PCB Tech - PCB routing strategy required for A/D converter

Analog (A/D) converters are derived from analog paradigms in which much of the physical silicon is analog. With the development of new design topologies, the paradigm evolved to include digital as the dominant component in low speed A/D converters. Despite the shift from analog to digital dominance in A/D converters, the PCB board wiring criteria remain unchanged. When wiring designers design mixed-signal circuits, critical wiring knowledge is still needed for effective wiring. In this paper, the A/D converters with successive approximation type and ∑-△ type A/D converters are taken as examples to discuss the PCB routing strategy required by A/D converters.

Successive approximation A/D converters have 8-bit, 10-bit, 12-bit, 16-bit, and 18-bit resolutions. Initially, the process and construction of these converters were bipolar with r-2R trapezoidal resistor networks. Recently, however, these devices have been transplanted to CMOS processes using the capacitive charge distribution topology. Obviously, this migration does not change the system routing strategy for these converters. Except for higher-resolution devices, the basic wiring method is consistent. For these devices, special care needs to be taken to prevent digital feedback from the serial or parallel output interfaces of the converter.

The converter uses a charge distribution formed by an array of capacitors.

In this block diagram, the sampler/hold, comparator, most of the digital-to-analog converter (DAC), and the 12-bit successive approximation type A/D converter are analog. The rest of the circuit is digital. Therefore, most of the energy and current required for this converter is used in the internal analog circuits. The device requires very little digital current, with only A few switches occurring with the D/A converter and digital interface.

These types of converters can have multiple ground and power connection pins. Pin names are often misleading because pin labels can be used to distinguish analog and digital connections. These labels are not intended to describe the system connection to the PCB, but to determine how digital and analog current flows out of the chip. Knowing this information, and knowing that the primary resources consumed on a chip are analog, it makes sense to connect power and ground pins on the same plane, such as the analog plane.

For these devices, two ground pins are usually invoked from the chip: AGND and DGND. The power supply has a lead pin. When implementing PCB wiring with these chips, AGND and DGND should be connected to the analog ground plane. Analog and digital power pins should also be connected to the analog power plane or at least to the analog power rail, and the appropriate bypass capacitance should be connected as close to each power pin as possible. Devices such as MCP3201 have only one ground pin and one positive power pin due to the limitation of the number of package pins. However, isolation increases the likelihood that the converter will be good and repeatable. For all of these converters, the power strategy should be to connect all ground, positive, and negative power pins to the analog plane. Also, the 'COM' or 'IN' pins associated with the input signal should be connected as close to the signal as possible.

For higher resolution successive approximation type A/D converters (16 - and 18-bit converters), additional care is required to isolate digital noise from "quiet" analog converters and the power plane.When these devices interface with the single chip microcomputer, external digital buffers should be used to obtain noiseless operation. Although these types of successive approximation A/D converters usually have an internal dual buffer on the digital output side, an external buffer is used to further isolate the analog circuit in the converter from digital bus noise.

For high resolution successive approximation type A/D converters, the power supply and ground of the converter should be connected to the analog plane. The digital output of the A/D converter should then be buffered using an external tri-state output buffer. In addition to their high drive capability, these buffers have the function of isolating the analog and digital sides. For high resolution successive approximation type A/D converters, the power supply and ground of the converter should be connected to the analog plane. The digital output of the A/D converter should then be buffered using an external tri-state output buffer. In addition to their high drive capability, these buffers have the function of isolating the analog and digital sides.

Cabling strategy for high ∑-△ TYPE A/D converter

The silicon area of high ∑-△ type A/D converters is mainly digital. In the early days of the converters, the paradigm shift encouraged users to use PCB planes to separate digital noise from analog noise. As with successive approximation A/D converters, these types of A/D converters may have multiple analog, digital, and power pins. Digital or analog design engineers generally prefer to separate these pins and connect them to different planes. However, this tendency is a mistake, especially when you are trying to solve the serious noise problem of 16-bit to 24-bit devices.

For A high-resolution ∑-△ TYPE A/D converter with A 10Hz data rate, the clock (internal or external clock) added to the converter may be 10MHz or 20MHz. This high frequency clock is used to switch on and off the modulator and run the oversampling engine. For these circuits, AGND and DGND pins are connected together on the same ground plane as for successive approximation A/D converters. Also, analog and digital power pins are 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 floor plan, which means at least two panels. On this double panel, the floor plan should cover at least 75% of the entire floor area. The purpose of the ground plane layer is to reduce grounding impedance and inductive reactance, and to provide shielding against electromagnetic interference (EMI) and radio frequency interference (RFI). If internal connection wiring is required on the ground plane side of the board, the wiring should be as short as possible and perpendicular to the ground current loop.

Conclusion

For low A/D converters, such as six-bit, eight-bit, or perhaps even ten-bit A/D converters, it is ok to keep the analog and digital pins unseparated. But as your choice of converters and resolutions increases, the wiring requirements become more stringent. High resolution successive approximation A/D converters and ∑-△ A/D converters need to be directly connected to the low noise analog ground and the power plane.