How important is the vertical resolution of Puyuan oscilloscope?

The vertical resolution of an oscilloscope refers to the vertical resolution of the analog-to-digital converter, which is used to measure how accurately the oscilloscope converts the input voltage into a digital value. It is usually expressed in terms of the number of A/D bits.

After sorting out the various brands and models of oscilloscopes currently on the market, the methods for improving resolution after ADC sampling can be summarized into three signal processing algorithms: averaging, linear phase FIR filtering, and group averaging. Here, the Antai test uses the Puyuan DS70000 digital oscilloscope as an example.

Puyuan DS70000 digital oscilloscope

1. Average

If the signal is periodic and repeatable, the oscilloscope collects n segments of repeated waveforms every time, aligns them according to the trigger position, adds them and divides them by n to obtain the average waveform of one segment.

The result of trigger position jitter is that each waveform is misaligned and added, and the waveform is distorted after averaging. When the noise of the signal itself is relatively large, the distortion phenomenon is more obvious.

2. Linear Phase FIR Filter (Linear Phase FIR Filter)

Noise is often spread widely across the frequency spectrum. Digital filtering can be used to suppress noise outside the signal bandwidth, thereby improving the signal-to-noise ratio and vertical resolution. One common filter type is linear phase FIR filtering. In the ERES function, you can directly choose how much bit resolution to enhance, which is actually selecting the bandwidth of the filter. The smaller the bandwidth, the more the signal-to-noise ratio improves and the more bits of resolution are enhanced.

3. Group Average

This algorithm is called high-resolution sampling mode in many oscilloscopes). Some oscilloscopes also call this linear averaging, or oversampling and linear noise reduction techniques. No matter what the name is, the essence is to perform signal processing on the ADC sampled data. The advantage of this algorithm is that it can also be used for non-periodic signals. The oscilloscope collects waveforms at the maximum sampling rate, divides every n sampling points of a waveform into a group, and averages the n sampling points in a group to obtain one data point and stores it in the acquisition memory. The waveform finally displayed on the oscilloscope is the data after the sample points are grouped and averaged. The number of sample points is extracted n times.

Group averaging improves resolution at the cost of compromised performance and waveform distortion:

1) High sampling rate and high vertical resolution cannot have both. For example, to achieve 12-bit resolution on an 8-bit, 40 GSa/s oscilloscope, the processed sampling rate can only reach 2 or 5 GSa/s. To achieve a processed sampling rate of 5GSa/s, the resolution can only be increased to 11bit.

2) Oscilloscope bandwidth and vertical resolution are also incompatible. In the previous example, to achieve 12bit resolution, the bandwidth drops from 4GHz to 500MHz.

3) For oscilloscopes working in HiRes mode, users cannot directly choose how much bit resolution to enhance, as it changes dynamically with the time scale of the oscilloscope.

4) The group average algorithm is effective in suppressing Gaussian white noise, but has no suppression effect on the noise caused by the INL (integral nonlinearity) of the oscilloscope ADC.

5) The steeper edges of the signal are prone to undersampling in HiRES mode. For example, the signal edge originally has 16 sampling points. In order to improve the 4-bit resolution, every 16 points need to be averaged. As a result, only 1 sample point remains on the edge. Many oscilloscopes use the sin(x)/x method for interpolation, which will cause Gibbs phenomenon in the case of undersampling, causing overshoot and ringing on the originally normal edges.

6) Group averaging will also significantly reduce the oscilloscope waveform update rate.

7) Group averaging can conditionally improve the resolution, but it cannot improve the accuracy of the oscilloscope. For example, the DC gain accuracy of an oscilloscope with a hardware 12bit ADC can reach ±0.5%. Using oversampling and linear noise reduction technology to upgrade the oscilloscope to a 12-bit resolution, the DC gain accuracy is still the level of an 8-bit ADC oscilloscope: ±2%