Datasheet

LTC2756
12
2756f
operaTion
Output Ranges
The LTC2756 is a current-output, serial-input precision
multiplying DAC with selectable output ranges. Ranges
can either be programmed in software for maximum
flexibility—the DAC can be programmed to any one of six
output ranges—or hardwired through pin-strapping. Two
unipolar ranges are available (0V to 5V and 0V to 10V), and
four bipolar ranges (±2.5V, ±5V, ±10V and –2.5V to 7.5V).
These ranges are obtained when an external precision 5V
reference is used. The output ranges for other reference
voltages are easy to calculate by observing that each range
is a multiple of the external reference voltage. The ranges
can then be expressed: 0 to 1×, 0 to 2×, ±0.5×, ±1×, ±2×,
and –0.5× to 1.5×.
Manual Span Configuration
Multiple output ranges are not needed in some applica-
tions. To configure the LTC2756 to operate in a single span
without additional operational overhead, tie the M-SPAN
pin directly to V
DD
. The active output range is then set via
hardware pin strapping of pins S2, S1 and S0 (rather than
through the SPI port); and Write and Update commands
have no effect on the active output span. See Figure 1
and Table 2.
Tie the M-SPAN pin to ground for normal SoftSpan
operation.
LTC2756
M-SPAN
S2
S1
S0
2756 F01
CS/LD SDI SCK
V
DD
V
DD
±10V
–
+
Figure 1. Using M-SPAN to Configure the LTC2756
for Single-Span Operation (±10V Range Shown)
Input and DAC Registers
The LTC2756 has two sets of double-buffered regis-
ters—one set for the code data, and one for the output
range of the DAC—plus one readback register, for a total
of five registers. Double buffering provides the capability to
simultaneously update the span (output range) and code,
which allows smooth voltage transitions when changing
output ranges.
Each set of double-buffered registers comprises an Input
register and a DAC register.
Input register: The Write operation shifts data from the
SDI pin into a chosen Input register. The Input registers
are holding buffers; Write operations do not affect the
DAC outputs.
DAC register: The Update operation copies the contents
of an Input register to its associated DAC register. The
contents of a DAC register directly updates the associated
DAC output voltage or output range.
Note that updates always include both Code and Span
register sets; but the values held in the DAC registers will
only change if the associated Input register values have
previously been altered via a Write operation.