DAC108S085_15 Datasheet, PDF (19/36 Pages) Texas Instruments – DAC108S085 10-Bit Micro Power OCTAL Digital-to-Analog Converter with Rail-to-Rail Outputs

DAC108S085
www.ti.com
SNAS423B – AUGUST 2007 – REVISED MARCH 2013
Updating DAC Outputs Simultaneously
When the DAC108S085 is first powered on, the DAC is operating in Write Register Mode (WRM). Operating in
WRM allows the user to program the registers of multiple DAC channels without causing the DAC outputs to be
updated. As an example, here are the steps for setting Channel A to a full scale output, Channel B to three-
quarters full scale, Channel C to half-scale, Channel D to one-quarter full scale and having all the DAC outputs
update simultaneously.
As stated previously, the DAC108S085 powers up in WRM. If the device was previously operating in Write
Through Mode (WTM), an extra step to set the DAC into WRM would be required. First, the DAC registers need
to be programmed to the desired values. To set Channel A to an output of full scale, write "0FFC" to the control
register. This will update the data register for Channel A without updating the output of Channel A. Second, set
Channel B to an output of three-quarters full scale by writing "1C00" to the control register. This will update the
data register for Channel B. Once again, the output of Channel B and Channel A will not be updated since the
DAC is operating in WRM. Third, set Channel C to half scale by writing "2800" to the control register. Fourth, set
Channel D to one-quarter full scale by writing "3400" to the control register. Finally, update all four DAC channels
simultaneously by writing "A00F" to the control register. This procedure allows the user to update four channels
simultaneously with five steps.
Since Channel A was one of the DACs to be updated, one command step could have been saved by writing to
Channel A last. This is accomplished by writing to Channel B, C, and D first and using the the special command
"Channel A Write" to update Channel A's DAC register and output. This special command has the added benefit
of updating all DAC outputs while updating Channel A. With this sequence of commands, the user was able to
update four channels simultaneously with four steps. A summary of this command can be found in Table 3.
Updating DAC Outputs Independently
If the DAC108S085 is currently operating in WRM, change the mode of operation to WTM by writing "9XXX" to
the control register. Once the DAC is operating in WTM, any DAC channel can be updated in one step. For
example, if a design required Channel G to be set to half scale, the user can write "6800" to the control register
and Channel G's data register and DAC output will be updated. Similarly, if Channel F's output needed to be set
to full scale, "5FFC" would need to be written to the control register. Channel A is the only channel that has a
special command that allows its DAC output to be updated in one command regardless of the mode of operation.
Setting Channel A's DAC output to full scale could be accomplished in one step by writing "BFFF" to the control
register.
USING REFERENCES AS POWER SUPPLIES
While the simplicity of the DAC108S085 implies ease of use, it is important to recognize that the path from the
reference input (V_REF1,2) to the DAC outputs will have zero Power Supply Rejection Ratio (PSRR). Therefore, it is
necessary to provide a noise-free supply voltage to V_REF1,2. In order to utilize the full dynamic range of the
DAC108S085, the supply pin (V_A) and V_REF1,2 can be connected together and share the same supply voltage.
Since the DAC108S085 consumes very little power, a reference source may be used as the reference input
and/or the supply voltage. The advantages of using a reference source over a voltage regulator are accuracy and
stability. Some low noise regulators can also be used. Listed below are a few reference and power supply
options for the DAC108S085.
LM4132
The LM4132, with its ±0.05% accuracy over temperature, is a good choice as a reference source for the
DAC108S085. The 4.096V version is useful if a 0V to 4.095V output range is desirable. Bypassing the LM4132
voltage input pin with a 4.7µF capacitor and the voltage output pin with a 4.7µF capacitor will improve stability
and reduce output noise. The LM4132 comes in a space-saving 5-pin SOT23.
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