PCB News - High-precision and resolution analog-to-digital converter PCB wiring technology

In the new design of the converter, most of the analog-digital converters have become digital. Even with such changes, the PCB circuit wiring design has not changed. This article will introduce the wiring method of using continuous approximation buffer type and Sigma-Delta type analog-to-digital converter.
At first, most of the analog-to-digital converters in the chip are still composed of analog circuits. Due to the improvement of the PCB design, slow analog-to-digital converters have mostly become digital. Even if it changes from analog to digital in the chip, the circuit board wiring work has not changed. This is still the case at present. When a wiring designer deals with mixed-signal circuits, he still needs basic wiring knowledge if he wants to make the wiring work well. This article will discuss the use of continuous approximation buffer (SAR) and Sigma-Delta analog-to-digital converter circuit board wiring methods.

Continuous approximation buffer type converter wiring

The resolution of the SAR analog-digital converter is 8-bit, 10-bit, 12-bit, 16-bit, and sometimes 18-bit. In the beginning, the manufacturing process and structure of these converters were two-carrier and R-2R ladder resistor networks, respectively. However, these components have recently become CMOS manufacturing processes that use capacitor charging and distribution technology, and the system wiring of these converters will not change with this transition. Except for high-resolution components, the basic way of wiring remains the same. These components need to pay more attention to avoid the digital feedback of the serial or parallel output interface of the converter.

To estimate the different block structure of the circuit system and the chip, the SAR converter is obviously an analog device.

In this block diagram, the sample/hold, comparator, most of the digital-to-analog converters and 12-bit SAR are all analog; the rest of the circuit is digital. As a result, the analog circuit in the converter consumes most of the power and current. Except for the small amount of switching current that occurs in the digital-to-analog converter and the interface, the digital circuit consumes very little current.

This type of converter has several ground and power pins. These pin names are often misunderstood as being able to distinguish digital or analog based on their pin names. However, the names of these pins do not clearly indicate that they are connected to the system and the circuit board. They are to distinguish how digital and analog current flows out of the chip. Knowing this information and understanding that the main component of the chip is analog, so that the power and ground lines are placed on the same plane, for example, the analog surface becomes meaningful.

These components usually have two ground pins pulled out from the chip: AGND and DGND. The power supply only uses one pin. When wiring the circuit board of this chip, 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 to the analog power trace, add appropriate bypass capacitors and Place as close to the ground and power pins as possible. The only reason why these components have only one ground pin and one power pin like MCP3201 is because of the limitation of the number of package pins. However, if the digital and analog pins are separated, the converter will have good accuracy and reproducibility.

The power wiring method of all converters is: connect all ground, positive and negative power pins to the analog surface. In addition, connect the "COM" or "IN" pins related to the input signal as close to the signal ground as possible.

High-resolution SAR converters (16 and 18-bit converters) need to consider separating the digital noise from the quiet analog converter and the power plane. When connecting these components to the microcontroller, an external digital buffer should be used to achieve a clean operating environment; although these types of SAR converters usually have internal double buffers on the digital output, the use of external buffers further reduces the The internal analog circuit is isolated from the digital bus noise. Appropriate power handling for this system.

Legend: When using a high-resolution SAR analog-to-digital converter, the converter power and ground should be connected to the analog surface. The digital output of the analog-digital converter should have a buffer, and an external three-state output buffer should be used. These buffers separate the analog side from the digital side and provide high drive capability.

Precise Sigma-Delta wiring method

The most accurate Sigma-Delta analog-to-digital converters in the chip are digital. When manufacturing this kind of converter in the early days, users separated the digital noise from the analog noise by the copper foil surface of the circuit board. The SAR analog-to-digital converter may have multiple analog ground pins, digital ground pins, and power pins. Once again, the general tendency of digital or analog design engineers is to connect these pins to various ground or power planes, respectively. Unfortunately, this tendency can be misleading, especially when solving the noise problem of 16- to 24-bit precision components.

A high-resolution Sigma-Delta converter with a data conversion rate of 10 Hz, and its frequency (internal or external) can be as high as 10 MHz or 20 MHz. This high frequency is used to maintain the operation of the modulator and the super-sampling engine circuit. As in the case of the SAR converter, the AGND and DGND pins of this component are connected to the same ground plane. In addition, the analog and digital power pins should be connected together, and it is better to be on the same layer of the circuit board. The analog and digital requirements on the power plane are the same as the high-resolution SAR converter.

The ground plane is necessary, which means that at least a double-layer board is required. On this double-layer board, the ground plane should cover at least 75% of the area. The purpose of this ground plane is to reduce grounding resistance and inductance, and to isolate electromagnetic interference and radio wave interference. If it is unavoidable that the signal traces pass through the ground plane of the circuit board, the signal traces should be as short as possible and perpendicular to the ground current return path.

in conclusion

There is no need to separate low-resolution analog-to-digital converters-for example, the analog and digital pins of a 6-bit, 8-bit, or even 10-bit converter. However, as the resolution/accuracy of the selected converter increases, the wiring conditions become more stringent. High-resolution SAR and Sigma-Delta analog-to-digital converters, these two components must be directly connected to the lower noise analog ground and power plane.

The above is the introduction of high-precision and resolution analog-to-digital converter PCB wiring technology. Ipcb also provides PCB manufacturers and PCB manufacturing technology.