AD2S44TM18B ,Low Cost, 14-Bit, Dual Channel Synchro/Resolver-to-Digital Converter
AD2S44-TM18B ,Low Cost, 14-Bit, Dual Channel Synchro/Resolver-to-Digital Converter
AD2S80AAD ,Variable Resolution, Monolithic Resolver-to-Digital ConverterSPECIFICATIONSAD2S80AParameter Conditions Min Typ Max UnitsSIGNAL INPUTSFrequency 50 20,000 HzVolta ..
AD2S80ABD ,Variable Resolution, Monolithic Resolver-to-Digital ConverterSPECIFICATIONSAD2S80AParameter Conditions Min Typ Max UnitsSIGNAL INPUTSFrequency 50 20,000 HzVolta ..
AD2S80ABD ,Variable Resolution, Monolithic Resolver-to-Digital Converterspecifications are guaranteed.
AD2S80AJD ,Variable Resolution, Monolithic Resolver-to-Digital ConverterGENERAL DESCRIPTIONquired and increases the reliability.The AD2S80A is a monolithic 10-, 12-, 14- o ..
AD8542 ,Dual Rail-to-Rail Input and Output, Single Supply Amplifier Featuring Very Low Supply Currentapplications with high source impedance. Sup-V– 4 5+IN Bply current is only 45 µA per amplifier, id ..
AD8542AR ,General Purpose CMOS Rail-to-Rail AmplifiersGENERAL DESCRIPTIONThe AD8541/AD8542/AD8544 are single, dual and quad rail-to-rail input and output ..
AD8542ARM ,General Purpose CMOS Rail-to-Rail AmplifiersFEATURESSingle Supply Operation: +2.7 V to +5.5 VSO-8 (R) SOT-23-5 (RT)Low Supply Current: 45 A/Am ..
AD8542ARM ,General Purpose CMOS Rail-to-Rail AmplifiersCHARACTERISTICSOffset Voltage V 16 mVOS–40°C ≤ T ≤ +125°C7mVA Input Bias Current I 460 pAB–40°C ≤ T ..
AD8542ARM ,General Purpose CMOS Rail-to-Rail AmplifiersCHARACTERISTICSOffset Voltage V 16 mVOS–40°C ≤ T ≤ +125°C7mVA Input Bias Current I 460 pAB–40°C ≤ T ..
AD8542ARM-REEL ,Dual Rail-to-Rail Input and Output, Single Supply Amplifier Featuring Very Low Supply CurrentCHARACTERISTICSOutput Voltage High V I = 1 mA 2.575 2.65 VOH L–40∞C £ T £ +125∞C 2.550 VA Output Vo ..
AD2S44TM18B-AD2S44-TM18B
Low Cost, 14-Bit, Dual Channel Synchro/Resolver-to-Digital Converter
Cy, ANALOG
DEVICES
AN-264
APPLICATION NOTE
ONE TECHNOLOGY WAY tt PD. BOX 9108 e NORWOOD, MASSACHUSETTS 02062-9106 0 617/329-4700
Dynamic Characteristics of Tracking Converters
by Mark Thomas
It is easy to get confused by the terms used to describe the
dynamic performance of tracking resolver-to-digital
converters:
"What is the bandwidth of this converter"
"Acceleration constant! - what does that tell me about
the performance"
"I want to increase the resolution. how will that affect
the tracking rate"
The aim of this application note is to give a simple expla-
nation of the meaning of the dynamic characteristic
specifications, and to explain their particular relevance in
the case of eynchro- and resolver-to-digital converters.
All symrhro/rxtsolver-to-digital converters manufactured
by Analog Devices utilize a Type 2 tracking loop in order
to convert the analog input signal into a digital output
signal.
Examination of the block diagram of a typical converter
will reveal that it essentially consists of six circuit elev
manta connected in a loop; these being the Control Trans-
former (Sine/Coeine Multipliers), ErrorAmp, Phase Sensi-
tive Detector, Integrator, Voltage Controlled Oscillator
and Up/Down Counter. The purpose of the converter is to
produce a digital "tprttstntatiort ofthe input (analog) sig-
nal. It is e ttloatttMooptxrtttrol system.
