AD5752 datasheet(29/31 Pages) AD

29 / 31 page

Data Sheet

AD5722/AD5732/AD5752

Rev. F | Page 29 of 31

REFIN line because any unwanted signals can couple through

to the DAC outputs.

Avoid crossover of digital and analog signals. Traces on

opposite sides of the board should run at right angles to each

other. This reduces the effects of feedthrough on the board. A

microstrip technique is by far the best method, but it is not

always possible with a double-sided board. In this technique,

the component side of the board is dedicated to a ground plane,

and signal traces are placed on the solder side.

GALVANICALLY ISOLATED INTERFACE

In many process control applications, it is necessary to provide

an isolation barrier between the controller and the unit being

controlled to protect and isolate the controlling circuitry from

any hazardous common-mode voltages that may occur. The

Coupler® family of products from Analog Devices, Inc., provides

voltage isolation in excess of 2.5 kV. The serial loading structure

of the AD5722/AD5732/AD5752 makes them ideal for isolated

interfaces because the number of interface lines is kept to a

minimum. Figure 45 shows a 4-channel isolated interface to the

AD5722/AD5732/AD5752 using an ADuM1400. For further

information, visit www.analog.com/iCouplers.

ENCODE

DECODE

ENCODE

DECODE

ENCODE

DECODE

VIA

VIB

VIC

VID

VOA

VOB

VOC

VOD

ENCODE

DECODE

ADuM1400*

MICROCONTROLLER

SERIAL CLOCK OUT

SERIAL DATA OUT

SYNC OUT

CONTROL OUT

TO SCLK

TO SDIN

TO SYNC

TO LDAC

*ADDITIONAL PINS OMITTED FOR CLARITY.

Figure 45. Isolated Interface

VOLTAGE REFERENCE SELECTION

To achieve optimum performance from the AD5722/AD5732/

AD5752 over their full operating temperature range, a precision

voltage reference must be used. Thought should be given to the

selection of a precision voltage reference. The voltage applied to

the reference inputs is used to provide a buffered positive and

negative reference for the DAC cores. Therefore, any error in

the voltage reference is reflected in the outputs of the device.

There are four possible sources of error to consider when

choosing a voltage reference for high accuracy applications:

initial accuracy, temperature coefficient of the output voltage,

long-term drift, and output voltage noise.



Initial accuracy error on the output voltage of an external

reference can lead to a full-scale error in the DAC. To

minimize these errors, a reference with low initial accuracy

error specification is preferred. Choosing a reference with

an output trim adjustment, such as the ADR421, allows a

system designer to trim out system errors by setting the

reference voltage to a voltage other than the nominal. The

trim adjustment can also be used to trim out temperature-

induced errors.



The temperature coefficient of a reference output voltage

affects INL, DNL, and TUE. A reference with a tight

temperature coefficient specification should be chosen to

reduce the dependence of the DAC output voltage on

ambient conditions.



Long-term drift is a measure of how much the reference

output voltage drifts over time. A reference with a tight

long-term drift specification ensures that the overall

solution remains relatively stable over its entire lifetime.



Reference output voltage noise needs to be considered in

high accuracy applications that have relatively low noise

budgets. It is important to choose a reference with as low

an output noise voltage as practical for the required system

resolution. Precision voltage references such as the ADR431

(XFET® design) produce low output noise in the 0.1 Hz to

10 Hz range. However, as the circuit bandwidth increases,

filtering the output of the reference may be required to

minimize the output noise.

MICROPROCESSOR INTERFACING

Microprocessor interfacing to the AD5722/AD5732/AD5752 is

via a serial bus that uses a standard protocol compatible with

microcontrollers and DSP processors. The communications

channel is a 3-wire (minimum) interface consisting of a clock

signal, a data signal, and a synchronization signal. The AD5722/

AD5732/AD5752 require a 24-bit data-word with data valid on

the falling edge of SCLK.

For all interfaces, the DAC output update can be initiated

automatically when all the data is clocked in, or it can be

performed under the control of LDAC. The contents of the

registers can be read using the readback function.

AD5722/AD5732/AD5752 to Blackfin® DSP Interface

Figure 46 shows how the AD5722/AD5732/AD5752 can be

interfaced to the Analog Devices Blackfin DSP. The Blackfin has

an integrated SPI port that can be connected directly to the SPI

pins of the AD5722/AD5732/AD5752 and the programmable

I/O pins that can be used to set the state of a digital input such

as the LDAC pin.

SYNC

ADSP-BF531

AD5722/

AD5732/

AD5752

SCLK

SDIN

SPISELx

SCK

MOSI

LDAC

PF10

Figure 46. AD5722/AD5732/AD5752 to Blackfin Interface

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