Mach-DSP User’s Manual
Document Number: MACH-DSP-9021
Page 10
www.ScannerMAX.com
It occurs often that clients mistakenly believe they should connect their “ground” from
the source to pins 2 and pin 5 of the Analog Input connector of the Mach-DSP. (After all,
pins 2 and 5 are labeled “ground” above…) While this will sort of work, it will also very
likely cause inaccurate scanning as well as cross-talk between X and Y axes. The
reason is not obvious, and is covered in the book
LASER SCANNERS: Technologies
and Applications
, by William R. Benner, Jr (designer of all ScannerMAX products
including the Mach-DSP servo driver). See the chapter that covers
Making the Right
Connections for Perfect Laser Images
.
Nevertheless,
if you have a single-ended source, in most cases the best way to
connect it to the Mach-DSP is
to connect the + output of the DAC board to pin 1 (the
+X-axis command input) and/or pin 4 (the +Y-axis command input), and connect the
“ground” from the signal source to
both
pin 3 (the -X-axis command input)
and
pin 6 (the
-Y-axis command input) on the Analog Command Input connector. Doing this will take
maximum advantage of the differential receiver in the Mach-DSP, allowing it to remove
any noise picked up between the signal source and the Mach-DSP, and more
importantly, allowing it to subtract out any “ground bounce” and other phantom power
supply ground currents.
In most cases, do not connect to pins 2 and 5 on the Analog Input connector
It is very rare indeed that clients should make any connections to pins 2 and 5 of the
Analog Input connector. (Again, for all of the reasons why, please read the book
mentioned above.) Pins 2 and 5 are connected to the other pins labeled “ground” on the
Mach-DSP, including the Scanner interface and Power Supply connector. In most
cases, best performance will be had using a single-point-grounding-scheme, with that
single point being located at (or very close to) the power supply.
Voltage level of the input signal, and corresponding scan angle
The “Input Scale” is fully adjustable using the Mach-DSP Servo Control software
application, and it is possible to accommodate input voltage levels ranging from ±1V to
±10V differential, and have that correspond to any scan angle up to the maximum
provided by the scanner connected (greater than 100 degrees optical peak-to-peak with
ScannerMAX scanners).
However, to take full advantage of the dynamic range provided by the 16-bit Analog-to-
Digital converter front end of the Mach-DSP, the input signal should have a voltage level
of ±10V differential. (When using a single-ended source, this corresponds to a voltage
range of –10V to +10V at the + axis inputs, and 0V at the – axis inputs.) Of course the
Mach-DSP can be special ordered for other full-scale signal ranges. If you have any
questions or concerns about this, please contact us to discuss your requirements.
Input voltage range and common-mode rejection
The inputs are true differential, and as long as each of the + and – inputs do not exceed
20V, the system will provide high common-mode rejection.
