BusWorks Model XT1531‐000
4 CH Current 4 CH Digital I/O w/USB & Modbus
Acromag, Inc. Tel: 248‐295‐0880
‐
54
‐
Current Outputs…
Excitation for the analog outputs
is required at TB3 to operate
them.
WARNING:
If you make the
mistake of connecting 24V
excitation to the 12V excitation
terminals, you will damage the
12V TVS and switching regulator.
If that happens, you can connect
24V to the 24V excitation
terminals to continue operation.
Reference Test Conditions:
0‐
20mA output; 25
C ambient
temperature; 24VDC supply,
24VDC analog and digital
excitation.
Output Resolution:
A 16‐bit voltage DAC with a 2.76267V reference drives
2*Vref=5.52534V at a full‐count of 32767 to a V/I converter with its output current
equal to the DAC v
oltage Vo divided by 249Ω. Thus, with a raw DAC count of +32767
corresponding to 5.52534V, the DAC Vo for 20mA of output is 0.020*249 = 4.98V.
This corresponds to a DAC count of 32767* (4.98V/5.52534V)=29533. Thus, a raw
DAC count of 0‐29533 yields an output current of 0‐20mA. This range is calibrated
using 1mA as the low calibration point and this has a corresponding raw DAC count of
29533*1/20=1477. The optional 4‐20mA range is treated as a sub‐range of 0‐20mA
with 4mA corresponding to a raw count of 2953
3*4/20=5907. From a programmer’s
perspective, to simplify control, both current output ranges are normalized to 0‐
30000 for 0‐100% of output range, or 0‐20000 with legacy support
enabled.
Normalization to 30000 limits the maximum program count to a signed
16‐bit integer
of 32767. Thus, 20mA*32767/30000=21.8447mA, slightly less than the
5.52534/249=22.1901mA the DAC is capable of driving. The raw DAC count that
corresponds to the normalized maximum is 32767*21.8447/22.1901=32257. The
effective resolution is the lowest of either the raw DAC count, or the normalized
count. For normalization to 30000, the effective resolution is dominated by the 16‐
bit raw DAC resolution at 1 part in 29533. But for normalization to 20000, the
effective resolution is dominated by the normalized resolution of 1 part in 20000. For
the 4‐20mA range and normalization to 30000, we get a maximum current of
20mA+(16mA*2767/30000) = 21.4757mA. For the 4‐20mA range and normalization
to 20000, we get a maximum current of 22.1901mA and this corresponds to a
normalized count of 20000+20000*2.1901/16=22738.
IDEAL RANGE LIMITS WRT DAC COUNTS, NORMALIZATION, AND 16‐BIT INTEGERS
Range
Limits
Internal 16‐Bit raw
D/A Count
1
Normalized 30K
Program Count
Normalized 20K
Count w/Legacy
0‐20mA
Output Range
ZERO
0
0 (0.000mA)
0 (0.000mA)
+FS
29533
30000 (20.000mA)
20000 (20.000mA)
Resolution
1 in 29533
1 in 30000
1 in 20000
+OVR
32257 (21.8447mA)
32767 (22.1901mA)
32767 (21.8447mA)
22190 (22.1901mA)
4‐20mA Output Range (Sub‐Range of raw 0‐20mA range)
ZERO
5907
0 (4.000mA)
0 (4.000mA)
+FS
29533
30000 (20.000mA)
20000 (20.000mA)
Resolution
1 in 23626
1 in 30000
1 in 20000
+OVR
31712 (21.4757mA)
32767 (22.1901mA)
32767 (21.4757mA)
22738 (22.1901mA)
1
Note:
This is the ideal internal count used by the firmware to control the DAC, not
the count that operators use to set the output level. Operators use the Normalized
counts to set the output level and that count will differ around the ideal values shown
above according to their
channel‐specific calibration, normalization, and range.
Output Calibration:
Manually calibrated by precisely driving the 0‐20mA output to
1mA and 20mA, and storing their raw DAC counts to build the equation of a line for
deriving any output current along the linear relationship between these points.
Output Digital‐to‐Analog Converter (DAC):
A 16‐bit converter from Analog Devices,
ADS5362BSTZ, operating in bipolar mode with a 2.76267V reference, yielding a 16‐bit
D/A output voltage range of ±5.52534V corresp
onding to a raw DAC count of ‐32768
to +32767 and a normalized program count of ±30000.
