Research on the Application of ADC in Data Acquisition System

Power line monitoring systems or modern three-phase motor control systems require accurate multi-channel simultaneous measurements within a wide dynamic range of about 70dB~90dB (depending on the specific application), and the sampling rate usually requires 16kbps or even higher.

Main noise and interference affecting DAS

Industrial Two types of noise/interference are defined in data sampling systems (DAS). The first type of noise originates from internal electronic components. Noise sources include ADC conversion processing noise and harmonic distortion, buffer amplifier noise and distortion, and reference noise. The second type of interference originates from outside the system and includes external electromagnetic noise, power supply noise/ripple, I/O port crosstalk, and digital system noise and interference. Figure 1 lists the different noise sources.

Figure 1 Effects of different noise and interference sources on system resolution and accuracy

The power line DAS signal processing chain includes CT, PT measurement transformer, anti-aliasing low-pass filter (LPF), buffer amplifier, and simultaneous sampling ADC and CPU. The simultaneous sampling ADC is the core circuit of the system, which is used to measure the voltage and current signals adjusted in the standard industrial input dynamic range (such as +5V, ±5V or ±10V).

Table 1 lists the 1LSB values ​​and quantization noise of these devices. These values ​​provide designers with the total noise and interference that can be tolerated by the DAS based on the number of ADC bits.

Table 1 Quantization value and quantization noise corresponding to ADC resolution

The total noise and ripple of the ADC input should be less than 1/2 LSB. At the same time, the quantization noise determines the basic noise floor of the system.

Note: In some designs, an overall noise of just 1mVRMS can cause the entire design to fail because the un-“calibrated” overall noise will cause the ADC accuracy to degrade.

Choosing the Correct Input Amplifier

Based on the above analysis, it is necessary to correctly select the input amplifier in the DAS signal processing chain. There are many devices to choose from, and the MAX130x and MAX132x series are one of the options. The input circuit of the MAX130x and MAX132x series ADCs has a very low impedance, as shown in Figure 2. Accordingly, in most applications, these devices require an input buffer to achieve 12-bit and 14-bit accuracy.

Figure 2 Typical input circuit for MAX130x and MAX132x ADCs

To achieve 12-bit to 16-bit accuracy, key factors to consider when selecting an amplifier include: bandwidth, slew rate, VP-P output, low noise, low distortion, and low offset. The buffer amplifier noise should be kept as low as possible, that is, much lower than the SNR (signal-to-noise ratio) of the ADC. The overall offset error of the amplifier, including drift, should be less than the required accuracy error over the entire temperature range. Each buffer amplifier should be carefully selected based on the specific application. It is not recommended to use general-purpose op amps for high-precision ADCs.

Input filter devices use a differential input structure and usually do not require an input buffer amplifier. For example, the MAX11046 series has extremely high input impedance and can be directly connected to low-impedance sensors. For example, the impedance of CT and PT measurement transformers is relatively low (10Ω~50Ω), so they can be directly connected to the MAX11046 input stage through a simple low-pass filter, as shown in Figure 3.

Figure 3 MAX11046 series device input circuit

In order to maintain the accuracy of the DAS, it is critical to choose the correct RSOURCE and CEXTERNAL. The RSOURCE resistor must be a metal film resistor with an accuracy of 1% or higher and a low temperature coefficient. It is recommended to choose components provided by some well-known manufacturers such as ROHM or Vishay.

For best results, CEXTERNAL capacitors should be selected from ceramic capacitors. These capacitors maintain their nominal value over a wide temperature and voltage range and are available in cost-effective SMT form factors from companies such as Kemet or Samsung.

ADC Reference Selection

The choice of ADC reference is very important for the performance of the entire DAS and is closely related to the resolution and accuracy requirements of the ADC. It is critical to maintain reasonable drift and initial accuracy over the entire temperature range. Taking the MAX11046 as an example, 1 LSB = 62.5µV. The temperature drift of the MAX11046 internal reference is ±10ppm/°C. Over the entire temperature range of 50°C, the reference drift can reach ±500ppm or ±2.048mV ​​(±33LSB). In applications with strict temperature drift requirements, it is best to use an external low-temperature drift reference such as the MAX6341 (1ppm/°C). When using an external reference, the reference input current of the MAX11046 is only ±10µA. The output current of the series reference can reach 10mA, so a single reference device can provide a reference for multiple high-performance ADCs, eliminating the reference differences between different devices.

