What Is an Active DAC?
A digital-to-analog converter (DAC) is a device that converts digital signals, such as binary code, into analog signals (Yazdani and Singh, 2022). The analog signals are a continuous representation of the discrete digital inputs. DACs are used in a wide range of electronic devices to convert stored or transmitted digital data into analog signals that can be used in the real world. For example, DACs are used to convert digital audio files into analog signals that can drive speakers and produce sound. They are a key component of digital media systems.
At their most basic level, DACs convert 1s and 0s from digital signals into corresponding voltage or current levels in analog signals. There are various circuit designs for DACs, but fundamentally they all use some type of electronic switching to generate an analog output that corresponds to the incoming digital code (Rashid, 2022). The resolution of the DAC determines how many discrete voltage or current levels it can produce.
Sources:
Rashid, M.H. (2022). What is a DAC: The Ultimate DAC Guide. Retrieved from https://en.wikipedia.org/wiki/Digital-to-analog_converter
Yazdani, A., & Singh, A. (2022). Digital Electronics: Principles, Devices and Applications. John Wiley & Sons.
What is an Active DAC?
An active DAC (Direct Attach Copper) is a type of cable that contains active electronics that condition the signal between two connected devices. This is in contrast to a passive DAC, which does not contain any active electronics (Source).
The main difference between an active and passive DAC is that the active DAC integrates electrical components like equalizers, amplifiers, and other active circuitry that can boost the signal, enable longer lengths, and compensate for signal loss (Source). This allows an active DAC to transmit data over longer distances, from 30 meters up to 100 meters.
A passive DAC is just a simple twinax copper cable assembly that relies solely on the transmitting and receiving devices to condition the signal. It does not contain any active components. Passive DACs are limited in length, typically no more than 7-10 meters (Source).
In summary, the key difference is that an active DAC integrates electrical components to boost and condition the signal, while a passive DAC does not contain any active electronics.
How an Active DAC Works
An active DAC contains additional circuitry and components that allow it to amplify and regenerate the electrical signal as it travels through the cable (Link). This provides several benefits compared to a passive DAC cable.
Inside an active DAC cable, there are small integrated circuits (ICs) spaced out along the length of the cable. These ICs contain signal amplifiers that boost the strength of the signal at regular intervals. This restores the signal and helps prevent signal loss or degradation as it travels down the lengthy copper cable (Link).
The integrated circuits are powered through additional copper wires within the cable that deliver DC power. This allows the active components to operate without requiring external power at the endpoints. High quality shielding helps minimize noise and interference on the data signals.
Active DACs contain transceiver chips at each end that encode and decode the signals. This allows them to transmit data at high speeds over longer distances compared to passive DACs (Link). The transceiver ICs also handle re-timing the signals to maintain synchronization.
Overall, the active components within an active DAC cable compensate for cable attenuation and dispersion effects that can degrade signals on passive DACs. This allows active DACs to transmit data reliably over greater lengths and at higher speeds. The components rebuild the signal integrity at set intervals for delivery with minimal loss between endpoints.
Advantages of Active DACs
Active DACs offer several key advantages over passive DACs:
Faster speeds – Active DACs incorporate electronic components that condition and boost the signal, allowing for higher bandwidth and data transfer rates compared to passive DACs. According to Pactech, active DACs can achieve speeds up to 25 or 28Gbps, whereas passive DAC max out at 10Gbps.
Increased accuracy – The signal conditioning and regeneration performed by an active DAC’s integrated circuitry results in lower bit error rates and more accurate data transfer over longer distances.
Lower noise – Active DACs actively reduce jitter, insertion loss, cross-talk and other sources of noise that can interfere with signal integrity. This results in a cleaner, higher quality signal.
Overall, active DACs enhance performance for high speed applications where fast, reliable and accurate data transmission over copper cables is required. The improved signal integrity allows longer maximum distances compared to passive DACs. As this source summarizes, active DACs provide advanced signal conditioning without the need for additional hardware at the switch or host.
Disadvantages of Active DACs
While active DACs offer several advantages, they also come with some downsides to consider:
Active DACs are more complex than passive DACs, requiring active electronic components inside the cable. This added complexity increases the potential points of failure (1). The active electronics inside active DAC cables also draw more power compared to passive cables. Higher power consumption generates more heat that needs to be dissipated, adding design challenges (2).
Due to the active electronics inside the cable, active DACs are heavier and bulkier than passive DAC cables. The thicker cables are more difficult to manage and route through tight spaces (3).
Sources:
(1) https://www.precisionot.com/trade-offs-to-consider-with-dacs-aecs-and-aocs/
(2) https://www.fiber-optic-components.com/advantages-and-disadvantages-of-direct-attach-twinax-cable.html
(3) https://www.fiber-optic-components.com/advantages-and-disadvantages-of-direct-attach-twinax-cable.html
Types of Active DACs
There are several types of active DACs, with the main types being:
R-2R DACs
R-2R DACs consist of a ladder network of resistors with equal values of R and 2R. This resistor ladder acts as a weighted resistor network to convert the digital input code to an analogue output voltage. R-2R DACs are simple to construct but have low accuracy and linearity [1].
