Test probe
From Wikipedia, the free encyclopedia
A test probe (test lead, test prod) is a physical device used to connect electronic test equipment to the device under test (DUT). They range from very simple, rugged devices to complex probes that are sophisticated, expensive, and fragile.
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[edit] Voltmeter probes
Voltmeter probes usually consist of single wires that are equipped on one end with a connector that fits the user's voltmeter and on the other end with a rigid plastic section (the probe itself) that allows the user to safely hold the probe while being protected from the danger of electric shock. Within the plastic body of the probe, the wire is connected to a rigid, pointed metal tip that makes the actual contact with the DUT.
Voltmeter probes are usually colored red (for the positive probe) and black (for the negative probe). Either probe may be replaced with a wire ending in an alligator clip, allowing a connection to the DUT that does not need to be held. Some probes allow an alligator clip to be screwed onto their ends, covering the metal point.
Ordinary voltmeter probes can be used for voltages up to about 1,000 volts and currents of a few amps. Depending upon the accuracy required, they can be used for frequencies ranging from DC to a few kilohertz.
[edit] High voltage probes
By inserting a large resistor in series with the probe, and by providing massive amounts of electrical insulation, it is possible to create a probe that allows an ordinary voltmeter to measure very high voltages (up to about 50 kV). The value of the resistor must be chosen to form an appropriate voltage divider with the input resistance of the voltmeter. Because of the very high value of the resistor (many megohms), high voltage probes are only useful for measuring DC; the RC circuit that is formed with the parasitic capacitance of the voltmeter input will attenuate any frequencies above DC.
[edit] Oscilloscope probes
Because of the high frequencies involved, oscilloscopes do not normally use simple wires to connect to the DUT. Instead, a specific scope probe is used. Scope probes use a coaxial cable to transmit the signal from the tip of the probe to the oscilloscope, preserving those high frequencies that are so important to accurate oscilloscope operation.
Scope probes fall into two main categories: passive and active.
[edit] Passive scope probes
Passive scope probes contain no active electronic parts, such as transistors, so they require no external power.
The most common design inserts a 9 megohm resistor in series with the probe tip. The signal is then transmitted from the probe head to the oscilloscope over a highly specialized coaxial cable that is designed to minimize capacitance and ringing. The resistor serves to minimize the loading that the cable capacitance would impose on the DUT. In series with the normal 1 megohm input impedance of the oscilloscope, the 9 megohm resistor creates a 10x voltage divider so such probes are normally known as either low cap(acitance) probes or 10X probes.
Because the oscilloscope input has some parasitic capacitance in parallel with the 1 megohm resistance, the 9 megohm resistor must also be bypassed by a capacitor to prevent it from forming a severe RC low-pass filter with the 'scope's parasitic capacitance. The amount of bypass capacitance must be carefully matched with the input capacitance of the oscilloscope so that the capacitors also form a 10x voltage divider. In this way, the probe provides a uniform 10x attenuation from DC (with the attenuation provided by the resistors) to very high AC frequencies (with the attenuation provided by the capacitors).
In the past, the bypass capacitor in the probe head was adjustable (to achieve this 10x attenuation). More modern probe designs use a laser-trimmed thick-film electronic circuit in the head that combines the 9 megohm resistor with a fixed-value bypass capacitor; they then place a small adjustable capacitor in parallel with the oscilloscope's input capacitance. Either way, the probe must be adjusted so that it provides uniform attenuation at all frequencies. This is referred to as compensating the probe. Compensation is usually accomplished by probing a square wave and adjusting the compensating capacitor until the oscilloscope displays the most accurate waveshape. Newer, faster probes have more complex compensation arrangements and may occasionally require further adjustments.
100x passive probes are also available, as are some designs specialized for use at very high voltages (up to 25 kV).
Passive probes usually connect to the oscilloscope using a BNC connector. Most 10x probes impose a loading of about 10-15 pF and 10 megohms on the DUT, with 100x probes imposing a lighter load.
[edit] Z0 probes
Z0 probes are a specialized type of low-capacitance passive probe used in low-impedance, very-high-frequency circuits. Very similar in design to ordinary passive probes, they are designed to connect to oscilloscopes with 50 ohm (rather than 1 megohm) input impedance. Because of this, these probes therefore use a 450 ohm series resistor and 50 ohm coaxial cable (rather than the 9 megohm resistor and specialized coaxial cable of the 10x probe).
The Z0 name refers to the fact that the coaxial cable matches the characteristic impedance of the oscilloscope. This provides far better high-frequency performance than any ordinary passive probe can achieve, but at the expense of the 500 ohm load offered by the probe tip to the DUT. Parasitic capacitance at the probe tip is very low, though, so for very high-frequency signals, the Z0 probe can actually offer lower loading than any ordinary passive probe.
Z0 probes have now been largely supplanted by active probes.
