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Data Device Corporation
www.ddc-web.com
SD-14550 Series
Rev. H
GENERAL SETUP CONSIDERATIONS
The following recommendations should be considered when
using the SD-14550 Series converters:
1) The power supply is +5 V DC.
2) Direct inputs are referenced to AGND.
3) Connect (close to hybrid) pin 31 (Analog Ground) to pin 7
(GND).
4) Connect a 33 F/10 VDC tantalum filter capacitor externally
between pin 5 (filter point) to pin 7 (ground).
PROGRAMMABLE RESOLUTION
Resolution is controlled by pins 27 and 28. The resolution can be
changed during converter operation so that the appropriate res-
olution and velocity dynamics can be set as needed. To insure
that a race condition does not exist between counting and
changing the resolution, the resolution control is latched inter-
nally. Refer to TABLE 2 for resolution control.
BIT (BUILT-IN-TEST)
This output is a logic line that will flag an internal fault condition
or LOS (Loss-Of-Signal). The internal fault detector monitors the
internal loop error and, when it exceeds approximately ±100
LSBs, will set the line to a logic 0; this condition will occur during
a large-step input and will reset to a logic 1 after the converter
settles out. (The error voltage is filtered with a 500 s filter.) BIT
will set for an over velocity condition because the converter loop
cannot maintain input/output sync. BIT will also be set if a total
LOS (loss of all signals) occurs. Additionally, in the SD-14550XS
version, BIT will set when a Loss-of-Reference (LOR) condition
occurs.
THEORY OF OPERATION
The SD-14550 Series of converters are based upon a single
chip CMOS custom monolithic. They are implemented using the
latest IC technology, which merges precision analog circuitry
with digital logic to form a complete high performance tracking
synchro/resolver-to-digital converter.
CONVERTER OPERATION
FIGURE 1 is the functional block diagram of the SD-14550
Series. The converter operates with a single +5 V DC power sup-
ply and internally generates a negative voltage of approximate-
ly 5 volts. This negative voltage comes out on pin 5 (filter point)
— see GENERAL SETUP CONSIDERATIONS.
The converter is made up of three main sections; an input front-
end, an error processor, and a digital interface. The converter
front-end differs for synchro, resolver and direct inputs. An elec-
tronic Scott-T is used for synchro inputs, a resolver conditioner
for resolver inputs, and a sine and cosine voltage follower for
direct inputs. These amplifiers feed the high accuracy Control
Transformer (CT). Its other input is the 16-bit digital angle
φ. Its
output is an analog error angle, or difference angle, between the
two inputs. The CT performs the ratiometric trigonometric com-
putation of SIN
θCOSφ - COSθSINφ = SIN(θ - φ) using amplifiers,
switches, logic, and capacitors in precision ratios.
The converter accuracy is limited by the precision of the com-
puting elements in the CT. In these converters ratioed capacitors
are used in the CT, instead of the more conventional precision
ratioed resistors. Capacitors used as computing elements with
op-amps need to be sampled to eliminate voltage drifting.
Therefore, the circuits are sampled at a high rate to eliminate
this drifting and at the same time to cancel out the op-amp off-
sets.
The error processing is performed using the industry standard
technique for type II tracking R/D converters. The DC error is
integrated yielding a velocity voltage which in turn drives a volt-
age controlled oscillator (VCO). This VCO is an incremental inte-
grator (constant voltage input to position rate output) which
together with the velocity integrator forms a type II servo feed-
back loop. A lead in the frequency response is introduced to sta-
to reduce the gain and ripple at the carrier frequency and above.
TABLE 2. RESOLUTION CONTROL (A AND B)
RESOLUTION
B
A
10 bit
0
12 bit
0
1
14 bit
1
0
16 bit
1