9 often overlooked ADC specifications, take a look!

In any device selection, it's impossible to perfectly balance all relevant technical specifications. This is especially true for Analog-to-Digital Converters (ADCs). While resolution, signal-to-noise ratio (SNR), and harmonic distortion are commonly focused on by engineers, there are other important parameters that should not be overlooked. Brad Brannon, an application engineer at ADI Systems, highlights nine ADC specifications that are often ignored but play a critical role in system performance. **Resolution** Resolution is the number of bits the ADC outputs, but it doesn’t necessarily reflect real-world performance. Some datasheets provide effective number of bits (ENOB), which gives a better indication of actual converter performance based on measured SNR. A more useful metric is noise spectral density (NSD), expressed in dBm/Hz or nV/√Hz. NSD helps you match the ADC with the analog front-end, ensuring optimal performance. Relying solely on resolution can lead to suboptimal choices. **Spurious and Harmonic Performance** Many users focus on spurs and harmonics, but these are independent of resolution. However, converter designers often optimize their design to align harmonics with the resolution, making it essential to consider both aspects when evaluating performance. **Power Supply Rejection (PSR)** PSR measures how well the ADC suppresses power supply noise from appearing in the output. If PSR is limited, power line noise may only be reduced by 30–50 dB. In applications like medical or industrial systems, where power supply noise is common, PSR becomes a crucial factor in determining the required filtering and decoupling. **Common-Mode Rejection (CMR)** CMR measures how well the ADC rejects common-mode signals—signals that appear equally on both inputs. Differential input structures help suppress even-order distortion, but CMR values are often not specified. Most ADCs have a CMR of 50–80 dB, which is vital in environments with high electromagnetic interference. **Clock Slew Rate** The clock slew rate is the minimum rate needed to ensure proper ADC operation. If the clock is too slow, sampling instants become uncertain, leading to increased noise. Always check the minimum slew rate requirement to maintain expected performance. **Aperture Jitter** Aperture jitter refers to internal timing uncertainty in the ADC. It limits the noise performance, especially at higher frequencies. External jitter adds to internal jitter, so using a clean, stable clock is essential for high-frequency applications. **Aperture Delay** Aperture delay is the time between the start of a sample and the actual sampling. While usually small, it matters when precise timing is required. For most applications, it’s not a concern unless exact sampling instants are critical. **Conversion Time and Conversion Delay** Conversion time applies to SAR ADCs and refers to the time between the conversion command and the output becoming valid. Conversion delay is used for pipelined ADCs and is related to the number of pipeline stages. Understanding both is key to optimizing system timing. **Wake Time** To save power, ADCs often enter a low-power mode during inactivity. However, upon waking, internal references and clocks need time to stabilize. During this period, output data may not meet specifications, so wake-up behavior must be considered in power-sensitive designs. **Output Load** ADCs, especially those with CMOS outputs, specify drive capability. Overloading the output can cause voltage drops and reduce performance. Using LVDS outputs can minimize switching current and improve signal integrity, making them ideal for high-performance systems. **Monotonicity** A non-monotonic ADC produces digital codes that don’t increase linearly with the analog input. This can cause instability in closed-loop systems. Ensuring monotonic performance is essential for control applications. **PCB Layout** While not always specified, PCB layout significantly impacts ADC performance. Poor decoupling, long traces, or inadequate grounding can introduce noise and degrade signal integrity. Proper layout and decoupling are critical for reliable ADC operation. By paying attention to these often-overlooked specifications, engineers can make more informed decisions and achieve better overall system performance.

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