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Analog-to-Digital Conversion
Published in David R. Martinez, Robert A. Bond, Vai M. Michael, High Performance Embedded Computing Handbook, 2018
James C. Anderson, Helen H. Kim
The offset error of an ADC, which is similar to the offset error of an amplifier, is defined as a deviation of the ADC output code transition points that is present across all output codes (Bowling 2000). This error has the effect of shifting, or translating, the ADC’s actual transfer function away from the ideal transfer function shown in Figure 7-3. For example, by comparing the conceptual ADC’s transfer function of Figure 7-2 with that of the ideal transfer function defined by Figure 7-3, it is apparent that the conceptual ADC’s offset error is -1/2 LSB. In practice, the ideal transfer function may be defined by either Figure 7-2 (known as the mid-riser convention) or Figure 7-3 (known as the mid-tread convention) depending on the manufacturer’s specifications for a particular ADC (IEEE 2000). Once the offset error has been characterized, it may be possible to compensate for this error source by adjusting a variable “trimming” resistor in the analog domain, or by adding (or subtracting) an appropriate offset value to the ADC output in the digital domain. Note that changes in the offset error as a function of time create a dynamic offset error, the effects of which may be mitigated through a variety of design techniques described later.
Analog-to-Digital Converters
Published in John G. Webster, Halit Eren, Measurement, Instrumentation, and Sensors Handbook, 2017
Figure 88.3 shows the transfer function of an ideal 3 bit ADC with some offset error. The straight line joining the centers of the code transitions is raised, or offset, by 0.6 LSB, and the bottom graph shows the resulting error. Figure 88.4 shows an ideal 3 bit ADC with a +25% gain error. The slope of the line through the code transitions is 1.25 times the ideal slope of 1.00. If the slope of the line was 0.75 instead, the gain error would be −25%. The bottom graph shows the error resulting from excessive gain. Offset errors can be compensated for simply by adding a correcting voltage in the analog circuitry or by adding a constant to the digital data. Gain errors can be corrected by analog circuitry like potentiometers or voltage-controlled amplifiers or by multiplying the digital data by a correction constant.
Application of Operational Amplifiers
Published in Bogdan M. Wilamowski, J. David Irwin, Fundamentals of Industrial Electronics, 2018
Carlotta A. Berry, Deborah J. Walter
Unlike an ideal op amp, when the input voltage to the practical op amp is zero, the output is not zero. The input offset voltage is the differential input voltage that is needed to make the output zero when the input is zero. A typical value for the input offset voltage is 2 mV. The output when the input is zero is the output dc offset voltage. The way to compensate for this value in a realistic design is to add a small voltage source at the inverting or non-inverting amplifier of the opposite magnitude and polarity. It may be necessary to use a potentiometer with the op amp input set to zero to find the exact value to cancel out the input offset voltage.
A 261mV Bandgap reference based on Beta Multiplier with 64ppm/0C temp coefficient
Published in International Journal of Electronics Letters, 2022
R. Nagulapalli, N. Yassine, S. Barker, P Georgiou, K. Hayatleh
Minimising the systematic offset requires a very high gain generated via the opamp feedback loop formed by OP1, M3 and Rs (Leung & Mok, 2003; Nagulapalli, Hayatleh, Barker, Zourob et al., 2018). Apart from obtaining high gain with stable loop dynamics, the self-bias in the opamp could give the best results by utilising device mismatch. Hence a low voltage self-bias folded cascode opamp has been designed, where input common-mode voltage is very low (~50-100 mV). The opamp tail current transistor is biased from node A (Figure 3) to implement self-biasing while avoiding the external opamp bias requirement. Whenever the temperature changes, the bias current in the bandgap increases due to it being proportional to absolute temperature (Shrivastava et al., 2015). Hence, if the opamp is biased with a fixed bias current, there will be a systematic offset. Fortunately, the self-bias technique tracks the opamp current with the temperature (Hamouda et al., 2013).
Influence of surface distresses on smartphone-based pavement roughness evaluation
Published in International Journal of Pavement Engineering, 2021
L. Janani, V. Sunitha, Samson Mathew
The external tri-axis accelerometer was calibrated using the offset and sensitivity method (Tuck 2012). Offset is the difference between the actual as well as the specified output value. Sensitivity is the minimum physical parameter input that is required to create a detectable output range (Brown 2013). A departure from ideal sensitivity value is called the sensitivity error. When the accelerometer is placed on a flat surface with its front facing up; the x-, y-, and z-axes point from left to right, rear to front and top to bottom of the accelerometer, respectively. Ideally, the values of x-, y-, and z-axis accelerations should be 0, 0 and 9.81 m/s2, respectively. Here, 9.81 m/s2 represents the acceleration due to gravity. Similarly, when the accelerometer is placed on a flat surface with its front facing down, the ideal acceleration values for x-, y-, and z-axis should be 0, 0 and −9.81 m/s2. For portrait up position, i.e. when the head of the accelerometer is facing up on a flat position, the ideal acceleration for y-axis should be 9.81 m/s2 and for portrait down position, the same will be −9.81 m/s2. In landscape right position, i.e. when left edge of the accelerometer is facing up, the ideal x-axis acceleration should be 9.81 m/s2 and the same will be −9.81 m/s2 for landscape left position.
Hardware design implementation issues of the estimator-based controller using FPGA
Published in International Journal of Electronics, 2019
Kanthimathi R., Kamala J., Jaibalaganesh T., Vasuhi S.
For example, an offset error can be added with the measured value which is obtained from ADC to get the actual value. The offset error is 00000111 (0.14 V). Measured and actual values of ADC and 16-bit representation of analogue voltage are given in Table 4. From the table, 3.5 LSB variations can be observed between measured and actual values. Offset error is rectified through 16-bit developed architecture using VHDL coding.