Analog-to-digital (A/D) converters have their origins in the analog paradigm, where the bulk of the physical silicon is analog. With the development of new design topologies, this paradigm has evolved to digital predominance in low-speed A/D converters. Although the A/D converter on-chip is dominated by analog to digital dominated, the routing rules of the
PCB board have not changed. When routing designers design mixed-signal circuits, critical routing knowledge is still required for effective routing. This article will take successive approximation A/D converters and sigma-delta A/D converters as examples to discuss the PCB layout strategies required for A/D converters.
Successive approximation A/D converters are available in 8-bit, 10-bit, 12-bit, 16-bit and 18-bit resolution. Initially, the process and construction of these converters was bipolar with an R-2R resistor ladder. Recently, however, these devices have been migrated to CMOS processes using capacitive charge distribution topologies. Clearly, this migration did not change the system wiring strategy for these converters. Except for the higher resolution devices, the basic routing method is the same. For these devices, special care needs to be taken to prevent digital feedback from the converter's serial or parallel output interface. In terms of circuits and on-chip resources dedicated to different domains, analog dominates successive approximation A/D converters. Figure 1 is a block diagram of a 12-bit CMOS successive approximation A/D converter.
In this block diagram, the sample/hold, comparator, most of the digital-to-analog converter (DAC), and the 12-bit successive approximation A/D converter are all analog. The rest of the circuit is digital. Therefore, most of the energy and current required by this converter goes to the internal analog circuitry. This device requires very little digital current and only a small amount of switching occurs in the D/A converter and the digital interface. These types of converters can have multiple ground and power connection pins. Pin names are often misleading because analog and digital connections can be distinguished by pin numbers. These numbers are not intended to describe the system connections to the PCB board, but rather to determine how digital and analog currents flow out of the chip. Knowing this information, and knowing that the main resource consumed on-chip is analog, it makes sense to connect the power and ground pins on the same plane (like the analog plane).
For these devices, two ground pins are usually derived from the chip: AGND and DGND. The power supply has a pin out. When using these chips for PCB board routing, AGND and DGND should be connected to the analog ground plane. The analog and digital power pins should also be connected to the analog power plane or at least the analog power rails with appropriate bypass capacitors as close to each power pin as possible. Devices like the MCP3201 have only one ground pin and one positive power supply pin due to package pin count limitations. However, isolating ground increases the likelihood that the converter will be good and repeatable. For all these converters, the power strategy should be to connect all ground, positive and negative power pins to the analog plane. Also, the 'COM' pin or 'IN' pin associated with the input signal should be connected as close to the signal ground as possible. For higher resolution successive approximation A/D converters (16- and 18-bit converters), additional care is required to isolate digital noise from "quiet" analog converters and power planes. When interfacing these devices with microcontrollers, external digital buffers should be used for noise-free operation. Although these types of successive approximation A/D converters typically have internal double buffers on the digital output side, external buffers are used to further isolate the analog circuitry in the converter from digital bus noise.
The majority of the silicon area of a high sigma-delta A/D converter is digital. In the early days of production of such converters, this shift in paradigm prompted users to use the PCB plane to isolate digital noise from analog noise. Like successive approximation A/D converters, these types of A/D converters may have multiple analog ground, digital ground, and power supply pins. Digital or analog design engineers generally prefer to separate these pins and connect them to different planes. However, this tendency is wrong, especially when you are trying to solve the severe noise problems of 16-bit to 24-bit devices. For a high-resolution sigma-delta A/D converter with a 10Hz data rate, the clock (internal or external) applied to the converter may be 10MHz or 20MHz. This high frequency clock is used to switch the modulator and run the oversampling engine. For these circuits, the AGND and DGND pins are connected together on the same ground plane as in the successive approximation A/D converter. Also, the analog and digital power pins are preferably connected together on the same plane. The requirements for analog and digital power planes are the same as for high-resolution successive approximation A/D converters. There must be a ground plane, which means at least a double-sided board. On this double-sided board, the ground plane should cover at least 75% of the entire board area. The purpose of the ground plane layer is to reduce ground impedance and inductive reactance, and to provide shielding against electromagnetic interference (EMI) and radio frequency interference (RFI). If internal connection traces are required on the ground plane side of the board, keep the traces as short as possible and perpendicular to the ground current return.
For low A/D converters, such as six-, eight-, or even possibly ten-bit A/D converters, it is fine that the analog and digital pins are not separated. But as your choice of converters and resolutions increases, so do the wiring requirements. High-resolution successive approximation A/D converters and sigma-delta A/D converters require direct connections to
PCB board low-noise analog ground and power planes.