Delta-Sigma DACs
Delta-sigma DACs utilize delta-sigma modulation and oversampling techniques to achieve higher resolution than traditional R-2R DACs. They contain a delta-sigma modulator and a reconstruction filter. Delta-sigma DACs offer improved accuracy but require complex digital signal processing [2].
Segmented DACs
Segmented DACs contain multiple R-2R ladders to obtain higher resolution. The outputs of each ladder are summed to generate the analog output. Segmented DACs provide good linearity and resolution but require complex circuitry [3].
Active DAC Applications
Active DACs are used in a wide range of applications that require the conversion of digital signals to analog signals. Some of the most common applications include:
Audio – Active DACs are commonly used to convert digital audio signals to analog for playback through amplifiers and speakers. High-end audio equipment like CD players, Hi-Fi tuners, DJ mixers and more use precision active DACs to achieve top quality audio reproduction [1].
Video – Digital video signals need to be converted to analog for display on analog monitors, projectors and TVs. Active DACs perform this conversion while preserving video signal fidelity. They are used in DVD players, display adapters, video processing equipment and more [1].
Communications – In digital communications systems, active DACs are used to convert digital data to analog signals for transmission over analog channels. They are found in modems, mobile phones, routers, radio transmission systems and more [2].
Instrumentation – Active DACs convert digital signals from sensors, microcontrollers and measurement systems into analog voltages/currents. They interface digital circuitry with analog instruments, actuators and control systems [3].
Medical Equipment – Medical imaging systems, patient monitors, infusion pumps and other medical equipment use active DACs to generate analog waveforms for stimulation, display and control purposes.
Active DAC Specifications
Active DACs are characterized by certain key specifications including resolution, sampling rate, signal-to-noise ratio, distortion, and more. These specifications determine the performance and capabilities of the DAC.
Resolution refers to the number of bits of data the DAC can process. Higher resolution DACs with more bits (e.g. 24-bit) can encode a greater dynamic range and more precisely reproduce an analog waveform compared to lower resolution DACs (Pactech, 2022).
Sampling rate is the number of samples per second taken by the DAC to convert digital signals to analog. Higher sampling rates like 192 kHz allow the reproduction of higher frequencies up to 96 kHz. This results in better audio quality.
Signal-to-noise ratio (SNR) measures how much noise is present relative to the desired signal. A higher SNR indicates less noise and distortion, so an active DAC with a SNR of 115 dB or more is considered high performance.
Total harmonic distortion (THD) specifies the distortion present when converting digital to analog signals. Lower THD such as <0.0015% will result in more accurate conversion and clean audio playback.
Other key specifications include jitter, crosstalk, output voltage, and more. Comparing the specifications of different active DAC models assists in selecting the right DAC for a given application.
Selecting an Active DAC
When selecting an active DAC, it’s important to match the specifications to your application’s needs. Key factors to consider include:
Speed – Active DACs support speeds from 10Gbps up to 100Gbps. Consider the bandwidth needs of your application.
Reach – Active DAC cables can span longer distances than passive DACs, from 3 meters up to 15 meters typically. Measure the required length for your setup.
Connectors – Active DAC connectors include SFP+, QSFP28, QSFP56, and proprietary. Select the connector type that matches your hardware.
Power – Active DACs require power from one or both ends. Ensure power is available from host devices.
Environment – If cables will be exposed to harsh environments, choose an active DAC with sturdy shielding and connectors.
By carefully matching the specifications of the active DAC to the speed, reach, connectors, power, and environmental needs, you can select the right DAC for optimal performance in your application.
Conclusion
In summary, an active DAC is a type of digital-to-analog converter that includes an amplifier built into the chip. This integrated amplifier provides the current needed to drive the output load, eliminating the need for an external op amp and gaining stage.
Active DACs provide a number of advantages over passive DACs, including better linearity, reduced noise, easier design requirements, and space savings. However, they also have some disadvantages like higher power consumption and cost. There are several types of active DAC architectures to choose from, each with their own performance trade-offs.
When selecting an active DAC, key specifications to consider are resolution, sampling rate, signal-to-noise ratio, distortion, and power consumption. The best active DAC for an application will depend on the specific performance requirements and design constraints. With their integrated amplification, active DACs are an excellent choice for many applications where high-speed, high-performance digital-to-analog conversion is needed.
In conclusion, active DACs provide integrated amplification that can simplify system design while enabling excellent linearity and low noise performance. By understanding active DAC architectures, specifications, and trade-offs, designers can leverage these devices to create high-quality analog outputs from digital sources.
