Host Processor Operation
3-3
EVM Operation
3.2
Host Processor Operation
The host processor basically drives the DAC, so proper DAC operation
depends on the successful configuration of the host processor and the EVM
board. In addition, properly written code is also required to operate the DAC.
A custom cable can be made specific to the host interface platform. The EVM
allows interface to the host processor through the J2 header connector for the
serial control signals and the serial data input. The output can be monitored
through the J4 header connector. An interface adapter card is also available
for specific DSP starter kit, as well as an MSP430 motherboard as mentioned
in Chapter 1 of this manual.
The EVM includes an optional signal conditioning circuit for the DAC output
through an external operational amplifier, U2. This is set to a unity gain
configuration by default. Regardless, the raw output of the DAC can be probed
through W13 pin 2 so that it can be compared with the output of U2 if
necessary. The output terminal, J4, is provided mainly for mechanical stability
when stacking or plugging into an interface card or the MSP430 motherboard.
But it is also used to monitor the different output configuration of the DAC
through U2 easily by shorting the respective pins of W13. In addition, it
provides easy access for monitoring up to three DAC7731 EVMs in daisy chain
or cascading fashion with the option of using U2 for each of the EVMs stacked
together.
The following sections describe the different configurations of the output
amplifier, U2.
3.2.1
Unity Gain Output
The buffered output configuration is used to prevent loading the DAC7731 and
closely matches the raw output of the DAC with possible slight distortion
because of the feedback resistor and capacitor. You can tailor the feedback
circuit to closely match the desired wave shape by desoldering R7 and C11
and replacing with desired values. You can also remove R7 and C11
altogether, and solder a zero-ohm resistor in place of R7 if desired.
Table 3 - 2 shows the jumper setting for the unity gain configuration of the DAC
external output buffer in unipolar or bipolar mode.
Table 3 - 2. Unity Gain Output Jumper Settings
Jumper Setting
Reference
Unipolar
Bipolar
Function
W5
2 - 3
1 - 2
Supplies the voltage for the negative rail of op amp.
W13
3 - 4
3 - 4
DAC output is channeled to the output terminal J4 - 6.
W15
Open
Open
Disconnect negative input of op amp from GND.