All closed-loop control systems can be ttlasgifittti accord-
ing to the nature ofthoir stetady-atatrt errors (i.e., the state
where all transient signals have decayed), and in the par-
ticular case of the Type 2 system, the steady-state error is
zero for both a stationary input signal and a constantly
varying (constant velocity) input signal. The only time
when the error signal is present is during periods of accel-
. oration or deceleration.
The advantage of using a Type 2 loop is that the digital
output will always represent the analog input (within the
device eocurecy specification) under the conditions of e
stationary input and also a constantly varying (constant
, velocity) input, i.e.,there ie not velocity lag.
When considering the dynamic performance of the con-
verter, the key specifications which will be of interest to
the system deeignerwill usually be found under the head-
ing "Dynamic Characteristics"; the apoeifieations being
Bandwidth, Acceleration Constant, Reference Frequency,
Tracking Rate, Velocity Scaling, Settling Time and Resolu-
tion. Let us look at each oftheee in tum.
Bandwidth
Being a Type 2 tracking loop, the converter is essentially
a second order low peas filter. It has a predictable dynam-
ie response, the critical frequency and damping character-
istics being set try the values of the components which
make up the loop.
Until very recently, the bandwidth of the converters
has been set by the designer and could not be varied
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by the user. This meant that the user could not optimize
the performance of the device in order to suit the
system requirements. However, with the release by Ana-
log Devices of the monolithic converter, the user can, for
the first time, optimize the bandwidth and other dynamic
characteristics.
The bandwidth of the converter indicates the small signal
response of the loop, and is shown diagrammatically on
the data sheets " a plot of gain and phase against
frequency.
The plots show how the converter will respond if a small
signal, sinusoidally varying about a fixed point, is applied
to the input ot the device. It is important to stress that the
gain and phase plots only indicate the response of the titF
vice to small changes in the input signal; the sinusoidally
varying signal must not cause saturation in any compo-
nent part of the loop, and will in general have a maximum
amplitude of about 5 degrees. The output from the con-
verter will also be a small signal, sinusoidally varying
about a fixed point; the gain and phase relationship be-
tween the input and output signals are shown on the
plots. The frequency axis refers to the frequency of the
sinusoidally varying input signal, and not, it should be
stressed, to the frequency of the reference carrier. The
bandwidth figure quoted for the converter is defined "
the frequency of the sinusoidslly varying input signal, at
the point where the device has a gain of - 3dB. For exam-
ple, in the case of the IS74 hybrid converter, the
bandwidth quoted on the data sheet is 230Hz. If we apply
a sinusoidally varying input signal with a maximum
amplitude of t degree, oscillating at 230Hz about a fixed
point, we should observe a sinusoidally varying output
signal, with a maximum amplitude of about 0.7 degrees
(1/V5x 1 deg) and lagging by almost90degrrres.
Both the bandwidth and the transfer function from which
it is derived, are calculated spetcificatior" based on the in-
dividual transfer functions of the component parts of the
converter loop. They allow the system designer to
evaluate the frequency response of the converter and, in
combination with the other components of the system.
the frequency response of the overall system. The effect
of increasing the bandwidth is to allowthe convenerto re-
spond to faster (higher frequency) small changes in the
input signal; hence, the response time is shorter. How.
" " " Mm 2” 500 "
id "s.
ever, an increasing bandwidth also leads to an increase in
the susceptibility to noise, as the higher frequency noise
signals come into the range of frequencies "seen" by the
converter (remember that the converter is effectively a
low pass filter). One of the major detrimental effects of
this increase in the susceptibility to noise, is the appear-
ance of the noise on the analog velocity output signal.
AeetgttmtlottComttrtttt0U
This parameter specifies the ability of the converter to
withstand and track input accelerations. It is specified in
the data sheet with the dimension of "/secz" and is de-
fined as:
Input Acceleration
K‘ = Errorm t5ritriit7iiiTe
Once the K. is known, the acceleration is defined in any
units, e.g., with a K. equal to 227,555 Isec’, we can im-
mediately see that there will be 1 are minute of additional
error for an input acceleration of 227,555 are min/see', or
1 degree of additional error for an acceleration of 227,555
desgrtsett/sec2.
