In the process of analog-to-digital conversion of signals, it is normally accompanied by a problem with spurious signals, which will cause interference to the signal. To solve this problem and improve the reliability of the signal, the Data Acquisition Systems (DAS) will be the ideal solution. This article will introduce you to the causes of spurious signals, and the functions and features of ADI's AD7616 DAS.
Spurious signals affect system reliability
First, it is necessary to explain the term Spurious-Free Dynamic Range (SFDR). SFDR stands for the smallest power signal that can be distinguished from large interference signals. For the current high-resolution, precision analog-to-digital converter (ADC), SFDR generally mainly consists of the dynamic range between the fundamental frequency and the second or third harmonic of the target fundamental frequency. However, due to other aspects of the system, spurious signals may be generated and limit the performance of the system.
Although the current high-resolution Successive Approximation Register (SAR) ADCs and Σ-Δ ADCs can provide high resolution and low noise, it may be difficult for system designers to achieve the specified Signal-to-Noise Ratio (SNR) performance on the datasheet. It may be even more difficult to achieve the best SFDR, that is, to achieve a spur-free clean noise floor in the system signal chain. The spurious signals may originate from the improper circuit around the ADC, or it may be caused by external interference in the harsh operating environment.
Among the spurious signals in high-resolution and precision ADC applications, the more common ones are caused by the radiation of the DC-DC power supply on the controller board, or generated from the AC-DC adapter noise through the external reference source. In addition, there are also the spurious signals caused by the analog input cable or the coupling on the analog input cable, or caused by room lighting equipment.
These spurious signals can be categorized as input frequency dependent ones and fixed frequency ones. Input frequency dependent ones are related to harmonics or nonlinear characteristics and are mainly fixed-frequency spurious signals caused by power supplies, external reference sources, digital interfacing, and external interference. According to the application, if these types of spurious signals can be reduced or completely avoided, it will help to achieve the best signal chain performance. If the spurious signals can be improved, it will help improve the Electromagnetic Compatibility (EMC) capability and reliability of the end-system.
Spurious signals caused by the DC-DC power supply around the ADC
Since DC-DC switching regulators produce high ripple noise, it is usually recommended to use a Low Dropout regulator (LDO) as a solution to generate low noise power supply rails for precision ADCs in precision measurement systems. Fixed frequency or pulse width modulation switching regulators will produce switching ripple, which is generally with a fixed frequency from tens of kHz to a couple of MHz. The fixed frequency noise may be fed into the ADC conversion code through the ADC's Power Supply Rejection Ratio (PSRR) mechanism.
Some designers may use DC-DC switching regulators in precision ADC applications due to limited board space or budget issues. In order to achieve the ideal signal chain performance, they must limit the ripple noise or use a high PSRR ADC to ensure that the ripple noise is below the ADC noise floor. Otherwise, spurious signals may occur at the switching frequency of the ADC output spectrum, which may degrade the dynamic range of the signal chain.
DAS achieves higher SNR performance
To solve the spurious signals caused by the DC-DC power supply around the ADC, DAS would be an ideal solution. AD7616 launched by ADI is a 16-bit DAS that supports dual simultaneous sampling of 16 channels in power line monitoring.
AD7616
AD7616 is powered by a single 5 V power supply and can handle true bipolar input signals of ±10 V, ±5 V, and ±2.5 V. At the same time, each couple of channels can sample at a throughput rate of up to 1 MSPS and 90.5 dB SNR. Using the on-chip oversampling mode can achieve higher SNR performance (if the oversampling ratio (OSR) is 2, the SNR is 92 dB), the device has a high PSRR, and will be able to effectively rejecting/attenuating the switching ripple.
The input clamp protection circuit of AD7616 can tolerate voltages up to ±21 V. Regardless of the sampling frequency, the analog input impedance of AD7616 is always 1 MÙ. It adopts a single power supply mode, with on-chip filtering and high input impedance, so there is no need to drive an operational amplifier and an external bipolar power supply.
Fully integrated data acquisition solution
AD7616 devices have built-in analog input clamp protection, a dual 16-bit charge redistribution SAR ADC, a digital filter that supports burst mode, 2.5 V on-chip precision reference voltage source and reference voltage buffer, and a flexible high-speed serial and parallel interface.
AD7616 has an independently selectable channel input range. The VDRIVE supply voltage is 2.3 V to 3.6 V. It is a fully integrated data acquisition solution with a first-order anti-aliasing analog filter. The AD7616 is also compatible with Serial Peripheral Interface (SPI)/QSPI™/DSP/MICROWIRE, with optional Cyclic Redundancy Check (CRC) for error checking, support for on-chip self detect function, and an 80-pin LQFP package. AD7616 can be mainly used in power line monitoring, protective relays, multi-phase motor control, instrumentation and control systems, data acquisition systems, etc.
Full-featured evaluation board speeds up product development
EVAL-AD7616SDZ
To speed up the product development for customers, ADI has launched an evaluation board with AD7616. EVAL-AD7616SDZ/EVAL-AD7616-PSDZ is a single full-featured evaluation board designed to easily evaluate all the features of AD7616 and AD7616-P ADC. The evaluation board can be controlled by the EVAL-SDP-CB1Z through the 120-way high-speed system demonstration platform (SDP-B) connector (J10). The EVAL-SDP-CB1Z SDP-B board can be used to control the evaluation board through the USB port of the PC and the evaluation board software.
The full-featured evaluation board of AD7616 and AD7616-P support built-in power supply and independent working capability, and are compatible with the SDP-B controller, and also provide PC software for control and data analysis.
Conclusion
The spurious signals will cause poor system stability, but nowadays it can be solved by DAS. ADI's AD7616 can solve the problem of spurious signals caused by the DC-DC power supply around the ADC, achieving higher SNR performance. It has a high PSRR and helps improve the EMC capability and reliability of the end-system. It is the best choice for electronic product developers.