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In today s world of mixed-signal systems, many applications require analog quantities including but not limited to voltage, current, temperature, pressure, acceleration, pH, flow, and ECG to be measured and processed. The field of uses ranges from lab and medical equipment operating in controlled environments to industrial equipment running under harsh operating conditions. The analog signals to be measured can range from a few micro-volts in ECG systems to thousands of volts in electricity generation plants.
Unfortunately, there is no such thing as an ideal converter in the real world, where systems have to contend with errors that are introduced into the system and affect the ADC s output. The most important errors are offset and gain errors.
Refer graph below. This is a plot of an 8 bit ADC with a range of +2.5V. X axis denotes the input voltage and Y axis denotes the ADC counts. The blue line is the ideal ADC output. The red line is the actual ADC output. Notice the actual output is shifted from the ideal. This shift is called the offset error.
All operational amplifiers have a finite offset voltage at the input. This offset voltage gets added to the input signal, gets amplified by the amplifier s gain and manifests at the output. Apart from the amplifier stage, the ADC also has its own offset voltage which adds to the system error. Offset error is an additive error and can be easily removed from the system.
Graph below is the plot of the same 8 bit ADC with the +2.5V range. Note that the slope of the actual output is now different from the slope of the ideal output. This shift in slope is called the gain error.
These errors may be removed from a system using many calibration techniques like:
Correlated Double Sampling
Gain calibration using external reference.
PSoC 1, with its flexible analog resources and routing makes it very easy to implement all of the above calibration techniques. Depending upon the application, one or more of these methods can be combined to achieve maximum accuracy.