Introduction
Signal processing applications like wireless communications, audio processing, and machine learning frequently use encoder-decoder architectures to convert analog signals into digital data and vice versa. While there are many commercial encoder-decoder integrated circuits (ICs) available, designing a custom IC tailored to your specific application requirements can improve performance, reduce power consumption, and lower costs. In this article, we will walk through the key design considerations and steps involved in creating a custom encoder-decoder IC using modern design flows and fabrication processes.
Key Design Considerations
Several key factors need consideration when architecting a custom encoder-decoder IC:
Coding Scheme
The coding scheme defines how the analog signal gets converted into a digital code. Common schemes include pulse-code modulation (PCM), delta modulation (DM), and sigma-delta modulation. The coding scheme impacts parameters like signal-to-noise ratio (SNR), bandwidth, and circuit complexity.
Resolution
The resolution determines the number of bits used to represent each sampled value of the analog signal. Higher resolution improves the dynamic range and SNR but increases the data rate. Resolution requirements depend on the application – for example, CD-quality audio requires 16 bits while HD video may need 24 bits.
Sampling Rate
The sampling rate is the number of samples captured from the analog signal per second. According to the Nyquist theorem, the sampling rate must be at least twice the maximum frequency of the analog signal to enable accurate reconstruction. Higher sampling rates improve the SNR but also increase the data rate.
Bandwidth
The analog input bandwidth specifies the range of frequencies that need to be encoded. A larger bandwidth signal requires higher sampling rates and filtering to avoid aliasing. The output reconstruction filter bandwidth must also match the input signal.
Latency
Latency is the delay between the analog input and digital output of the encoder and vice versa for the decoder. Lower latency is critical for applications like wireless communications. The coding scheme choice affects the latency through the encoder and decoder.
Power Consumption
For battery-powered and size-constrained applications, low power consumption is essential. Complex coding schemes and higher sampling rates increase power draw which must be balanced against performance needs. Clock gating, power gating, and careful circuit design help reduce power use.
Technology Process
The IC fabrication process sets the voltage levels, transistor sizes, and passive component values available for implementation. Processes with smaller transistors tend to enable higher speeds and density at lower voltages but cost more. The process choice involves trade-offs between performance, power, area, and fabrication cost.
Encoder Architecture
The encoder architecture takes an analog input signal and converts it into a digital code for storage or transmission. Here is a typicalencoder block diagram:
The key components of the encoder are:
Analog Front End
This conditions the input signal through amplification, filtering, and sampling/holding to capture discrete voltage levels. The front end bandwidth, filter characteristics, and sample/hold parameters depend on the application.
Analog to Digital Converter (ADC)
The ADC quantizes the held sample voltage into a digital code based on the resolution. High-speed ADCs like successive approximation register (SAR) and sigma-delta designs help minimize conversion errors. Calibration circuitry can improve ADC linearity.
Digital Filter
An FIR or IIR digital filter processes the digitized samples to optimize the SNR, band-limit the signal, and compensate for analog front end non-idealities. An efficient implementation is necessary to sustain high sample rates.
Coding Block
This block implements the selected modulation scheme like PCM, DM, or sigma-delta on the filtered digital words. The coding drives the overall encoder resolution, noise shaping, and latency.
Clock Generation
A fast, low jitter clock drives the sampling of the ADC. Phase-locked loops (PLLs) create stable sampling clocks from a reference oscillator. Multiple clock domains can be used to optimize performance and power.
Bias Circuitry
Bias voltages and currents generated on-chip power the analog front end amplifiers and ADCs. Bandgap voltage references give a stable bias supply over temperature variations. The biases require careful design to support robust operation across process corners.
Decoder Architecture
The decoder architecture recovers the original analog signal from the incoming digital code. A typical decoder block diagram is shown below:
The core decoder components are:
Digital Filter
A digital filter processes the incoming codes to optimize the SNR and filter out quantization noise added during encoding. The filter coefficients and structure match the encoder filter.
Digital to Analog Converter (DAC)
The DAC converts the filtered digital words into equivalent analog voltage or current levels. High-performance DACs like current-steering and R-2R ladder architectures help maximize accuracy. Calibration and dynamic element matching minimize errors.
Analog Filter
This filter smoothes the reconstructed analog signal by removing high-frequency noise and images. The filter matches the encoder input filter bandwidth and characteristic.
Analog Back End
Amplifiers, buffers, and filters condition the decoder output to match the original signal levels and drive the required load. Gain/attenuation, filtering, and impedance conversion may be needed.
