Research on dynamic performance parameters of high-speed ADC and test methods based on DSP technology

High-speed ADC is an indispensable part of the signal processor, and its performance is also crucial to the overall performance of the signal processing system. Usually the technical parameters of ADC are provided by the manufacturer, which can be used as an important basis for the design, but the performance of the ADC module formed on the circuit board is also closely related to the peripheral circuit or input signal of the ADC, such as reference voltage source, sampling clock , Input operational amplifier and power supply, ground wire and signal wire interference, etc. Therefore, it is necessary to evaluate the dynamic performance of the high-speed ADC module online and analyze its impact on the performance of the signal processor system. This article introduces a method for online evaluation of the dynamic performance of the high-speed ADC module on the actual equipment of the signal processor. This method utilizes the data acquisition capability of the signal processor itself, by adding standard test signals to the analog input terminal, after AD conversion, the DSP of the signal processor reads the conversion results, and the data is read into the PC through the DSP simulation system, and then Use MATLAB software to perform spectrum analysis on the data, and finally calculate several dynamic parameters such as SNR and SENAD. This article also provides specific test results and analysis, and gives some opinions on the design of high-speed ADC modules.

One, dynamic performance parameters of high-speed ADC

The main parameters for evaluating ADC dynamic performance are defined as follows:

1, signal-to-noise ratio (dB)

Asignal is the root mean square value of the full-amplitude sinusoidal analog input signal, and Anoise is the root mean square of the sum of all noise sources.

2, signal-to-noise and distortion ratio (SINAD)

AHarmonic is the sum of the root mean square of the frequency components of each harmonic (except DC).

3, the number of effective bits (ENOB)

where N is the number of quantization bits of the conversion circuit, A measurement error is the average measurement noise, and A measurement error is the quantization error average.

4. Total harmonic distortion Among them, AF_IN is the root mean square value of the fundamental wave of the input signal, and AHD_2 down to AHD_N is the root mean square value of the second to Nth harmonic components in the frequency domain of the sampled signal.

5. Spurious-free dynamic range Among them, AF_IN is the root mean square value of the fundamental component of the input signal, and AHD_MAX is the root mean square value of the distortion harmonic component or spurious signal in the sampled waveform spectrum.

Through the definition of these parameters, it can be seen that most of the dynamic parameters of the high-speed ADC circuit can be expressed in the frequency domain; therefore, the corresponding dynamic performance parameters can be obtained by testing the high-speed ADC circuit in the frequency domain.

2. Traditional test methods

In the traditional test, a DAC that is more than 2 bits higher than the tested analog-to-digital conversion circuit is used to generate a single-frequency sine wave as the test signal of the tested analog-to-digital conversion circuit, and a DAC is also connected to the back end of the tested circuit to restore the waveform . Figure 1.

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This kind of test structure is simple and intuitive. However, in engineering practice, a DAC circuit must be added in order to evaluate the analog-to-digital circuit, which will cause difficulty in connecting with the actual module and introduce DAC errors. Therefore, online evaluation is not easy to adopt.

3. Test method based on DSP technology

A simple and accurate test structure can be constructed using DSP technology. The digital signal processor is used to collect and save the output data at the back end of the ADC circuit, and then the data is transferred to the PC using the JTAG interface of the DSP simulation device, and the Matlab software is used for Relevant frequency domain processing to obtain the actual conversion characteristic parameters of the high-speed analog-to-digital conversion circuit. The structure is shown in Figure 2.

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Here, DSP-based frequency domain analysis tools are used instead of analog test instruments, which can provide higher and reproducible test results. After collecting the data in the PC, the discrete signal waveform in the time domain is converted to the frequency domain through the DFT (FFT) algorithm in the Matlab platform. In the frequency domain, the relevant dynamic performance parameters are obtained according to the definition. In the ideal case of no distortion of the input signal, when the input analog quantity is a sine wave, the output spectrum should be an impulse function graph with a frequency equal to the input frequency. In fact, the quantization error of the ADC, various noises inside the converter, and even the noise of the test system, will be reflected in the noise background on the spectrogram. The basic function based on FFT signal analysis is FFT itself and power spectrum. In the FFT algorithm, it is assumed that the discrete time series can be extended in the entire time domain. All signals containing the discrete time series are periodic functions, and the period is related to the length of the time series. However, if the length of the time series is not an integer multiple of the signal period, that is, spectrum leakage will occur. Here is the input signal frequency; Fsample is the sampling frequency; Nwndows window function length; Nrecord sampling signal data length. In the test, the Hanning window function is generally used to reduce spectrum leakage.

