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Gadgeteer ILDA laser galvo control?


#1

Has anyone wired up an ILDA laser galvo X-Y projector using Gadgeteer hardware?

ILDA is an analog bipolar differential voltage signalling system. Two wires, ranging -5V to +5V each, combined differential voltage ranging from -10V to +10V.

The 10 bit DAC on most analog out pins seems to fall short for this application - only 1024 levels = only 1024 scanlines. Is there a way to “finesse” more resolution, perhaps as an artifact of the differential between two pins? (2 x 1024 is better than 1 x 1024) Or are there higher resolution DAC modules available somewhere?

Thanks,
-Danny


#2

DACs aren’t a common module and are probably somewhat “specialised” so you will have to look at a DAC chip that meets your needs. You might be well served by something like this and an Extender/breakout module https://www.sparkfun.com/products/8736


#3

@ Brett -
Ok, thanks for the tip. The sparkfun module is a 12 bit DAC. An improvement over 10 bits, but your suggestion got me to searching in the right direction.

The TI DAC8554 (http://www.ti.com/lit/ds/symlink/dac8554.pdf) looks pretty close to ideal for what I have in mind - 16 bit DAC, SPI serial interface, 5V range. Extra bonus: 4 independent DAC channels, plus commands to stage and commit all DACs to the next data values simultaneously. Sweet! The 50MHz max input clock rate feeding 4 channels would yield about 500KHz samples/sec, more than enough for my target.

That doc also provides guidance for how to use the DAC8554 to output bipolar signals, which is also what I need to drive ILDA. By itself, it looks like the DAC8554 output ranges from 0 to 5V. The doc shows how to wire up a TI 703 opamp to make it swing from -5V to +5V, And lo and behold, TI makes a 4 channel version of the 703, too.

Time to break out the breadboard!
-Danny


#4

@ Brett -
P.S. Looks like Digikey only carries surface mount ICs for the DAC8554 and OPA4704.

Who might carry DIP versions?


#5

@ Brett - Hey! Look what I found: The Gadgeteer Music Module (https://www.ghielectronics.com/docs/85/music-module) has an 18 bit DAC buried in the spec sheet! :>

The DSP can be fed a stream of LPCM to output 2 16 bit 3.6v analog waveforms. I’d still need to wire up an op amp to bring the output to +/-5v, and the range is still only 16 bits.

To get the full 18 bit range the DAC is capable of it looks like I’d have to write to the DAC registers directly, which may require coding a custom DSP module. Hmm. 16 is fine.