCLK edges are applied before SYNC is brought

high, the input data is also invalid. The input register addressed is

updated on the rising edge of SYNC. For another serial transfer to

take place, SYNC must be brought low again. After the end of

the serial data transfer, data is automatically transferred from the

input shift register to the addressed register.

When the data has been transferred into the chosen register of

the addressed DAC, all DAC registers and outputs can be

updated by taking LDAC low.

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