After obtaining the frequency spectrum of the sampled output signal, to obtain the relevant test parameter values, the fundamental frequency of the signal and the position of each harmonic must be determined. The following shows the relationship between the frequency components and their mirror images as shown in Figure 3. Let Fo be a frequency in the Nyquist interval (DC～Fs/2). Let Fk be the mirror image of Fo in the frequency band, which is also called the kth mirror image of Fo.

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Figure 3 Schematic diagram of the relationship between frequency components and their image components

The relationship between all mirror images can be expressed as follows:

It can be seen from the above relationship that for odd and even image components, their corresponding frequency components can be found in the Nyquist interval, that is to say, any harmonic component beyond the Nyquist interval can be Find its image component in the Nyquist interval. If the harmonic is in the Nyquist interval, the harmonic position can be directly calculated; if the harmonic is outside the Nyquist interval, the above image method must be estimated to obtain the harmonic data.

Four, ADC module test based on signal processor

We conducted an online test on the high-speed ADC module of the circuit board of the signal processor. The part enclosed by the dashed frame in Figure 4 is the analog-to-digital conversion circuit of the signal processor. DSP21065L stores the collected data in the internal storage area. After the collection is completed, the data is transferred to the PC through the JTAG interface of the DSP simulation system. The relevant digital signal processing is performed in Matlab, and the conversion performance of the conversion circuit is determined according to the processing results. Corresponding evaluation analysis. According to the obtained performance index of the analog-to-digital conversion circuit, the influence of the conversion circuit on the signal processor can be analyzed.

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The AD conversion chip AD9225 in the picture is 12 bits, the conversion rate is 25MSPS, and the chip has a sample and hold circuit and a reference voltage source. The basic performance is as follows:

LSNR71dB (finput=2.5MHz)

LSINAD 70.7dB (finput=2.5MHz)

LTHD -82Db (finput=2.5MHz)

INL: DNL:

4.1 Part of the test data

The test signal is a full-amplitude sine wave, 8192 points of data are sampled each time, and 8192 points FFT are performed to obtain the sampled data spectrum. Under different sampling frequency and signal frequency, do 5 experiments respectively, and take the average value.

4.2 Test data analysis

12-bit conversion circuit, its theoretical signal-to-noise ratio should reach 74dB, but only 60～70dB in the actual test. Explain that there are other noise sources besides quantization noise. It is found in the experiment that if the analog power supply and the digital power supply in the conversion circuit are powered by the same power supply, the signal-to-noise ratio will be reduced by about 5-6dB compared to when the two are powered separately. Improving the grounding condition can also effectively improve the conversion dynamic performance. For example, when the grounding condition is not good, the signal-to-noise ratio at 5MHz sampling at 1.54MHz is only 59.7dB, but after improving the grounding condition, it is increased to 61.64dB, an increase of 1.94 dB. The experimental data shows that the harmonic distortion is more serious. Our analysis is mainly due to the low purity of the signal source. The test result is close to the parameters given in the AD9225 technical manual, which shows the reliability of the method. From the point of view of the SNR value, since the test is for the entire high-speed analog-to-digital conversion circuit, the result obtained reflects the dynamic conversion characteristics of the entire circuit. Due to the influence of each auxiliary circuit, the performance of the conversion circuit is lower than the parameters given in the device manual. Based on the analysis of the test results, the following reference opinions are put forward for the design of high-speed analog-to-digital conversion circuits:

L The data latch is used for isolation drive between the back end of the conversion device and the data bus to reduce the noise on the back end data bus from entering the analog circuit and reduce the conversion performance.

L The analog power supply and the digital power supply should be isolated to reduce the noise on the analog side.

L Choose a suitable op amp to reduce harmonic distortion.

L The analog input signal line should be as wide as possible to reduce signal distortion.

L When selecting devices. The conversion of the selected device should be higher than the required 2 effective digits.

L The conversion rate of the selected ADC chip should be much higher than the sampling speed.