Data Sheet AD2S44
Rev. B | Page 9 of 12
SCALING FOR NONSTANDARD SIGNALS
A feature of these converters is that the signal and reference inputs
can be resistively scaled to accommodate nonstandard input signal
and reference voltages that are outside the nominal ±10% limits of
the converter. Using this technique, it is possible to use a standard
converter with a personality card in systems where a wide range
of input and reference voltages are encountered.
The accuracy of the converter is affected by the matching accu-
racies of resistors used for external scaling. For resolver format
options, it is critical that the value of the resistors on the S1 (A)/
S1 (B) to S3 (A)/S3 (B) signal input pair be precisely matched to
the S4 (A)/S4 (B) to S2 (A)/S2 (B) input pair. For synchro options,
the three resistors on the S1, S2, and S3 pins must be matched. In
general, a 0.1% mismatch between resistor values contributes an
additional 1.7 arc minutes of error to the conversion. In addition,
imbalances in resistor values can greatly reduce the common-
mode rejection ratio of the signal inputs.
To calculate the values of the external scaling resistors, add
2.222 kΩ for each volt of signal in series with the S1, S2, S3, and
S4 pins (no resistor is required on the S4 pins for synchro options)
and add 3 kΩ extra per volt of reference in series with the R
LO
pins and the R
HI
pins.
DYNAMIC PERFORMANCE
θ
IN
θ
OUT
K
a
S
2
1 + sT
1
1 + sT
2
02947-006
Figure 6. Transfer Function of AD2S44
The transfer function of the converter is as follows:
Open-loop transfer function
2
1
a
IN
OUT
sT
sT
s
K
+
+
×=
θ
θ
1
1
2
Closed-loop transfer function
a
2
a
1
1
IN
OUT
KTsKssT
32
1
+++
=
θ
where:
K
a
= 62000 sec
–2
.
T
1
= 0.0061 sec.
T
2
= 0.001 sec.
The gain and phase diagrams are shown in Figure 7 and Figure 8.
FREQUENCY (Hz)
GAIN (dB)
6
3
0
–15
–12
–9
–6
–3
10 100
02947-007
Figure 7. Gain Plot
FREQUENCY (Hz)
PHASE (Degrees)
180
135
90
45
–180
–135
–90
–45
0
10 100
02947-008
Figure 8. Phase Plot
ACCELERATION ERROR
A tracking converter employing a Type II servo loop does not
suffer any velocity lag. However, there is an additional error
due to acceleration. This error is defined using the acceleration
constant (K
a
) of the converter
K
a
= Input Acceleration/Error in Output Angle
The numerator and denominator must have consistent angular
units. For example, if K
a
is expressed in sec
–2
, the input accelera-
tion is to be specified in degrees/sec
2
and the output angle error is
to be specified in degrees. Alternatively, the angular unit of measure
can also be in units such as radians, arc minutes, or LSBs.
