13
LTC1588/LTC1589/LTC1592
1588992fa
APPLICATIO S I FOR ATIO
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with low output voltage initial tolerance, like the LT1236
(±0.05%), minimizes the gain error caused by the refer-
ence; however, a calibration sequence that corrects for
system zero- and full-scale error is always recommended.
A reference’s output voltage temperature coefficient af-
fects not only the full-scale error, but can also affect the
circuit’s INL and DNL performance. If a reference is
chosen with a loose output voltage temperature coeffi-
cient, then the DAC output voltage along its transfer
characteristic will be very dependent on ambient condi-
tions. Minimizing the error due to reference temperature
coefficient can be achieved by choosing a precision
reference with a low output voltage temperature coeffi-
cient and/or tightly controlling the ambient temperature
of the circuit to minimize temperature gradients.
As precision DAC applications move to 16-bit and higher
performance, reference output voltage noise may contrib-
ute a dominant share of the system’s noise floor. This in
turn can degrade system dynamic range and signal-to-
noise ratio. Care should be exercised in selecting a voltage
reference with as low an output noise voltage as practical
for the system resolution desired. Precision voltage refer-
ences, like the LT1236, produce low output noise in the
0.1Hz to 10Hz region, well below the 16-bit LSB level in 5V
or 10V full-scale systems. However, as the circuit band-
widths increase, filtering the output of the reference may
be required to minimize output noise.
Table 5. Partial List of LTC Precision References Recommended
for Use with the LTC1588/LTC1589/LTC1592 with Relevant
Specifications
INITIAL TEMPERATURE 0.1Hz to 10Hz
REFERENCE TOLERANCE DRIFT NOISE
LT1019A-5, ±0.05% 5ppm/°C12µV
P-P
LT1019A-10
LT1236A-5, ±0.05% 5ppm/°C3µV
P-P
LT1236A-10
LT1460A-5, ±0.075% 10ppm/°C20µV
P-P
LT1460A-10
LT1790A-2.5 ±0.05% 10ppm/°C12µV
P-P
Grounding
As with any high resolution converter, clean grounding is
important. A low impedance analog ground plane and star
grounding techniques should be used. I
OUT2
must be tied
to the star ground with as low a resistance as possible.
When it is not possible to locate star ground close to I
OUT2
,
a low resistance trace should be used to route this pin to
star ground. This minimizes the voltage drop from this pin
to ground caused by the code dependent current flowing
to ground. When the resistance of this circuit board trace
becomes greater than 1Ω, a force/sense amplified con-
figuration should be used to drive this pin (see Figure 2).
This preserves the excellent accuracy (1LSB INL and DNL)
of the LTC1588/LTC1589/LTC1592.
An Isolated 16-Bit Subsystem Using the LTC1592
The circuit in Figure 4 is a complete example of an optically
isolated analog output subsystem that supports most of
the legacy ranges that are still common in industrial
environments. This circuit uses only two optoisolators,
the load pulse (CS/LD) being derived from a series of
transitions on the data line (SDI) after the clock (SCK) is
halted high. If a single chip microcontroller with an auto-
mated SPI interface is to be used, the SPI port can transfer
the 24 bits as three bytes. Subsequently, the data output
port pin can be reassigned to general purpose port opera-
tion and exercised to produce a number of transitions to
generate the load pulse. Alternatively, the entire sequence
can be programmed bit by bit with a general purpose port.
Figure 5 shows the timing.
The DC/DC converter, Figure 3 based on the LT
®
3439
ultralow noise transformer driver provides a compact
means of powering this circuit, and allows the output to
deliver output current that is only limited by the LT1468
capabilities. The output capability of the DC/DC converter
itself is 80mA at ±12V and is available as demo board
DC511A. This circuit as shown requires approximately
130mA of the 5V supply (no load). The total surface area
required is less than 2 square inches.
