Does the oscilloscope probe affect the measurement of capacitive load?

As the signal frequency or conversion rate increases , The capacitive component of impedance becomes the main factor . result , Capacitive load becomes the main problem . In particular, the capacitive load will affect the rise time and fall time on the fast conversion waveform and the amplitude of high-frequency components in the waveform , So what are the effects of oscilloscope probe on measuring capacitance load ？

Effect on rise time

To illustrate the capacitive load , Let's consider a pulse generator with a very fast rise time , Pictured 4.3 Shown , The rise time of the pulse on the output of the ideal generator is zero (t r=0). however , The resistance and capacitance related to the impedance load of the signal source change the zero rise time .

RC The integral network has been produced 2.2RC Of 10-90% Rise time . This is derived from the general time constant curve of the capacitor . Value 2.2 yes C adopt R Charge , Change the pulse amplitude from 10% Up to 90% The required RC Time .

In the figure 4.3 Under the circumstances ,50 Ohmic and 20 pF Of the signal source impedance 2.2 ns Pulse rise time . This 2.2RC Value is the fastest rise time that a pulse can have .

4.3. The rise time of the pulse generator depends on its RC load

When detecting the output of the pulse generator , The input capacitance and resistance of the oscilloscope probe are added to the value of the pulse generator , Pictured 4.4 Shown , It's increased 10 Megohm and 11 pF Typical oscilloscope probe . Because the oscilloscope probe 10 The megohm resistance is much larger than that of the generator 50 Ohmic resistance , Therefore, the resistance of the oscilloscope probe can be ignored . however , The capacitance of the probe is roughly equal to the load capacitance , Add directly to get 31 pF Load capacitance . This improves 2.2RC Value , As a result, the measured rise time is increased to 3.4 ns, The rise time before detection is 2.2 ns.

Specify the ratio of capacitance to known or estimated source capacitance by using an oscilloscope probe , The influence of probe tip capacitance on rise time can be estimated . Use the figure 4.4 The value in , The percentage change in rise time can be estimated as follows ：

C Probe tip /C1 x 100%=11 pF/20 pF x 100%=55%

4.4. The capacitance of the probe is increased RC value , At the same time, the measured rise time is improved

It is clear from above that , Probe selection 、 In particular, the choice of oscilloscope probe capacitance will affect the rise time measurement . For passive probes , Generally speaking , The greater the attenuation ratio , The lower the head capacitance . From the table 4.1 This can be seen in , Some examples of probe capacitance of various passive probes are introduced .

surface 4.1. Probe tip capacitance

When a smaller head capacitance is required , Active power shall be used FET Input probe . According to the specific oscilloscope active probe model , Less than or equal to... Can be provided 1 pF Head capacitance of .

Effect on amplitude and phase

In addition to affecting the rise time , The capacitive load also affects the amplitude and phase of the high-frequency components in the waveform . Remember this , All waveforms are composed of sinusoidal components .50 MHz The square wave has more than 100 MHz The effective harmonic component of . Therefore, it is not only necessary to consider the load effect on the basic frequency of the waveform , Moreover, the load effect on the frequency that exceeds several times of the basic frequency shall be considered .

The load depends on the total impedance at the probe tip . This is called Z p,Z p It is composed of resistance R p And reactance components X p form . Reactance mainly consists of capacitance , However, the inductance unit can be designed in the probe , To partially offset the capacitive load .

Generally speaking ,Z p It will decrease as the frequency increases . Most probe instrument manuals compile probes R p data , The document includes the display Z p Relation curve with frequency . chart 4.5 It is an example of an ordinary active probe . Be careful ,1 The megohm impedance amplitude is fixed at approximately 100 kHz. This is achieved by carefully designing the relevant resistance unit of the oscilloscope probe 、 Capacitor unit and inductance unit realize .

4.5. The typical input impedance of an active probe varies with frequency

chart 4.6 Another example of a probe curve is illustrated . under these circumstances , It shows typical 10 Megohm passive probe Rp and X p Relationship with frequency . Dotted line (Xp) The capacitance reactance changes with frequency . Be careful ,X p stay DC It starts to fall , but R p until 100 kHz The obvious roll down starts at . By carefully designing the relevant R、C and L unit , The total load can be offset again .

4.6. A typical 10 Megohm passive probe Xp and Rp Relationship with frequency

If the impedance curve of the oscilloscope probe is not obtained , The following formula can be used to estimate the worst case load ：

X p=1/2πfC

among ：

X p= Capacitive reactance

f= frequency

C= Probe tip capacitance

Such as , The head capacitance is 11 pF Standard passive 10 Capacitance reactance of megohm probe (X p) stay 50 MHz The time is about 290 ohm . According to the signal source impedance , This load may have a great impact on the signal amplitude ( Through simple shunt action ), It may even affect the operation of the detected circuit .