Ratiometric Conversion
Ratiometric Conversion is the A/D conversion process in which the binary result is a ratio of the supply voltage or reference voltage, the latter being equal to full-scalevalue by default. The PIC16C7X is a ratiometric A/D converter where the result depends on VDD
or VREF. In some A/Ds, an absolute reference is provided resulting in “absolute conversion”.
Sample and Hold
In sample and hold type A/D converters, the analog input has a switch (typically a FET switch in CMOS) which is opened for a short duration to capture the analog input voltage onto an on-chip capacitor. Conversion is typically started after the sampling switch is closed.
Track and Hold
Track and Hold is basically the same as sample and hold, except the sampling switch is typically left on. Therefore the voltage on the on-chip holding capacitor “tracks” the analog input voltage. To begin a conversion, the sampling switch is closed. The PIC16C7X A/D falls in this category.
Sampling Time
Sampling Time is the time required to charge the on-chip holding capacitor to the same value as is on the analog input pin. The sampling time depends on the magnitude of the holding capacitor and the source impedance of the analog voltage input.
Offset Error (or Zero Error)
Offset Error is the difference between the first actual (measured) transition point and the first ideal transition point as shown in Figure 6. It can be corrected (by the user) by subtracting the offset error from each conversion result.
Full Scale Error (or Gain Error)
Full Scale Error is the difference between the ideal full scale and the actual (measured) full scale range (Figure 7). It is also called gain error, because the error changes the slope of the ideal transfer function creating a gain factor. It can be corrected (by the user) by multiplying each conversion result by the inverse of the gain.
Integral Non-Linearity (INL), or Relative Error
The deviation of a transition point from its corresponding point on the ideal transfer curve is called “Integral Non-Linearity” (Figure 8). The maximum difference is reported as the INL of the converter. It is important to note that Full Scale Error and the Offset Error are normalized to match end transition points before measuring the INL.
Reference:
Ratiometric Conversion is the A/D conversion process in which the binary result is a ratio of the supply voltage or reference voltage, the latter being equal to full-scalevalue by default. The PIC16C7X is a ratiometric A/D converter where the result depends on VDD
or VREF. In some A/Ds, an absolute reference is provided resulting in “absolute conversion”.
Sample and Hold
In sample and hold type A/D converters, the analog input has a switch (typically a FET switch in CMOS) which is opened for a short duration to capture the analog input voltage onto an on-chip capacitor. Conversion is typically started after the sampling switch is closed.
Track and Hold
Track and Hold is basically the same as sample and hold, except the sampling switch is typically left on. Therefore the voltage on the on-chip holding capacitor “tracks” the analog input voltage. To begin a conversion, the sampling switch is closed. The PIC16C7X A/D falls in this category.
Sampling Time
Sampling Time is the time required to charge the on-chip holding capacitor to the same value as is on the analog input pin. The sampling time depends on the magnitude of the holding capacitor and the source impedance of the analog voltage input.
Offset Error (or Zero Error)
Offset Error is the difference between the first actual (measured) transition point and the first ideal transition point as shown in Figure 6. It can be corrected (by the user) by subtracting the offset error from each conversion result.
Full Scale Error (or Gain Error)
Full Scale Error is the difference between the ideal full scale and the actual (measured) full scale range (Figure 7). It is also called gain error, because the error changes the slope of the ideal transfer function creating a gain factor. It can be corrected (by the user) by multiplying each conversion result by the inverse of the gain.
Integral Non-Linearity (INL), or Relative Error
The deviation of a transition point from its corresponding point on the ideal transfer curve is called “Integral Non-Linearity” (Figure 8). The maximum difference is reported as the INL of the converter. It is important to note that Full Scale Error and the Offset Error are normalized to match end transition points before measuring the INL.
Reference:
- AN546, Microchip Technology Inc. Sumit Mitra, Stan D’Souza, and Russ Cooper,
