Differential probes for the basic series of oscilloscope probes

As a professional hardware design and testing engineer, we use a variety of different digital oscilloscopes for the measurement of the relevant electrical signal volume every day. There are also a wide range of probes that match these oscilloscopes, including passive probes (including high-voltage probes, transmission line probes), active probes (including active single-ended probes, active differential probes, etc.), current probes, light probes, etc. Each probe has its advantages and disadvantages, so each has its own applicable occasions. Among them, the active probe is widely used in high-speed digital measurement field due to its advantages of high bandwidth, small input capacitance and small ground loop, but the high price of active probe, small dynamic range, electrostatic sensitivity and calibration trouble, therefore, each engineer using Oscilloscope's entry-level probe is usually passive probe. The most common 500Mhz passive voltage probes are suitable for general circuit measurements and fast diagnostics, and can accommodate most low-speed digital signals, TVs, power supplies, and other typical oscilloscope applications.

1. Differential Measurement characteristics

The probe can be divided into passive probes and active probes in general, while wide bandwidth oscilloscopes and active probe users also need to choose between single-ended probes and differential probes. The pair of lines that carry the differential signal is called a differential route. This article is mainly about the differential probe. The most obvious advantages of differential signals compared to ordinary single-ended signal lines are in the following three areas:

1. Strong anti-jamming ability, because the coupling between two differential lines is very good, when the external noise interference, almost simultaneously coupled to two lines, and the receiving side is only concerned about the difference between the two signals, so the external common mode noise can be the maximum extent offset.

2. Can effectively suppress EMI, the same reason, because the two signal polarity opposite, their external radiation electromagnetic field can cancel each other, the more tightly coupled, vent to the external electromagnetic energy less.

3. Timing positioning is accurate, because the differential signal switching is located at the intersection of two signals, rather than the ordinary single-ended signal depends on the high and low two threshold voltage judgment, thus affected by the process, the temperature is small, can reduce the timing error, but also more suitable for low-amplitude signal circuit. The current popular LVDS refers to this small amplitude differential signal technology.

The structural characteristics of the differential signal require the corresponding test equipment must also be a differential topology, the differential probe thus become the mainstream of modern oscilloscope accessories. 1 is a typical active differential probe circuit structure diagram:

For high-frequency signal testing, the main advantage of the active differential probe is that the low input capacitance and the ability to suppress common-mode noise are much higher than the single-ended probe, and its drawbacks are mainly in high prices and the need for additional power supplies. For example, the Wavelink series high-bandwidth differential probes from the Force branch are representative of this type of probe.

　　2. Differential probe with high common-mode rejection ratio

What is the common-mode rejection ratio, in short, is the differential amplifier circuit in the signal common-mode component of the suppression ability, which is defined as the amplifier to the differential mode signal voltage magnification ADM and the common mode signal voltage magnification ratio of ACM, the English full name is Commonmoderejectionratio, Usually expressed in shorthand CMRR.

We can define this: the average voltage of two inputs to the ground is the common-mode voltage VCM, the gain after the difference amplifier is the common-mode gain ACM, and the relative voltage difference between the two inputs is the differential-mode-voltage VDM, whose gain after the differential-mode amplifier is Adm. The CMRR calculation formula is as follows:

The larger the differential-mode signal voltage gain ADM, the smaller the common-mode gain ACM, the greater the CMRR. The greater the ability of the differential amplifier to suppress the common-mode signal, the better the amplifier's performance. When the operating amplifier circuit is completely symmetrical, the common mode signal voltage amplification acm=0, the common mode rejection ratio ccmr→∞, this is the ideal situation, in fact, the circuit is completely symmetrical is not exist, the common mode rejection ratio can not tend to infinity.

What factors affect the common-mode rejection ratio of the transducer?

Circuit symmetry-the symmetry of the circuit determines the amplitude of the residual common mode interference of the amplified signal, the worse the circuit symmetry, the smaller the common-mode rejection ratio, the less the ability to suppress the common-mode signal (interference).

Signal frequency or DV/DT

Mismatch of any probe or instrument input.

