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ADW71205YSTZ Datasheet(Arkusz danych) 11 Page - Analog Devices

Numer części ADW71205YSTZ
Szczegółowy opis  12-Bit RDC with Reference Oscillator
Pobierz  20 Pages
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AD2S1205
Rev. A | Page 11 of 20
ON-BOARD PROGRAMMABLE SINUSOIDAL
OSCILLATOR
An on-board oscillator provides the sinusoidal excitation signal
(EXC) and its complement signal (EXC) to the resolver. The fre-
quency of this reference signal is programmable to four standard
frequencies (10 kHz, 12 kHz, 15 kHz, or 20 kHz) by using the
FS1 and FS2 pins (see
). FS1 and FS2 have internal pull-ups,
so the default frequency is 10 kHz. The amplitude of this signal
is centered on 2.5 V and has an amplitude of 3.6 V p-p.
Table 5
Table 5. Excitation Frequency Selection
Frequency Selection (kHz)
FS1
FS2
10
1
1
12
1
0
15
0
1
20
0
0
The frequency of the reference signal is a function of the CLKIN
frequency. By decreasing the CLKIN frequency, the minimum
excitation frequency can also be decreased. This allows an
excitation frequency of 7.5 kHz to be set when using a CLKIN
frequency of 6.144 MHz, and it also decreases the maximum
tracking rate to 750 rps.
The reference output of the AD2S1205 requires an external buffer
amplifier to provide gain and additional current to drive the
resolver. See Figure 6 for a suggested buffer circuit.
The AD2S1205 also provides an internal synchronous reference
signal that is phase locked to its Sin and Cos inputs. Phase errors
between the resolver’s primary and secondary windings may
degrade the accuracy of the RDC and are compensated for by using
this synchronous reference signal. This also compensates for the
phase shifts due to temperature and cabling, and it eliminates the
need for an external preset phase-compensation circuit.
SYNTHETIC REFERENCE GENERATION
When a resolver undergoes a high rotation rate, the RDC tends
to act as an electric motor and produces speed voltages in
addition to the ideal Sin and Cos outputs. These speed voltages are
in quadrature to the main signal waveform. Moreover, nonzero
resistance in the resolver windings causes a nonzero phase shift
between the reference input and the Sin and Cos outputs. The
combination of the speed voltages and the phase shift causes a
tracking error in the RDC that is approximated by
Frequency
Reference
Rate
Rotation
Shift
Phase
Error
×
=
(6)
To compensate for the described phase error between the resolver
reference excitation and the Sin/Cos signals, an internal synthetic
reference signal is generated in phase with the reference frequency
carrier. The synthetic reference is derived using the internally
filtered Sin and Cos signals. It is generated by determining the
zero crossing of either the Sin or Cos (whichever signal is
larger), which improves phase accuracy, and evaluating the phase
of the resolver reference excitation. The synthetic reference reduces
the phase shift between the reference and Sin/Cos inputs to less
than 10° and can operate for phase shifts of ±45°.
CHARGE-PUMP OUTPUT
A 204.8 kHz square wave output with a 50% duty cycle is available
at the CPO pin of the AD2S1205. This square wave output can
be used for negative rail voltage generation or to create a VCC rail.
CONNECTING THE CONVERTER
Ground is connected to the AGND and DGND pins (see Figure 5).
A positive power supply (VDD) of 5 V dc ± 5% is connected to
the AVDD and DVDD pins, with typical values for the decoupling
capacitors being 10 nF and 4.7 μF. These capacitors are then
placed as close to the device pins as possible and are connected
to both AVDD and DVDD. If desired, the reference oscillator
frequency can be changed from the nominal value of 10 kHz
using FS1 and FS2. Typical values for the oscillator decoupling
capacitors are 20 pF, whereas typical values for the reference
decoupling capacitors are 10 μF and 0.01 μF. As outlined in the
Loss of Signal Detection section 68 kΩ resistors between the Sin
and SinLO inputs and the Cos and CosLO inputs can be used to
ensure loss of signal detection when all four inputs from resolver
are disconnected.
In this recommended configuration, the converter introduces a
VREF/2 offset in the Sin and Cos signal outputs from the resolver.
The SinLO and CosLO signals can each be connected to a different
potential relative to ground if the Sin and Cos signals adhere to the
recommended specifications. Note that because the EXC and EXC
outputs are differential, there is an inherent gain of 2×.
shows a suggested buffer circuit. Capacitor C1 may be used in
parallel with Resistor R2 to filter out any noise that may exist on the
EXC and
Figure 6
EXC outputs. Care should be taken when selecting the
cutoff frequency of this filter to ensure that phase shifts of the
carrier caused by the filter do not exceed the phase lock range
of the AD2S1205.
The gain of the circuit is
))
1
/(
1
(
)
/
(
ω
C1
R2
R1
R2
n
CarrierGai
×
×
+
×
=
(7)
and
×
×
+
×
⎛ +
×
=
IN
REF
OUT
V
ω
C1
R2
R1
R2
R1
R2
V
V
))
1
/(
1
(
1
(8)
where:
ω
is the radian frequency of the applied signal.
VREF, a dc voltage, is set so that VOUT is always a positive value,
eliminating the need for a negative supply.




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