Why need 10-bit A/D Converter?

Digital Surge/Impulse Tester converts a response high voltage decay waveform into digitized signal with the A/D (analog-to-digital) converter for waveform analysis. Generally, electronegative voltage is applied into coil windings, and the resulting decay waveform oscillates over both negative and positive areas. Therefore, the A/D converter needs to pick up bipolar voltage, and actual voltage accuracy of the bit wide is halved. 

Also, range switching (i.e. graduated gain switching) is required to trap widespread applied voltage, and significant bit will be decreased again to about a half in the worst case. This means that only 6-bit voltage accuracy is available when an 8-bit A/D converter is employed for the Digital Impulse Tester. 6-bit has 64 steps and its resolution is only 1.56%. (For example, the resolution is 15.6V at applied voltage of 1kV.)

In recent years, higher quality and higher accuracy is focused in all the products. We do not think that an Impulse Tester with the 8-bit A/D converter (1.56% resolution) can achieve a satisfactory level of performance in product tests. Actually, some “One Turn Short” is detected by less than 0.5% difference in the Differential Area evaluation. We have realized high accuracy testing by employing the 10-bit A/D converter.

Basic Idea of Digital Theory

The principle behind digitazing an analog signal or wave is that any line, be it straight or curved, can be represented by a set of points, providing the points are close together enough to accurately reproduce the original wave.

Fig. 1 Fig. 2 Fig. 3

Please refer to the figures above. Suppose that the waveform (Fig .1) is a portion of some analog transient wave , and that in (Fig. 2) a grid is superimposed upon (Fig. 1). Each intersection point on the grid represents amplitude at a point in time. All of the intersection points between the grid and the wave (Fig. 1) will effectively represent a rough contour of (Fig. 1), seen in (Fig. 3). One can readily see that if the grid is made smaller and more samples (intersection points) utilized, one approaches a representation of the original wave. These are what a faster sampling rate (100MHz) and better voltage resolution of the A/D converter (10-bit) do.

The theory behind digitazation is that the wave is no longer an analog signal; it is a series of points. For highly accurate comparison testing between the master waveform and the test waveforms, it is absolutely necessary to have more number of sampling points. The SDT-06, that employs 10-bit A/D converter, has voltage resolution of 1024 dots. This has four times as many points as an 8-bit impulse tester.

Evaluation by the 2nd Zero Crossing Point

The 2nd Zero Crossing Point is very effective for evaluation of coil windings because it is less subject to unnecessary noises. Comparing the value of a master coil at the 2nd zero crossing point and that of test coils, the misalignment at the x-coodinate will be shown in permillage (1/1000) if any. When the value of a test coil is smaller than that of master, permillage of misalignment will be in negative value while it will be in positive value when the value of a test coil is bigger than that of master.

The sampling speed (resolution of time axis) of the A/D converter is 100MHz (10-nano sec.) The SDT-06 detects the zero crossing point with 1-nano degree of accuracy by arithmetic processing of angle to find the point at the second intersection of a waveform and the zero line.

Fig. 4 (2nd Zero Cross Point) Fig. 5 (extended figure)
The black dots are located on the 100MHz sampleing lines (10-nano sec.)
The red dot is the point obtained by arithmetic processing (1-nano sec.)