AD5757 Analog Devices, AD5757 Datasheet - Page 41

• Manufacturer Part Number: AD5757
• Description: Quad Channel, 16-Bit, Serial Input, 4-20mA Output DAC, Dynamic Power Control, HART Connectivity
• Manufacturer: Analog Devices

Specifications of AD5757

• Resolution (bits): 16bit
• Dac Update Rate: 60kSPS
• Dac Settling Time: 15µs
• Max Pos Supply (v): +33V
• Single-supply: No
• Dac Type: Current Out
• Dac Input Format: SPI

Data Sheet
TRANSIENT VOLTAGE PROTECTION
The AD5757 contains ESD protection diodes that prevent dam-
age from normal handling. The industrial control environment
can, however, subject I/O circuits to much higher transients. To
protect the AD5757 from excessively high voltage transients,
external power diodes and a surge current limiting resistor (R
are required, as shown in Figure 63. A typical value for R
The two protection diodes and the resistor (R
priate power ratings.
Additional protection can be provided using transient voltage
suppressors (TVSs), also referred to as transorbs. These compo-
nents are available as unidirectional suppressors, which protect
against positive high voltage transients, and as bidirectional
suppressors, which protect against both positive and negative
high voltage transients. Transient voltage suppressors are avail-
able in a wide range of standoff and breakdown voltage ratings.
The TVS should be sized with the lowest breakdown voltage
possible while not conducting in the functional range of the
current output.
It is recommended that all field connected nodes be protected.
MICROPROCESSOR INTERFACING
Microprocessor interfacing to the AD5757 is via a serial bus that
uses a 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 latch
signal. The AD5757 requires a 24-bit data-word with data valid
on the falling edge of SCLK.
The DAC output update is initiated on either the rising edge of
LDAC or, if LDAC is held low, on the rising edge of SYNC . The
contents of the registers can be read using the readback function.
AD5757-TO-ADSP-BF527 INTERFACE
The AD5757 can be connected directly to the SPORT interface
of the ADSP-BF527, an Analog Devices, Inc., Blackfin® DSP.
Figure 64 shows how the SPORT interface can be connected to
control the AD5757.
CONVERTER)
DC-TO-DC
(FROM
C
4.7µF
DCDC
Figure 63. Output Transient Voltage Protection
R
FILTER
10Ω
C
0.1µF
FILTER
AD5757
V
BOOST_x
AGND
I
OUT_x
D1
D2
R
P
P
) must have appro-
R
P
LOAD
is 10 Ω.
Rev. B | Page 41 of 44
P
)
LAYOUT GUIDELINES
Grounding
In any circuit where accuracy is important, careful consideration
of the power supply and ground return layout helps to ensure
the rated performance. The printed circuit board on which the
AD5757 is mounted should be designed so that the analog and
digital sections are separated and confined to certain areas of the
board. If the AD5757 is in a system where multiple devices
require an AGND-to-DGND connection, the connection should
be made at one point only. The star ground point should be
established as close as possible to the device.
The GNDSW
referred to as PGND. PGND should be confined to certain areas
of the board, and the PGND-to-AGND connection should be
made at one point only.
Supply Decoupling
The AD5757 should have ample supply bypassing of 10 μF
in parallel with 0.1 μF on each supply located as close to the
package as possible, ideally right up against the device. The
10 μF capacitors are the tantalum bead type. The 0.1 μF capaci-
tor should have low effective series resistance (ESR) and low
effective series inductance (ESL), 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
switching.
Traces
The power supply lines of the AD5757 should use as large a
trace as possible to provide low impedance paths and reduce the
effects of glitches on the power supply line. Fast switching signals
such as clocks should be shielded with digital ground to prevent
radiating noise to other parts of the board and should never be
run near the reference inputs. A ground line routed between the
SDIN and SCLK lines helps reduce crosstalk between them (not
required on a multilayer board that has a separate ground plane,
but separating the lines helps). It is essential to minimize noise
on the REFIN line because it couples through to the DAC output.
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 but not always possible
with a double-sided board. In this technique, the component
side of the board is dedicated to ground plane, whereas signal
traces are placed on the solder side.
ADSP-BF527
Figure 64. AD5757-to-ADSP-BF527 SPORT Interface
x
SPORT_TSCK
and ground connection for the AV
SPORT_DTO
SPORT_TFS
GPIO0
SYNC
SCLK
SDIN
LDAC
AD5757
CC
AD5757
supply are