PSoC
®
3 / PSoC 5 - Accurate ADC Measurement
May 31, 2011 Document No. 001-60263 Rev. *E 1
Associated Part Family: CY8C38XX ,CY8C36XX,
CY8C34XX, CY8C55XX
Associated Application Notes: None
Application Note Abstract
Accuracy is the degree of closeness between a measured value and the actual value. AN60263 describes how the
high-resolution 20-bit delta sigma ADC in PSoC
®
3 / PSoC 5, with its 0.1% internal reference, measures voltage accurately. It
also shows how to get an offset of less than 1 LSb and a gain error of less than 0.003% at a fixed temperature.
Introduction
Accurate measurements are important in many industrial
and medical applications. Most digital measuring devices
work in a similar way: a transducer changes the physical
quantity to be measured to a voltage. Examples of
physical quantities are pressure, temperature, and
acceleration. The voltage is sent through signal
conditioning circuitry, if necessary, and then to an analog
to digital convertor (ADC). The digital value is displayed on
an LCD. PSoC 3 / PSoC 5 combines the signal
conditioning circuitry, ADC, microcontroller, and LCD
driver into a single chip.
The transducer, signal processing circuitry, ADC, and the
finite word length of the controller define the accuracy of
the physical quantity that is measured. This application
note describes how an ADC has an effect on the
measurement accuracy and how to adjust the
PSoC 3 / PSoC 5 delta sigma ADC for maximum
accuracy.
Figure 1. Typical Measurement Signal Chain
Transducer
Signal
Conditioning
Analog to Digital
Convertor
MicroController
Physical
quantity
Single or
differential
Voltage output
Single or
differential
Voltage output
Processing the digital
output and display the
result
The following factors have an effect on the accuracy of an
ADC:
Resolution
Offset error
Gain error
Differential non-linearity (DNL)
Integral non-linearity (INL)
Resolution
Resolution is the minimum measurement that can be
made with a device. In an ADC, it is the minimum voltage
that it can measure. It is given by the following equation.
n
2
Range VoltageInput
Resolution
Equation 1
where n is the number of bits in the convertor.