Mach-DSP User’s Manual
Document Number: MACH-DSP-9021
Page 21
www.ScannerMAX.com
User-defined analog output signals
For most users, the most important thing to understand about these signals is that the
voltage level and actual servo signal that is assigned to be output by each user-defined
analog output can be chosen in the
Oscilloscope and Analog Output Controls tool
,
found in the Mach-DSP Servo Control Application program. This is described in Chapter
8 of this manual.
When these signals are merely directed to an external oscilloscope, to probe the
internal workings of the Mach-DSP servo, probably no additional information is needed.
However, when these signals are used by external equipment, such as safety-analysis
circuitry or data acquisition systems, the additional details presented below may be
critical for the overall success of the system.
The user-defined analog outputs are derived from a 16-bit digital to analog converter.
As such, they have limited resolution in both time and range/delta. Note that, with the
exception of a single 1K-ohm buffer resistor placed in between each DAC output and
the associated Test Interface Connector pins, no filtering or other signal conditioning is
performed on these outputs. This has several implications, including:
Since the DACs are 16-bits, the smallest possible voltage delta is approximately
153 microvolts (+/-5V range over 65536 DAC counts)
All DAC outputs are updated at the same time (simultaneous sampling), and all
outputs are updated at the sample rate that is used by the servo, which is user-
adjustable between 100kHz and 300kHz, with 160kHz being typical.
Albeit nearly all DACs experience some “glitching” between major code
transitions, the DAC used on the Mach-DSP (Texas Instruments part number
DAC8728) was specifically chosen to be a “low-glitch” type (4nV-S, 5mV max).
Given the points described above, it may be desirable to low-pass filter these signals –
at the very least to remove glitches, or possibly as an anti-aliasing portion of the data-
acquisition system. Moreover, apparent “code repeating” and “sample repeating” will be
sensed if these signals are fed into an ADC that has greater than 16-bit resolution, or
one that digitizes a higher sample rate than the servo is operating. This too may be
mitigated by the use of low-pass filtering.
Scanner body temperature sensing via Thermistor
An NTC thermistor may be connected between pins 6 and 15 and then attached to the
X-Y mount. This will give the Mach-DSP servo an accurate measurement of the
scanner’s body temperature. (Without the thermistor, the Mach-DSP will assume that
the body temperature is a constant 30 degrees C, which may lead to incorrect estimates
of the overall scanner coil temperature). The Mach-DSP’s temperature calculation will
be based on a thermistor having a 10K nominal resistance and
β
value of 3300K. (Note
that thermistor manufacturers report
β
values differently, and therefore the
β
values
shown in thermistor datasheets are not a consistent indicator of performance…)
Nevertheless, our testing revealed that 10K /
β
=3300 matches pretty well with Cantherm
part number CWF4B103G3380, as well as Vishay part number NTCALUG01T103G501.
