Application in testing power supply ripple and noise for SDS2000

SHENZHEN, China - Jan. 23, 2014 - PRLog -- It is important to adopt correct method to measure the ripple and noise. Some enginners gets hundreds mV for the ripple/noise value, which are dozens of times higher than the real value. That is caused by incorrect measuring method.

The elements related with the accuration are as below:

1.    The background noise and quantization error of oscilloscope;

2.    Using the probe with high attenuation rate to test low voltage

3.    The distance between the probe GND and the two testing points are too long

4.    The current channel set of oscilloscope (50Ω/1MΩ, DC/AC coupling)

Requirement on the instrument

Support bandwidth limit or digital filter function: an oscilloscope is generally equipped with 20MHz bandwidth limit function. Some have advanced full bandwidth limit function, such as SDS2000 series. Alternative impedance: 50Ω or 1MΩ.

The impedance and power between chips are usually in mΩ level.  If the impedance of the oscilloscope channel matches with coaxial cable’s impedance(50Ω) when a  high frequency noise is transferred from the coaxial-cable to the oscilloscope, there is no reflection. If the impedance of oscilloscope channel is 1MΩ, the power supply’s noise will be reflected according to the transmision theory. Thus the noise value will be tested higher than the 50Ω, when the input impedance is 1MΩ.

Probe Requirements

To make a shorter ground wire, you can use a short testing PIN, or make a shorter ground wire by yourself: firstly take off the original ground line from the probe, then make a new one with recommended moderately in strength CAT5 copper wire, or tin wire, aluminum wire.

Selecting 1X(non-attenuation) ratio, under which the bandwidth can be limited at 6MHz/10MHz, reducing the interference from the high frequency ripple/noise. On the contrary, selecting 10X will not act on filter function, but attenuate the ripple /noise sharply to case difficult observation.

The following are correct steps for SDS2000 power supply measurement:

1.    Open the bandwidth limit function to avoid the influence of high frequency ripple/noise on digital circuit. Meanwhile set the input impedance to 50Ω.

2.    Set to AC coupling mode to avoid no ripple/noise signal display resulting from overlarge DC signal, making a more convient measurement. In DC coupling mode, some oscilloscopes’bias voltage is not enough at less than or 2mV div.

The following are bias voltage range for SDS2000 series oscilloscope:

2mV – 100mV: ±1V

1.02mV – 1V: ±10V

1.02V – 10V: ±100V

3.    Ensure a short ground wire Probe. Otherwise, much EMI noise will be reflected into signal circle of the probe. As shown in figure 1, the oscilloscope will display a wrong value with EMI noise. To obtain a accurate value, the circle needs to be smaller, which can be realized by firstly remove the probe cover and the ground wire, and then tie around the ground cover of the probe with ground wire. Thus  will effectively limit length of groud wire to less than 1cm. (It is very important to reduce the noises from other devices since the ripple/noise signal is very weak).

Method of measuring a ripple/noise signal

As shown in figure 2, measure the ripple/noise of DC power supply.

Firstly, set the impedance to 1MΩ, as shown in figure 3:

Then set the impedance to 50Ω, as shown in figure 4:

The engineers in the technical group also find: the power supply noise tested under 1MΩ are usually higher than under 50Ω. In summary, the input impedance is recommended to set to 50Ωwhen measuring low power supply noise.