This section will illustrate two examples of loading effects caused by probing circuits. In each example the effects that occur as a result of probing the circuit would cause the device to fundamentally change behavior or cease to function entirely.
A LC circuit, otherwise known as a tank circuit, contains an inductor and a capacitor in parallel. The end effect of this circuit is that the inductor coil emits a resonance frequency at a given value determined by the inductor and capacitor. The frequency is governed by Equation 3.
Equation 3. This equation governs the resonance frequency of a LC circuit.
This circuit is used in commercial RFID tags, so that will be the example to show this loading affect. Figure 5 shows a very common LC circuit in an RFID chip.
Figure 5. LC circuits are used in RFID tags. This is a very common RFID LC circuit.
The engineer designing or testing this circuit might want to probe the line containing the capacitor. If the engineer attaches a SP500X probe at the high potential point of this circuit, the capacitance of the probe will be added in parallel with C1 between high potential and ground as shown in Figure 6.
Figure 6. The probe’s input capacitance will be added to the circuit if it is not probed in a way that prevents current flow.
The additional capacitance of the probe will cause the resonance frequency of the LC circuit to change according to Equation 4.
Equation 4. Additional capacitance introduced by the SP500X probe will change the resonance frequency of the LC circuit to 0.93 times its original frequency.
Due to this change in frequency, the RFID tag will now emit a frequency much different than the intended transmitter frequency which will not build up enough energy to be detected by the sensor or functionally characterized for correct operation.
The oscillator circuit in Figure 7 contains a resistor with a 10 Megohm value in parallel with a CMOS inverter. Probes have input resistance of 10 Megohm to prevent significant current flow through the probe and to avoid affecting the circuit under test. In this case the circuit under test includes a high-resistance element.
Figure 7. A watch oscillator circuit can be functionally simplified in representation to show how resistive loading can affect its operation.
An engineer may be interested in the potential at the junction of CTRA In, the 10 Megohm resistor, and the power supply of the crystal oscillator as shown in Figure 8. This probe point would put the 10 Megohm input resistance of the probe in parallel with the 10 Megohm resistor which will create a voltage divider. The crystal oscillator in this circuit expects to operate with a given voltage. If the oscillator receives half of the voltage it expects, it could operate sporadically or not operate at all.
Figure 8. Probing in parallel with the 10 Megohm resistor in the crystal oscillator circuit will create a voltage divider that could cause it to stop functioning.
1:1 (one-to-one) probes, also known as 1x probes, connect the 1 MΩ impedance input of the oscilloscope to the circuit being measured. They are designed for minimum loss and easy connection, but otherwise they are equivalent to using a cable to connect the scope. Figure 4 shows the circuit diagram for a high-impedance scope input connected to a circuit under test. The circuit under test is modeled as a voltage source with a series resistor. The 1:1 probe (or cable) will introduce a significant amount of capacitance which appears in parallel with the input of the scope. A 1:1 probe may have around 40 to 60 pF of capacitance, which is usually larger than the oscilloscope input capacitance.
The construction of 1:1 probes does not allow for the same level of performance that you would expect in an attenuating probe as will be explained in the 10:1 Probe section.
10:1 probes (also called 10x probes, divider probes, or attenuating probes) have a resistor and capacitor (in parallel) built into the probe. Figure 8 shows the circuit for the 10:1 probe connected to a high-impedance input of an oscilloscope. If R1C1 = R2C2, then this circuit has the amazing result that the effect of both capacitors exactly cancel. In practice, this condition may not be met exactly but can be approximated. The capacitor is usually made adjustable and can be tweaked for a near perfect match. Equation 5 shows the relationship of Vs to VIN under these conditions.
Equation 5. Attenuating probes like 10X probes use the voltage divider principal described in this equation.
This equation is reminiscent of the voltage divider equation. R2 is the input resistance of the scope's high input impedance (1 MW) and R1 = 9R2. Equation 6 show the result of Equation 5 using a 10X probe.
Equation 6. A 10X probe results in 1/10 the voltage at the oscilloscope input.
So the net result is a probe and scope input combination that has a much wider bandwidth than the 1:1 probe, due to the effective cancellation of the two capacitors. The penalty that is incurred is the loss of voltage. The oscilloscope now sees only one-tenth of the original voltage (hence the name 10:1 probe). Also notice that the circuit being measured sees a load impedance of R1 + R2 = 10 MW, which is much higher than with the 1:1 probe. Some probes are designed to be conveniently switched between 1:1 and 10:1 operation.
Figure 9. The effect of the capacitors in a passive probe is cancelled when C1 is adjusted properly.
With a 10:1 probe, both the resistive and capacitive loading effects are reduced (relative to a 1:1 probe). Although the input capacitance of the scope is ideally canceled, there is a remaining capacitance due to the probe, CPROBE. This capacitance, which is specified by the manufacturer, will load the circuit under test.
The factor of 10 loss in voltage is not a problem as long as the voltage that is being measured is not so small that dividing it by 10 makes it unreadable by the scope. This means that the scope's sensitivity and the signal voltage may be factors in deciding whether to use a 10:1 probe. On most oscilloscopes, the user must remember that a 10:1 probe is being used and must multiply the resulting measurements by a factor of 10. This is a nuisance so some scopes include two scale markings: one valid for a 1:1 probe and the other valid for a 10:1 probe. Other scopes have gone one step farther and automatically adjust the readings by the correct amount when an attenuating probe is used.
Note that some 10:1 probes have a resistor across the probe input so that the resistive loading is 1 Megohm. These probes do not represent an improvement in resistive loading over the 1:1 probe, but they do have less capacitive loading.
Other Attenuating Probes
Attenuating probes come in a number of values like 50: 1 and 100: 1 probes. The general principles of these probes are the same as the 10:1 divider probe: voltage level and bandwidth are traded off to obtain wider bandwidth, more loss is incurred in the probe and less voltage is supplied to the input of the scope. This may require a more sensitive scope for low-level measurements. There are also certain 50Ω impedance passive probes that have wider bandwidths but limited applications.