AD5762R
Preliminary Technical Data
LAYOUT GUIDELINES
1
µCONTROLLER
ADuM1400
In any circuit where accuracy is important, careful consider-
ation of the power supply and ground return layout helps to
ensure the rated performanceꢀ The printed circuit board on
which the AD5762R is mounted should be designed so that the
analog and digital sections are separated and confined to
certain areas of the boardꢀ If the AD5762R is in a system where
multiple devices require an AGND-to-DGND connection, the
connection should be made at one point onlyꢀ The star ground
point should be established as close as possible to the deviceꢀ
The AD5762R should have ample supply bypassing of 1ꢂ µF in
parallel with ꢂꢀ1 µF on each supply located as close to the
package as possible, ideally right up against the deviceꢀ The 1ꢂ
µF capacitors are the tantalum bead typeꢀ The ꢂꢀ1 µF capacitor
should have low effective series resistance (ESR) and low
effective series inductance (ESI) such as the common ceramic
types, which provide a low impedance path to ground at high
frequencies to handle transient currents due to internal logic
switchingꢀ
V
V
V
V
V
IA
OA
OB
OC
OD
ENCODE
ENCODE
ENCODE
ENCODE
DECODE
DECODE
DECODE
DECODE
SERIAL CLOCK OUT
TO SCLK
TO SDIN
TO SYNC
TO LDAC
V
V
V
IB
IC
ID
SERIAL DATA OUT
SYNC OUT
CONTROL OUT
1
ADDITIONAL PINS OMITTED FOR CLARITY
Figure 43. Isolated Interface
MICROPROCESSOR INTERFACING
Microprocessor interfacing to the AD5762R is via a serial bus
that uses standard protocol compatible with microcontrollers
and DSP processorsꢀ The communications channel is a 3-wire
(minimum) interface consisting of a clock signal, a data signal,
and a synchronization signalꢀ The AD5762R requires a 2.-bit
data-word with data valid on the falling edge of SCLKꢀ
The power supply lines of the AD5762R should use as large a
trace as possible to provide low impedance paths and reduce
the effects of glitches on the power supply lineꢀ Fast switching
signals, such as clocks, should be shielded with digital ground
to avoid radiating noise to other parts of the board, and should
never be run near the reference inputsꢀ A ground line routed
between the SDIN and SCLK lines helps reduce cross-talk
between them (not required on a multilayer board, which has a
separate ground plane, however, it is helpful to separate the
lines)ꢀ It is essential to minimize noise on the reference inputs,
because it couples through to the DAC outputꢀ Avoid crossover
of digital and analog signalsꢀ Traces on opposite sides of the
board should run at right angles to each otherꢀ This reduces the
effects of feed through on the boardꢀ A microstrip technique is
recommended, but not always possible with a double-sided
boardꢀ In this technique, the component side of the board is
dedicated to ground plane, while signal traces are placed on the
solder sideꢀ
For all the interfaces, the DAC output update can be done
automatically when all the data is clocked in, or it can be done
under the control of LDACꢀ The contents of the DAC register
can be read using the readback functionꢀ
AD5762R to MC68HC11 Interface
Figure .. shows an example of a serial interface between the
AD5762R and the MC68HC11 microcontrollerꢀ The serial
peripheral interface (SPI) on the MC68HC11 is configured for
master mode (MSTR = 1), clock polarity bit (CPOL = ꢂ), and the
clock phase bit (CPHA = 1)ꢀ The SPI is configured by writing to the
SPI control register (SPCR) (see the 68HC11User Manual)ꢀ SCK of
the MC68HC11 drives the SCLK of the AD5762R, the MOSI
output drives the serial data line (DIN) of the AD5762R, and the
MISO input is driven from SDOꢀ The SYNC is driven from one of
the port lines, in this case PC7ꢀ
GAꢀVANICAꢀꢀY ISOꢀATED 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 might occurꢀ
Isocouplers provide voltage isolation in excess of 2ꢀ5 kꢁꢀ The
serial loading structure of the AD5762R makes it ideal for
isolated interfaces, because the number of interface lines is kept
to a minimumꢀ Figure .3 shows a .-channel isolated interface
to the AD5762R using an ADuM1.ꢂꢂꢀ For more information,
go to wwwꢀanalogꢀcomꢀ
Rev. PrA | Page 30 of 33