To further complicate the issue, mixed-signal ICs have both analog and digital ports, adding to the confusion with respect to proper grounding techniques.

Even high resolution, so-called “low frequency” industrial measurement ADCs (such as the AD77xx-series), with throughputs of 10 Hz to 7.5 kHz, operate on 5-MHz or higher-frequency clocks and offer resolutions to 24 bits. Sigma-delta (Σ-Δ) ADCs also require high-speed clocks because of their high oversampling ratios. For example, a medium-speed 12-bit successive-approximation (SAR) ADC may operate from a 10-MHz internal clock, while the sampling rate is only 500 kSPS. While certainly true of DSPs, it is also true for ADCs and DACs.Īll sampling ADCs (those employing an internal sample-and-hold circuit) suitable for signal processing applications operate with relatively high-speed clocks with fast rise and fall times (generally a few nanoseconds), so they must be treated as high-speed devices, even though throughput rates may appear low. However, the following two examples show that, in practice, most of today’s signal processing ICs are really “high-speed,” and must, therefore, be treated as such in order to maintain high performance. With respect to ADCs and DACs, the sampling (or update) frequency has generally been used as the distinguishing speed criterion. In the past, “high-precision, low-speed” circuits have generally been viewed differently than so-called “high-speed” circuits. Maintaining a wide dynamic range with low noise in a hostile digital environment is dependent upon using good high-speed circuit design techniques, including proper signal routing, decoupling, and grounding. Requirements for processing analog signals that have a wide dynamic range impose the need to use high-performance ADCs and DACs.

Today’s signal processing systems generally require mixed-signal devices, such as analog-to-digital converters (ADCs) and digital-to-analog converters (DACs), as well as fast digital signal processors (DSPs). Indeed, the single issue of quality grounding can-and must- influence the entire layout philosophy of a high-performance mixed-signal PCB design. Proper conductor routing and sizing, as well as differential signal handling and ground isolation techniques, enable control of such parasitic voltages.Īn important topic to be discussed is grounding techniques appropriate for a mixed-signal, analog/digital environment. These voltages can be due to external signal coupling, common currents, or, simply, excessive IR drops in ground conductors. Some other aspects of grounding that must be managed include the control of spurious ground and signal return voltages that can degrade performance. Since this factor is one of the more significant advantages to PCB-based analog designs, appreciable discussion here is focused on it. Fortunately, certain principles of quality grounding, especially the use of ground planes, are intrinsic to the PCB environment. Grounding is an issue for all analog designs, and it is a fact that proper implementation is no less essential in PCB-based circuits. Improper application of grounding strategies can cripple performance in high-accuracy linear systems. Unfortunately, it has also become the return path for the power-supply current in unipolar supply systems. Unfortunately, there is no “cookbook” approach that will guarantee good results, and there are a few things that, if not done well, will probably cause headaches.įor linear systems, the ground is the reference against which we base our signal. While the basic concepts are relatively simple, implementation is very involved. Grounding is undoubtedly one of the most difficult subjects in system design.

Power ground isolator