[edit] Active scope probes
Active scope probes use a small, usually FET-based amplifier mounted directly within the probe head. By doing this, they are able to obtain exceptionally low parasitic capacitance while still offering a high DC resistance. (It is common to see capacitance of one pF or less while maintaining 1 megohm resistance!) They then connect to the oscilloscope in the same fashion as Z0 probes (using 50 ohm coaxial cable terminated at the oscilloscope's input).
Active probes do have several disadvantages, however. These have kept them from entirely replacing passive probes:
- They are several times more expensive than passive probes
- They require power (but this is usually supplied by the oscilloscope)
- They have a limited dynamic range, often as low as 3 to 5 volts.
- They can be damaged by overvoltage or, sometimes, even ESD.
To overcome their often-limited dynamic range, many active probes allow the user to introduce an offset voltage. The total dynamic range is still limited, but the user may be able to adjust its centerpoint so that voltages in the range of, for example, zero to five volts may be measured rather than -2.5 to +2.5.
Because of their inherent low voltage rating, there is little need to provide a large amount of insulation to ensure operator safety against electric shock. This allows the probe heads on active probes to be extremely small, making them very convenient for use with modern high-density electronic packaging. Because of their size and excellent electrical characteristics, they are strongly preferred for troubleshooting digital electronics.
Before the advent of high-performance solid-state electronics a very few active probes were built using vacuum tubes as the amplifiers.
[edit] Differential probes
Differential probes are a specialized variation on the other probe families optimized for acquiring differential signals. To maximize the common-mode rejection ratio (CMRR), differential probes must provide two signal paths that are as nearly-identical as possible, matching in overall attenuation, frequency response, and time delay.
In the past, this was done by designing passive probes with two signal paths, leading to a differential amplifier stage at or near the oscilloscope. (A very few probes fitted the differential amplifier into a rather-bulky probe head using vacuum tubes.) With advances in solid-state electronics, it has become completely practical to put the differential amplifier directly within the probe head, greatly easing the requirements on the rest of the signal path (since it now becomes single-ended rather than differential and the need to match parameters on the signal path is removed). A modern differential probe usually has two metal extensions which can be adjusted by the operator to simultaneously touch the appropriate two points on the DUT. Very high CMRRs are thereby made possible.
[edit] Additional probe features
All scope probes contain some facility for grounding (earthing) the probe to the circuit's reference (return) voltage. This is usually accomplished by connecting a very short pigtail wire from the probe head to ground. Because inductance in the ground wire can lead to distortion in the observed signal, this wire should always be as short as possible. Some probes allow the use of a small ground foot instead of any wire, allowing the ground link to be as short as 10 mm.
Most probes allow a variety of "tips" to be installed. A small, pointed tip is the most common, but "hook tips" that hold onto the test point are also very commonly used. Specialized tips that have a small plastic insulating foot with indentations into it can make it easier to probe very-fine-pitch integrated circuits; the indentations mate with the pitch of the IC leads, stabilizing the probe against the shaking of the user's hand and thereby help to maintain contact on the desired pin. Various styles of feet accommodate various pitches of the IC leads.
Some probes contain a push button. Pressing the button will either disconnect the signal (and send a ground signal to the 'scope) or cause the 'scope to identify the trace in some other way. This feature is very useful when simultaneously using more than one probe as it lets the user correlate probes and traces on the 'scope screen.
Some probe designs have additional pins surrounding the BNC or use a more complex connector than a BNC. These extra connections allow the probe to inform the oscilloscope of its attenuation factor (10x, 100x, other). The oscilloscope can then adjust its user displays to automatically take into account the attenuation and other factors caused by the probe. These extra pins can also be used to supply power to active probes.
Some X10 probes have a "X1/X10" switch. The "X1" position bypasses the attenuator and compensating network, and can be used when working with very small signals that would be below the scope's sensitivity limit if attenuated by X10.
[edit] Interchangeability
Because of their standardized design, passive probes (including Z0 probes) from any manufacturer can usually be used with any oscilloscope (although specialized features such as the automatic readout adjustment may not work).
On the other hand, active probes are almost always vendor-specific due to their power requirements, offset voltage controls, etc. Probe manufacturers sometimes offer external amplifiers that allow their probes to be used with any oscilloscope.
[edit] Current probes
Current probes use Hall effect sensors to measure the electric current flowing within a wire without any need to cut or interrupt the wire. They are available for both voltmeters and oscilloscopes. Most current probes are self-contained, drawing power from a battery or the instrument, but a few require the use of an external amplifier unit. (See also: Clamp meter)
Some larger current probes use a current transformer, but their response does not extend down to DC nor up to frequencies in the kilohertz range. (See also: Rogowski coil)
[edit] Near-field probes
Near-field probes allow the measurement of an electromagnetic field. They are commonly used to measure electrical noise and other undesirable electromagnetic radiation from the DUT, although they can also be used to spy on the workings of the DUT without introducing much loading into the circuitry.
They are commonly connected to spectrum analyzers.
[edit] Temperature probes
Voltmeters commonly allow the connection of a temperature probe, allowing them to make contact measurements of surface temperatures. The probe usually consists of a thermistor with characteristics that are specific to the voltmeter being used. Thermocouples can also be used.
