Background: Analog to digital converters (ADCs)

Analog to Digital Converters (ADCs)

An analog-to-digital converter is an electronic circuit that transforms an analog electronic signal into a digital signal.

Types of ADCs

The PSoC provides two types of ADCs.

• Successive Approximation (SAR) ADCs (2 available simultaneously)
• Delta-Sigma ADC (1 available)

Successive Approximation (SAR) ADC

The SAR ADC uses a digital-to-analog converter (DAC) to approximate an analog input signal.

The SAR ADC works as follows:

• First, a controller tells the N-Bit DAC to generate an output voltage.

• The output of the DAC is then compared to V_in using a voltage comparator. This tells us if the initial guess is too high or too low.

• The controller then adjusts the output of the DAC and guesses again.

• This is repeated until V_in is found.

Pros:

• Lower cost than Flash ADC
• Can be built for high-resolution applications.

Cons:

• Slower speed at high accuracies due to the need to compare many values.

Delta-Sigma ADC

The Delta-Sigma ADC is based on a delta modulator---a feedback control system that produces an output signal from an input signal. The feedback path contains an integrator, which integrates the output signal to create a feedback signal x̂(t). The feedback signal is subtracted from the input signal x(t) to generate an error signal e(t) = x(t) - x̂(t). The error signal is fed into a voltage comparator, which compares the error signal to GND (0 V) and produces an output signal y(t), which is either V_ref or -V_ref.

The Delta-Sigma ADC works as follows:

• First, from the analog signal an estimate x̂(t) is subtracted to calculate an error signal e(t) = x(t) - x̂(t).

• The error is fed into a 1-bit voltage comparator (quantizer) which outputs:

y(t) =  V_ref, e(t) ≥ 0
y(t) = -V_ref, e(t) < 0

• This "tracks" the input signal.

• The accuracy depends on V_ref and the sampling time dt.

Pros:

• Very high resolution
• Almost completely digital

Cons:

• Can be slow if oversampled