Obviously, the larger the CMRR value, the better, usually around 60dB (1000:1), but the CMRR will gradually decrease as the frequency increases. Because the faster the signal edge is more prone to bias between the positive and negative ends, it will also bring more common-mode voltage, as shown in.

Why CMRR is important because if the CMRR indicator of the differential probe is not good, the common-mode voltage is added to the differential voltage, resulting in a measurement error.

Why is the CMRR indicator of a single-ended probe difficult to be high? The single-ended probe model indicates that there is a parasitic resistor and parasitic inductance between the probe amplifier and the earth ground, which constitute the characteristic impedance produced by the transmission line consisting of the probe cable shield and the earth ground. This characteristic impedance is important because when you add a common-mode signal to a single-ended probe, the ground inductance is combined with this characteristic impedance to make up a voltage divider. This voltage divider acts as a attenuation to the ground signal reaching the amplifier. Since the amplifier's signal and ground input signal are subject to varying attenuation, a net signal appears on the amplifier's input, allowing the amplifier to have an output signal. The larger the ground inductance, the lower the common-mode rejection, so it is important to keep the ground as short as possible when using a single-ended probe.

When you add a common-mode signal to a differential probe, the amplifier has the same signal at the positive and negative two inputs. The only output signal produced is the function of the amplifier's suppression characteristic, which is independent of the Wired inductor. Therefore, it is more accurate to measure with a differential probe when there is a large common-mode noise. This is a typical difference between a differential probe and a single-ended probe, unless the inductance of the ground connection of a single-ended probe is very small, which is difficult in practice. Therefore, the actual differential probe CMRR is generally superior to single-ended probes.

　　3. Safe Floating Land measurement

Power system testing often requires the measurement of three-phase power supply in the FireWire and FireWire, or the FireWire and 0 (medium) line of the relative voltage difference, many users directly using a single-ended probe to measure the voltage of two points, resulting in the phenomenon of probe burning occurs. This is because most of the oscilloscope's "signal common line" terminals are connected to a protective grounding system, often referred to as "grounding". The result is that all of the signals that are applied to the oscilloscope and provided by the oscilloscope have a common connection point. This common connection point is typically an oscilloscope enclosure that connects the source line of the third wire in the power cord of the AC power supply to a test point by connecting the probe ground. The ground of the single-ended probe is directly connected with the power supply line, and the consequence must be short. In this case, we need to measure the float.

The so-called "float" measurement, that is, two points of measurement is not at ground potential, which is a typical differential measurement. The voltage between the "signal common line" and the ground may rise to hundreds of volts.

In addition, many differential measurements also require the suppression of high common-mode signals to facilitate the evaluation of low-level differential signals, as well as unwanted hum and grounding loops from excess ground currents. Users often use measurement techniques that are potentially dangerous to solve these problems.

Disconnect the centerline from the ground by cutting the standard three-head AC socket ground or using an AC isolation transformer. Float the oscilloscope from the protective ground to reduce the effect of the ground loop. This method is not practical, because in the wiring of the building, the midline may have been connected to the ground, is unsafe measurement method, will bring L personal injury, instrument and circuit damage!

In addition, it violates industrial health and safety regulations and results in poor measurements. Moreover, the AC power supply instrument will have a large parasitic capacitance when it floats on the ground. Therefore, the floating measurement will be destroyed by oscillation.

All in all, the oscilloscope will "float" a very bad idea, which will result in:

– Damage to the device under test;

--Damage Oscilloscope

-Potential harm to the person

– resulting in poor measurement accuracy

　　How to Solve

The best solution is to use a differential probe with a high common-mode rejection ratio, since there is no grounding problem on the two inputs, the differential operation of the two input signals is done at the probe front-end amplifier, the signal transmitted to the oscilloscope channel is a differential voltage, and the oscilloscope does not need to remove the grounding end of the three-wire plug to achieve a safe For example, the ADP305 high-voltage differential probe is a safe measurement of the three-phase mains and the FireWire, FireWire and midline pressure difference between the best probe.

Differential probes for the basic series of oscilloscope probes