How does a switched capacitor ADC sample and convert analog signals using capacitor charging and discharging?
1 Answer
A switched capacitor analog-to-digital converter (ADC) is a type of ADC that uses the principles of capacitor charging and discharging to sample and convert analog signals into digital representations. It consists of capacitors, switches, and an internal clock. Here's a high-level overview of how it works:
Sampling Phase:
The switched capacitor ADC operates in two main phases: sampling and conversion. In the sampling phase, the input analog signal is sampled and held by a sampling capacitor.
The sampling capacitor is connected to the input signal during a specific phase of the internal clock. When the clock's sampling phase begins, the switches connect the sampling capacitor to the analog input, allowing it to charge up to the voltage level of the analog signal.
Hold Phase:
Once the sampling capacitor has charged to the input voltage, the clock switches to the hold phase. During this phase, the switches disconnect the sampling capacitor from the analog input, effectively holding the charge on the capacitor at the sampled voltage level.
The hold phase ensures that the sampled voltage remains constant while the conversion process takes place, preventing any changes in the input signal from affecting the result.
Conversion Phase:
The next step is the conversion phase, where the sampled analog voltage on the sampling capacitor is used to produce a digital representation.
The switched capacitor ADC utilizes a series of capacitors and switches configured as a binary-weighted capacitor array.
During the conversion phase, these capacitors are connected and disconnected to the sampled voltage in a specific sequence according to the ADC's internal clock.
By doing so, the capacitors effectively act as a digital-to-analog converter (DAC), generating a binary-weighted voltage that approximates the input voltage.
Successive Approximation:
The binary-weighted voltage is then compared to the sampled voltage on the sampling capacitor using a comparator.
The comparator's output indicates whether the binary-weighted voltage is greater or smaller than the sampled voltage.
Based on this comparison, the ADC performs a successive approximation algorithm to determine the most significant bits (MSBs) of the digital representation.
The ADC adjusts the binary-weighted capacitor array to get closer to the sampled voltage value, and the process is repeated for each successive bit of the digital output.
Digital Output:
After the successive approximation algorithm completes, the ADC has generated a digital representation of the input analog signal.
This digital output can be further processed or stored as needed.
Switched capacitor ADCs are commonly used in integrated circuits due to their relatively simple architecture, low power consumption, and compatibility with CMOS technology. However, they may have some limitations regarding speed and accuracy compared to other ADC types like the successive approximation ADC or the delta-sigma ADC.
Sampling Phase:
The switched capacitor ADC operates in two main phases: sampling and conversion. In the sampling phase, the input analog signal is sampled and held by a sampling capacitor.
The sampling capacitor is connected to the input signal during a specific phase of the internal clock. When the clock's sampling phase begins, the switches connect the sampling capacitor to the analog input, allowing it to charge up to the voltage level of the analog signal.
Hold Phase:
Once the sampling capacitor has charged to the input voltage, the clock switches to the hold phase. During this phase, the switches disconnect the sampling capacitor from the analog input, effectively holding the charge on the capacitor at the sampled voltage level.
The hold phase ensures that the sampled voltage remains constant while the conversion process takes place, preventing any changes in the input signal from affecting the result.
Conversion Phase:
The next step is the conversion phase, where the sampled analog voltage on the sampling capacitor is used to produce a digital representation.
The switched capacitor ADC utilizes a series of capacitors and switches configured as a binary-weighted capacitor array.
During the conversion phase, these capacitors are connected and disconnected to the sampled voltage in a specific sequence according to the ADC's internal clock.
By doing so, the capacitors effectively act as a digital-to-analog converter (DAC), generating a binary-weighted voltage that approximates the input voltage.
Successive Approximation:
The binary-weighted voltage is then compared to the sampled voltage on the sampling capacitor using a comparator.
The comparator's output indicates whether the binary-weighted voltage is greater or smaller than the sampled voltage.
Based on this comparison, the ADC performs a successive approximation algorithm to determine the most significant bits (MSBs) of the digital representation.
The ADC adjusts the binary-weighted capacitor array to get closer to the sampled voltage value, and the process is repeated for each successive bit of the digital output.
Digital Output:
After the successive approximation algorithm completes, the ADC has generated a digital representation of the input analog signal.
This digital output can be further processed or stored as needed.
Switched capacitor ADCs are commonly used in integrated circuits due to their relatively simple architecture, low power consumption, and compatibility with CMOS technology. However, they may have some limitations regarding speed and accuracy compared to other ADC types like the successive approximation ADC or the delta-sigma ADC.