The effect of the effective number of oscilloscopes on signal measurement

There are many key indicators to measure the quality of an oscilloscope test system, and the "effective number of bits" is essential for understanding the entire measurement system. This article will discuss the effective position of the oscilloscope ENOB, and the impact of ENOB on the actual signal test, and combined with the actual application, give the EXCEL solution method.
  1 Introduction


It can be seen from the structure of the digital oscilloscope system in Figure 1. After the signal enters the oscilloscope through the probe system, it enters the ADC after being conditioned by the attenuator and preamplifier. The attenuation circuit and the amplifier work together through the relay, and can be quickly switched to use when adjusting the vertical resolution. There are many parameters to measure ADC performance indicators, such as sampling rate, DNL, INL, signal-to-noise ratio, and effective bits. This article mainly discusses the effective bit index of ADC and the effective bit of oscilloscope system.
2. ADC effective bit definition


The effective bit index of ADC is calculated indirectly by FFT spectrum analysis of sine wave signal. Chart 2 is the frequency domain distribution diagram of the sine wave after AD sampling and FFT transformation. In addition to the main sine wave component, there are many noises, harmonics and spurious signals.
The effective bit of ADC is defined as


where: 1.76 is the quantization noise of an ideal ADC; 6.02 is the coefficient ratio that converts log2 to log10.
3. The effective bit ENOB of the oscilloscope


ENOB parameter is widely accepted by the industry as a universal index for judging the performance of A/D digital-to-analog conversion, oscilloscope system or other digital system. There are many errors in all digital systems, and these errors will affect the deviation of the voltage obtained by the system from the "ideal" voltage. For an ideal oscilloscope system, within the input bandwidth range, the effective number of bits of the system should be the same as the conversion resolution of A/D. In reality, all instruments cannot be ideal, and ENOB is usually lower than its A/D conversion resolution.
It can be seen from Figure 1 that the digital oscilloscope system architecture diagram shows that the front-end acquisition circuit and ADC sampling circuit have a greater impact on ENOB. In actual work, offset error, nonlinear error, gain error, random noise, and noise caused by ADC interleaving will affect ENOB.


Graph 3 Oscilloscope ENOB curve
4. The influence of ENOB on actual signal measurement
Chart 4 Two oscilloscopes measure the same signal
The ENOB index of the oscilloscope is good, the time error and frequency spur (usually caused by splicing error) are relatively small, and the broadband noise is relatively low;
ENOB does not consider phase inconsistency and frequency response distortion, etc.;
Ideally, all oscilloscopes have flat phase and frequency response curves and the same roll-off method. But in fact, the phase and frequency response curves are generally not found in the oscilloscope index manual. Similarly, ENOB does not consider the frequency response flatness or phase inconsistency. Chart 4 shows the measurement results of an input signal on two different oscilloscopes. The ENOB of the two oscilloscopes is the same, but the waveform displayed by oscilloscope 1 is closer to the real input signal, while the waveform displayed by oscilloscope 2 is more distorted.
ENOB does not consider the offset error that the oscilloscope may introduce;
Two oscilloscopes with the same ENOB may display the same waveform shape, but the voltage offset is different. For this, measuring the oscilloscope's noise floor under different biases or evaluating the DC gain index can provide a better evaluation method.
5. Oscilloscope ENOB measurement
The measurement of oscilloscope ENOB needs to consider the following factors:
1) The ENOB of the test source is larger than the ENOB of the oscilloscope under test.
2) Whether the measured signal is full of the scope of the oscilloscope will affect the test result of ENOB.
3) The frequency of the tested signal and the sampling rate of the oscilloscope will affect the ENOB test results. This is very important, and the following conditions must be met in the actual test. (Details can be found in the "ZLG Ligong Technology-Zhiyuan Electronics" WeChat public account article search "Don't miss it! These FFT dry goods are really useful")


In the formula:
N: prime number;
N: the number of FFT analysis points, here 4096 points are taken;
Fs: oscilloscope sampling rate;
Fin: The frequency of the measured signal.
According to the above points, the general idea and related steps of oscilloscope ENOB measurement are as follows:
1) The oscilloscope sets the appropriate vertical scale, time base scale, storage depth and sampling rate. The sampling rate can be changed by adjusting the time base gear and storage depth;


Chart 5 Screenshot of oscilloscope ENOB test
2) Export data report. SCV file;


Chart 6 Oscilloscope export .SCV file
Chart 7EXCEL process data to solve ENOB


In the case of measured signals with different amplitudes and frequencies, use EXCEL to process the data to solve the ENOB of the oscilloscope. The results are shown in Figure 8. Although ZDS2020B Plus uses an 8-bit ADC, the actual measured ENOB cannot reach the ideal state due to the influence of the measured signal amplitude;


Chart 8 Oscilloscope ENOB record table
ENOB is one of the indicators to measure the oscilloscope ADC digital-to-analog conversion performance, but a higher ENOB is not a necessary condition for the oscilloscope to be able to reproduce the input signal. This mainly depends on what signal is measured and whether ENOB will affect the measurement result. In practical applications, the ENOB of the oscilloscope can be improved to obtain higher measurement accuracy through the following points.
The oscilloscope selects a bandwidth limit filter, which can suppress high-frequency components such as interleaving errors and noise, so as to obtain a higher ENOB.
When measuring repetitive or low-frequency signals, use average or high-resolution acquisition modes, which can reduce broadband noise. The combined use of these modes can effectively perform higher-precision measurements.
Select the appropriate vertical range, the general waveform occupies 80%~90% of the screen.