Delta-Sigma ADC

Key Takeaways

• A delta-sigma (Σ-Δ) analog-to-digital converter (ADC) is well-suited for high-resolution and high-precision conversion applications.

• A delta-sigma ADC is often used where accuracy and precision are critical, even if it means sacrificing some speed.

• Delta-sigma ADCs are commonly used in PCB designs for applications requiring high-resolution, low-noise analog signal conversion into digital data.

Block diagram of a first-order delta-sigma ADC with a 1-bit digital-to-analog converter (DAC)

(Source: Katanzag)

A delta-sigma (Σ-Δ) analog-to-digital converter (ADC) is well-suited for high-resolution and high-precision conversion applications, such as audio equipment, digital audio converters, measurement instruments, and some communication systems, where the emphasis is on achieving high resolution and minimizing noise. The core of a delta-sigma ADC is a feedback loop that compares the incoming analog signal to a reference voltage. It then rapidly switches between two digital values (usually 1 and 0) to approximate the analog signal's behavior. This high-speed switching results in a stream of digital bits filtered and processed to produce a high-resolution digital output. Here's how a delta-sigma ADC works in more detail:

• Delta Modulation: Using a delta modulator, the Delta-Sigma ADC compares an incoming analog signal to a reference voltage and produces a one-bit digital output. At each sampling instant, the output bit indicates whether the input voltage is higher or lower than the reference voltage.

• Integration and Feedback: One-bit output is then fed back into an integrator, where it is integrated over time. The output of this integration stage is subtracted from the input signal, resulting in an error signal.

• Quantization: The error signal is quantized into a one-bit digital value again. This quantization process introduces noise and error into the system. The delta modulator continuously tries to minimize the error between the input and the quantized signal by adjusting the integration process.

• High-Speed Operation: This process is repeated at a very high sampling rate, typically much higher than the desired output data rate. The high sampling rate helps push quantization noise outside the bandwidth of interest.

• Decimation (Digital) Filter: A decimation filter (or digital filter) filters and downsamples the high-speed one-bit data stream into a multi-bit digital output. This filter removes the high-frequency quantization noise, leaving a high-resolution digital representation of the original analog signal.

The essential advantage of a delta-sigma ADC is its ability to achieve high resolution and low noise performance. It is often used in applications where accuracy and precision are critical, even if it means sacrificing some speed. However, it can be relatively slow compared to other ADC types like successive approximation ADCs or flash ADCs.

Delta-Sigma ADC and PCB Design

Delta-sigma ADCs are commonly used in PCB designs for applications requiring high-resolution, low-noise analog signal conversion into digital data. PCB design for a delta-sigma ADC involves carefully segregating analog and digital sections, ensuring clean and stable power supplies with precise voltage references, implementing noise-reduction techniques like signal isolation, controlled impedance traces, and grounding, and adhering to the manufacturer's layout guidelines. Delta-sigma ADCs have some unique characteristics and requirements that can affect PCB design, including:

Noise Sensitivity

They are highly noise-sensitive, both in the analog and digital domains. You should route sensitive analog traces away from noisy digital components and signals to minimize noise. Ground planes and proper grounding techniques are critical to reduce noise. Isolate analog and digital sections on the PCB to prevent cross-talk.

Clock Signal

Delta-sigma ADCs typically require a stable and accurate clock signal for proper operation. Ensure the clock signal is clean and jitter-free and meets the ADC's timing requirements. Use a low-noise, low-jitter clock source and isolate it from other noisy components.

Digital Filter

They often include a digital filter to improve resolution and reduce noise. You'll need to allocate space for this filter on the PCB. Ensure the digital filter components and traces are designed to minimize signal degradation and noise pickup.

Reference Voltage

Delta-sigma ADCs require a precise reference voltage. The reference voltage source should be stable and well-regulated. It's essential to provide proper decoupling and filtering for the reference voltage to prevent noise interference.

Power Supply

They can be sensitive to variations in the power supply voltage. Use low-noise, well-regulated power supplies for both the analog and digital sections of the ADC. Pay attention to the power supply decoupling capacitors to ensure stable voltage levels.

PCB Layer Stackup

The PCB layer stackup is crucial in managing signal integrity and noise isolation. Consider using a multilayer PCB with dedicated ground and power planes to provide solid reference planes for signals and minimize noise coupling between layers.

Signal Integrity

High-speed digital signals on the same PCB can introduce electromagnetic interference (EMI) and affect the performance of the delta-sigma ADC. Implement proper signal integrity techniques such as controlled impedance traces, differential signaling, and EMI shielding where necessary.

PCB Layout

Careful component placement and routing are critical for delta-sigma ADCs. Minimize trace lengths, avoid sharp corners, and maintain proper signal spacing to reduce parasitic capacitance and inductance. Follow the manufacturer's recommended layout guidelines.

Thermal Management

Delta-sigma ADCs can generate heat during operation. Adequate thermal management, such as using thermal vias and heat sinks, may be necessary to ensure the ADC operates within its specified temperature range.

Grounding

Implement a star grounding scheme where all ground connections converge at a single point to minimize ground loops and noise.

Delta-sigma ADCs can be sensitive to noise and require careful attention to various aspects of PCB design. These aspects include noise management, clock signals, reference voltage, power supplies, layer stackup, signal integrity, layout, and thermal considerations. It's crucial to consult the datasheet and guidelines provided by the ADC manufacturer to ensure optimal performance in your specific application.

Cadence Can Help with Your PCB Design

Once you’ve decided to use delta-sigma ADCs in your PCB design and are ready to place and route components in your layout, make sure you use OrCAD PCB Designer and the industry’s best PCB design and analysis software from Cadence. Leading electronics providers rely on Cadence products to optimize power, space, and energy needs for a wide variety of market applications. To learn more about our innovative solutions, talk to our team of experts or subscribe to our YouTube channel.