AD5755-1x AD [Analog Devices], AD5755-1x Datasheet - Page 32

• Manufacturer Part Number: AD5755-1x
• Description: Quad Channel, 16-Bit, Serial Input, 4-20mA & Voltage Output DAC, Dynamic Power Control, HART Connectivity
• Manufacturer: AD [Analog Devices]

AD5755-1
APPLICATIONS INFORMATION
PRECISION VOLTAGE REFERENCE SELECTION
To achieve the optimum performance from the AD5755-1 over
its 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 inpus is used to provide a buffered 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 refer-
ence could lead to a full-scale error in the DAC. Therefore, 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 ADR425, allows a system
designer to trim system errors out by setting the reference
voltage to a voltage other than the nominal. The trim adjust-
ment can also be used at temperature to trim out any error.
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.
The temperature coefficient of a reference’s 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.
In high accuracy applications, which have a relatively low noise
budget, reference output voltage noise needs to be considered.
Choosing a reference with as low an output noise voltage as
practical for the system resolution required is important.
Precision voltage references such as the ADR435 (XFET design)
produce low output noise in the 0.1 Hz to 10 Hz region.
However, as the circuit bandwidth increases, filtering the output
of the reference may be required to minimize the output noise.
Table 35. Some Recommended Precision References
Part
No.
ADR435
ADR425
ADR02
ADR395
AD586
Initial
Accuracy
(mV
Max)
±6
±6
±5
±6
±2.5
Long-Term
Drift (ppm
Typ)
30
50
50
50
15
Temp
Drift
(ppm/°C
Max)
3
3
3
25
10
0.1 Hz to 10
Hz Noise
(µV p-p
Typ)
3.4
3.4
15
5
4
Rev. PrD | Page 32 of 34
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 AD5755-1.
TRANSIENT VOLTAGE PROTECTION
The AD5755-1 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 AD5755-1 from excessively
high voltag etransients , external power diodes and a surge
current limiting resistor may be required, as shown in Figure
22. 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
diodes and resistor must have appropriate power ratings.
MICROPROCESSOR INTERFACING
Microprocessor interfacing to the AD5755-1 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 AD5755-1 require 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.
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
AD5755-1 is mounted should be designed so that the analog
and digital sections are separated and confined to certain areas of
the board. If the AD5755-1 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.
Figure 22. Output Transient Voltage Protection
AD5755
Preliminary Technical Data
AV
AV
GND
DD
DD
I
OUT
OUT
and GND. This will ensure
DD
– 2.5V and the two protection
R
P
R
LOAD