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AD2S44-TM12B Datasheet(PDF) 9 Page - Analog Devices |
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AD2S44-TM12B Datasheet(HTML) 9 Page - Analog Devices |
9 / 12 page 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 RLO pins and the RHI pins. DYNAMIC PERFORMANCE θIN θOUT Ka S2 1 + sT1 1 + sT2 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 K T s K s sT sT 3 2 1 1 + + + + = θ θ where: Ka = 62000 sec–2. T1 = 0.0061 sec. T2 = 0.001 sec. The gain and phase diagrams are shown in Figure 7 and Figure 8. FREQUENCY (Hz) 6 3 0 –15 –12 –9 –6 –3 10 100 Figure 7. Gain Plot FREQUENCY (Hz) 180 135 90 45 –180 –135 –90 –45 0 10 100 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 (Ka) of the converter Ka = Input Acceleration/Error in Output Angle The numerator and denominator must have consistent angular units. For example, if Ka is expressed in sec–2, the input accelera- tion is to be specified in degrees/sec2 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. |
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