AD5420BCPZ AD [Analog Devices], AD5420BCPZ Datasheet - Page 25

• Manufacturer Part Number: AD5420BCPZ
• Description: Single Channel, 16-Bit, Serial Input, Current Source DAC
• Manufacturer: AD [Analog Devices]

Preliminary Technical Data
APPLICATIONS INFORMATION
DRIVING INDUCTIVE LOADS
When driving inductive or poorly defined loads connect a
0.01µF capacitor between I
stability with loads beyond 50mH. There is no maximum
capacitance limit. The capacitive component of the load may
cause slower settling, though this may be masked by the settling
time of the AD5420.
TRANSIENT VOLTAGE PROTECTION
The AD5420 contains ESD protection diodes which prevent
damage from normal handling. The industrial control
environment can, however, subject I/O circuits to much higher
transients. In order to protect the AD5420 from excessively high
voltage transients , external power diodes and a surge current
limiting resistor may be required, as shown in Figure 38. The
constraint on the resistor value is that during normal operation
the output level at IOUT must remain within its voltage
compliance limit of AV
and resistor must have appropriate power ratings.
LAYOUT GUIDELINES
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
AD5420 is mounted should be designed so that the analog and
digital sections are separated and confined to certain areas of the
board. If the AD5420 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 AD5420 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 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
switching.
The power supply lines of the AD5420 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
Figure 38. Output Transient Voltage Protection
AD5420
AV
AV
GND
DD
DD
DD
I
– 2.5V and the two protection diodes
OUT
OUT
and GND. This will ensure
R
P
R
LOAD
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avoid 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 feed through 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, while 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 might occur. The
i Coupler® family of products from Analog Devices provides
voltage isolation in excess of 2.5 kV. The serial loading structure
of the AD5420 make it ideal for isolated interfaces because the
number of interface lines is kept to a minimum. Figure 39 shows
a 4-channel isolated interface to the AD5420 using an
ADuM1400. For further information, visit
http://www.analog.com/icouplers.
*ADDITIONAL PINS OMITTED FOR CLARITY
MICROPROCESSOR INTERFACING
Microprocessor interfacing to the AD5420 is via a serial bus that
uses 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 AD5420 require a 24-bit data-word with
data valid on the rising edge of SCLK.
For all interfaces, the DAC output update is initiated on the
rising edge of LATCH. The contents of the registers can be read
using the readback function.
Controller
Serial Clock Out
Serial Data Out
Control out
SYNC Out
ADuM1400 *
V
V
V
V
IA
IB
IC
ID
Figure 39. Isolated Interface
ENCODE
ENCODE
ENCODE
ENCODE
DECODE
DECODE
DECODE
DECODE
V
V
V
V
OA
OD
OB
OC
AD5420
To SCLK
To SDIN
To LATCH
To CLEAR