PCB Design and Layout Considerations

Noise suppression using low-pass filter

There is considerable noise/interference on the power line at any time. The noise mainly comes from the cable/transmission system, and it is the capacitance/inductance that couples external noise sources to the power line. Noise and interference are also related to the dynamic characteristics of the power line. As shown in Figure 1, each CT and PT isolation/measurement transformer operates at a frequency of 50Hz/60Hz. In fact, these transformers have a very wide (100kHz) bandwidth and only provide attenuation/filtering for signals with frequencies of 100kHz or higher.

Another significant noise/interference source comes from the electronic components of the DAS system on the PCB. These components include the CPU and the power subsystem, especially when using a switching power supply. This means that each input channel of the ADC requires an anti-aliasing filter and noise suppression low-pass filter. The filtering components should be placed as close to the ADC input as possible.

Maintain signal integrity with grounding and shielding

The PCB traces carrying signals from the connector to the ADC input are subject to noise, interference, and crosstalk between channels. Taking special grounding and signal shielding measures for these analog signal lines will directly affect the integrity of the input signal. Figure 4 shows an example of PCB layout to protect analog signals.

Figure 4. Path from connector to MAX11046 analog input.

Note that the MAX11046 has extremely high channel-to-channel isolation. To achieve high isolation, a coplanar microstrip line structure is used.

General Rules for PCB Routing

There are several important PCB layout rules to follow to achieve optimal performance in multi-channel, simultaneous sampling DAS applications:

• Use a PCB with a ground plane.

• Ensure that analog and digital lines are separated from each other.

• Do not run digital signal lines and analog signal lines in parallel.

• Avoid running digital signal lines under the ADC package.

• Use separate ground planes with digital signals on one side and analog signals on the other.

• Keep the ground return impedance of the power supply low and the leads as short as possible to achieve noise-free operation.

Connect 0.1µF ceramic capacitors between the AVDD and DVDD pins and ground. The capacitors should be placed close to the device to reduce parasitic inductance.

• Add at least a 10µF decoupling capacitor to each AVDD and DVDD pin on the PCB.

• Use two power planes to connect all AVDD and DVDD connections separately.

• The AVDD power plane and DVDD power plane on the analog interface side of the MAX11046 should be kept away from the digital interface pins of the device.

(a) Histogram obtained by testing the user's DAS PCB. The PCB layout is unreasonable.

(b) Output histogram obtained after testing the user's PCB after improvement

(c) Maxim DAS output histogram

Figure 5 Test results

Test Results

Figure 5 provides some test results of an industrial prototype of a multi-chip, multi-channel, simultaneous sampling DAS based on the MAX11046. A precision 2.048V DC reference is applied to the MAX11046 input of the DAS. The ADC output conversion results have a range of ±32768. Figure 5(a) shows the test results of a user-provided PCB prototype that violates the layout principles of power supply layout and input signal integrity. The test data and histogram show that noise/interference reduces the effective bits of the DAS to about 11.5. The unpredictability of the measurement is also reflected in the unstable histogram test template.

Figure 5(b) shows the test result after the user's PCB layout was improved, adopting the power/ground layout rules and processing solutions introduced in this article to maintain input signal integrity. From the test results and histograms, it can be seen that the performance has improved and the effective bits of the DAS system have reached 13.5. During this test period, the histogram is repeatable, reflecting the stability of the measurement.

Figure 5(c) shows the test results of Maxim DAS under the same test conditions and in the same laboratory. The test results and histogram show that the effective bits of DAS reach about 14. In this test, the histogram has very good repeatability, reflecting the stability of the test and the advantages of Maxim layout and design configuration.

Conclusion

In order to achieve the DAS design specifications and the specifications published in the ADC data sheet, strict design principles need to be followed. These design considerations include: LPF filter, low noise buffer and reference selection, component layout, PCB layout, and power supply noise/ripple filtering. Paying attention to these design principles can achieve excellent results for the new generation of high-performance ADCs.