Reference Voltages

Several of the options discussed in the Degrees of Freedom page may involve use of reference voltages. Each ADC and DAC requires an input reference voltage. For the parts we are using, this corresponds to one more than the highest digital value. That is, just above the highest voltage which the device can measure or produce.

Most of the ADC and DAC channels use a system-wide, software-controlled reference voltage. This voltage, which we will call REFS, is generated by the DACB output on the emitter's DAC (using 3.3V as its reference voltage). Because we perform chirp emission, echo capture, and echo replay at different times, we can set REFS as needed for each task.

Note: The MCP4922 DAC has a "Gain Selection Option" bit; setting this doubles the voltage range of the device.

Emitter

In the emitter's DAC, input REFA is controlled by REFS. Output DACB is controlled by the Arduino software, within the limit set by REFA.

Microphones

In both microphone's ADCs, the VREF is controlled by REFS. This can be used to compensate for the expected decrease in reflection volume over increasing time (and thus, distance).

More generally, the plan is to set the VREF high enough that we get at least a few values at the limits (0, 1023), but we don't see many successive values there.

Speakers

In the speaker volume DAC, input REFB is controlled by REFS. Output DACB is controlled by the Arduino software, within the limit set by REFB. This lets the Arduino control the speaker volume dynamically during replay.

In the speaker output DACs, inputs REFA and REFB (reference voltages for the left and right speakers) are controlled by the Arduino software, within the limit set by REFA and REFB (i.e., REFS).


This wiki page is maintained by Rich Morin, an independent consultant specializing in software design, development, and documentation. Please feel free to email comments, inquiries, suggestions, etc!

Topic revision: r5 - 25 May 2016, RichMorin
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