Clock Generation
The decoder clock matches the encoder sampling clock frequency and phase to synchronize decoding. Typically the encoder clock is recovered from the digital input stream. PLLs filter jitter from the extracted clock.
Bias Circuitry
Bias voltages and currents needed by the DAC, filters, and amplifiers are generated on-chip. Bandgap references provide a temperature-independent bias source. Careful design ensures sufficient bias headroom over process and temperature ranges.
IC Implementation Flow
Once the encoder and decoder architectures are defined, the next step is the physical implementation of the design in an IC. A typical IC design flow involves:
- Functional Verification: The designs are modeled in a hardware description language (HDL) like VHDL or Verilog and simulated pre- and post-synthesis to verify correctness.
- Synthesis: The RTL code is synthesized into a gate-level netlist optimized for the target fabrication process technology. Common logic synthesis tools are Design Compiler and Genus.
- Floorplanning: The core encoder and decoder blocks are placed on the die area and power/ground networks planned. This step ensures good routability.
- Place and Route: The logic gates and standard cells are placed and the interconnections routed based on the floorplan. Timing optimizations occur in this stage.
- Layout: The placed components and routing are assembled into the photomask layout artwork defining each IC layer. Parasitic extraction generates netlists with wire delays for simulation.
- Verification: The placed and routed design goes through final timing, power, and functional verification to sign-off the tapeout.
- Mask Fabrication: The verified layout data generates the photomasks for IC fabrication.
- Fabrication: The masks pattern the transistors, interconnects, and other elements onto the silicon wafer through processes like CMOS.
- Packaging: The finished dies are packaged into IC packages with pins or balls for system integration.
Leading EDA tools for IC implementation include Cadence Innovus, Synopsys IC Compiler, and Mentor Graphics Calibre.
Key Design Trade-Offs
Some of the key design trade-offs encountered when architecting a custom encoder-decoder IC include:
- Coding scheme complexity vs. latency/power
- Resolution vs. bandwidth vs. SNR
- Sampling rate vs. power consumption
- Digital logic complexity vs. chip area
- Analog filter order vs. area/noise
- Technology process node vs. speed vs. cost
- On-chip bias/reference generation vs. external components
Careful modeling, analysis, and simulation are needed to strike the right balance between cost, performance, and power for the target application.
Conclusion
This article provided an overview of designing a custom encoder-decoder IC tailored for signal processing applications. Key concepts included:
- Defining coding scheme, resolution, sampling rate and other parameters
- Architecting efficient encoder and decoder functional blocks
- Leveraging modern IC implementation tools for rapid design
- Balancing trade-offs like speed vs. power based on application needs
With careful planning and optimization, a custom encoder-decoder IC can deliver significant benefits in performance, power, and cost efficiency over standard solutions. The digital design techniques coupled with high-performance analog/mixed-signal circuit design enable quality results. For complex applications, an ASIC encoder-decoder is a compelling approach.
Frequently Asked Questions
What are some examples of encoder-decoder ICs in commercial use today?
Some common examples include audio codec chips used in devices like smartphones, SD/HD video encoders/decoders, wireline modem codec ICs, and wireless baseband processors implementing LTE, 5G, WiFi, Bluetooth and GNSS standards. Companies like Qualcomm, Intel, NVIDIA, Broadcom, Maxim and Texas Instruments offer these chips.
What process nodes are best suited for integration of high-speed data converters?
Modern CMOS processes below 40nm are well-suited for high-performance analog/mixed-signal ICs like data converters. FinFET processes provide good analog/RF performance down to around 5nm. Some vendors also offer specialized processes specifically optimized for analog, RF, and mixed-signal circuits.
How can testing/calibration be implemented on encoder-decoder ICs?
On-chip DAC/ADC calibration circuitry using techniques like digital ramp generators, sine wave fitting, and redundancy helps test and calibrate data converters at speed. Built-in self-test (BIST) architectures and JTAG boundary scan allow testing the digital logic. Production testing validates performance across process corners.
What are some techniques to minimize power consumption?
Clock gating, multi-threshold voltage libraries, and power gating help reduce digital power. For analog, bias currents can be optimized, power supplies turned off when inactive, and cadence techniques used. Voltage scaling, parallel architectures, and simplified algorithms also help minimize power use.
How important is the choice of the IC package?
The IC package provides critical pinouts/balls, power/ground connections, thermal dissipation, and EMI shielding. Parameters like number of I/Os, supply current, heat, and noise need matching to the package options available in the target process technology. This ensures signal and power integrity.
