Texas Instruments DAC8881, User Manual, Page: 13 / 25

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EVM Operation
3.3.3 SDI Signal
The SDI signal is the serial data input that is loaded into the DAC serial shift register with respect to
SCLK.
3.3.4 LDAC Signal
The LDAC signal is the control input signal necessary to load the DAC register with the contents of the
input register. This signal is active low and can be triggered synchronously or asynchronously.
3.3.5 RST Signal
The RST signal is the control input necessary to reset the device to a known state determined by the state
of the RSTSEL pin when the RST pin is asserted. If RSTSEL is tied to DGND, the DAC latch is cleared
(0V) and V
OUT
is minimum scale (i.e., V
REF
L). If RSTSEL is tied to V
DD
, the DAC latch is set to mid-scale
and V
OUT
is equal to (V
REF
H – V
REF
L)/2. This pin is active low.
3.3.6 PDN Signal
The PDN signal is the control input provided for the hardware power-down function of the device. This
signal is active high, so when the PDN pin is driven high, the device goes into power-down mode, which
reduces its power consumption. The DAC8881 voltage output pin is connected to ground through an
internal 10k
Ω resistor while in power-down mode.
3.4 Analog Output
The DAC8881 voltage output is buffered internally and offers a force and sense output configuration to
allow the loop around the output amplifier to be closed as close to the load as possible. The EVM closes
this loop by default with R1 installed. So if there is a need to close the loop at the load, R1 must be
removed and jumpers W2 and W7 used to connect V
OUT
and R
FB
to the connector J4. The selected pins of
the J4 connector, as dictated by the jumper positions of the W2 and W7 jumpers, can then be connected
to the load to close the loop.
The EVM includes an external operational amplifier, U2, as an option for other output signal conditioning
circuitry for the DAC output. Although the buffered output of the DAC8881 can be monitored through J4
pin 2, the optional external buffer output (if the DAC output is connected to it) can be monitored through
the TP3 test point.
The external op-amp, U2, is set to the unity gain configuration by default to maintain the DAC8881
unipolar output mode of operation. But it can be modified by simple jumper settings to achieve other
modes of operation. The following sections describe the different configurations of the output amplifier, U2.
3.4.1 Bipolar Output Operation
Using the external V
REF
H to offset the DAC output and extend the range of operation to achieve a bipolar
mode of operation is possible by configuring the output op-amp, U2, properly. This configuration is
described in
Table 4 .
Table 4. Bipolar Output Operation Jumper Settings
Reference Jumper Setting Function
W9 1-2 Connects DAC output (V
OUT
1) to the non-inverting input of the output op-amp, U2.
2-3 Connects DAC output (V
OUT
2) to the non-inverting input of the output op-amp, U2.
W3 CLOSE Connects V
REF
H to the inverting input of the op-amp, U2.
W5 1-2 Supplies power (V
SS
) to the negative rail of the op-amp, U2, for a wider range of operation.
W12 OPEN Disconnects negative input of the op-amp from the gain resistor, R12.
13 SLAU257A – September 2008 – Revised November 2009 DAC8881 Evaluation Module
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