However, the acceleration figure quoted in the apetgfiea-
tion is not the maximum acceleration which the converter
can withstand. This maximum figure is governed by the
maximum allowable output swing of the internal error
amplifier. If this maximum acceleration is exceeded, then
the digital output will irrecoverably lose track with the
input. In general, the maximum acceleration will be
reached when the error between the input and digital out-
put is approximately 5 degrees. Therefore, a converter
can withstand an acceleration in deg/sec2 of about five
tlmes K. value.
Reference Frequency
In general, the higher the bandwidth of the converter, the
higher is the required reference frequency, which will also
result in a faster possible tracking rate and shorter settling
time. The only limitations on the maximum frequency the
reference can have are the limits set by the components
used in the converter loop. All converters have options
available which will function with e 2.6kHz reference fre-
quency. and many hybrids can operate up to 10kHz refer-
ence. The new monolithic converters will function with a
reference frequency fo 20kHz. In order for the demod-
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ulator to reject higher order multiples of the reference fre-
quency, the reference frequency of the converter should
be at least 2.5 times the closed-ioop bandwidth; hence,
possible bandwidth is limited by reference frequency
Tracking Rate
This is defined as the maximum angular speed for which
the converter output will be able to keep track with the
converter input. It should be noted that the tracking rate
figure quoted in the data sheet actually indicates the
guaranteed minimum limit to a range of tracking rates ac-
ceptable to the converter. This converter will, therefore,
keep track in both directions with inputs varying from sta-
tionary up to the tracking rate figure quoted in the data
sheet. However, due to component tolerances within the
converter loop, the maximum limit of this tracking rate
range may well exceed the figure quoted, and can vary
from device to device. The higherthe reference frequency
at which the converter is intended to operate, then the
higher will be the maximum possible tracking rate. How-
ever, the actual limit for the maximum tracking rate is set
by the integrator and Voltage Controlled Oscillator (VCO).
The maximum recommended rate for the VCO sets the
maximum possible tracking rate for the converter, e.g., if
the maximum recommended VCO rate is 1MHz,then with
a 12-bit converterthe maximum possible tracking rate will
be sete 1,000,000/4096 revs/sec.
The input to the VCO is obtained from the output of the
integrator, and so the maximum voltage swing available
at the integrator output will also limit the maximum track-
ing rate. in general, if the power supply rails are lowered,
the maximum voltage swing available from the integrator
will be lowered and the maximum possible tracking rate
will also decrease. (N.B. this does not happen in all con-
verters.) Signal voltage levels which are outside the
specified limits for the converter will also degrade the
dynamic performance, and the overall accuracy of the
device.
VethtettySeaWtg
Closely linked to tracking rate is the scaling of the analog
velocity signal output. The VCO rate is fixed for a given
input current by the VCO scaling factor which relates the
VCO output frequency to the input current. The input cur-
rent is scaled by the velocity scaling resistor which is set
by the manufagtumrforthtt majority of converters.
However, in the case of the monolithic converters, the
value of this velocity scaling resistor may be selected by
the user, allowing the velocity output to be scaled down
to suit the user’s requirements.
Settiing Time
If a step input is applied to the converter, a finite time will
be required for the intemel loop to null and produce the
required digital output. In all the data sheets, the settling
time is given " the time required for the converter to set-
tle within 1LSB of the quoted accuracy in response to a
step input of 179 degrees ii.e., the worst possible case).
Therefore, this spotty-tion gives an indication of the
large signal response of the converter.
A typical settling time curve for steps of less than 179 de-
grees is shown below.
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It can be seen that for step inputs within approximately 5-
10 degrees, the response is linear, and can be derived
from the small signal response of the converter ii.e., see
bandwidth). Beyond this region, the internal error voltage
will exceed the linear range of the converter.
Resolution
The VCO output clocks the up/down counter; therefore,
for a given VCO output rate, increasing resolution will
lead to a lengthening ofthe settling time ofthe output bits.
The converters available have either a Ith, 12-, 14- or 16-
bit output resolution, or in particular cases, the resolution
may be varied to have any of these four output resolu-
tions. Increasing the resolution from N to N +2 bits will
lead to a factor of 4 decrease in the tracking rate and an
increase in the settling time.
