Active probe on the op-amp for the oscilloscope. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Broadband amplifiers with high input impedance, low input capacitance, and low output impedance are used in a variety of applications. One of the applications is input probes for oscilloscopes and other measuring equipment. As shown in this article, modern op-amps from Analog Device make it possible to solve this problem with simple means.

An oscilloscope is one of the most versatile instruments that allows you to measure a wide variety of electrical signal parameters, and often greatly simplify the procedure for setting up electronic devices. In some cases, it is simply irreplaceable. However, many are familiar with the situation when connecting an oscilloscope to a custom device leads to a violation of its modes. This is primarily due to the capacitance and resistance of the input of the oscilloscope and its connecting cable introduced into the circuit under study.

Most oscilloscopes used by radio amateurs have a high input impedance (1 MΩ) and an input capacitance of 5 ... 20 pF. In combination with a connecting shielded input cable about a meter long, the total capacitance increases to 100 pF or more. For devices operating at frequencies above 100 kHz, this capacitance can have a significant impact on measurement results.

To eliminate this drawback, radio amateurs use an unshielded wire (if the signal level is high enough) or a special active probe, which includes an amplifier with a high input impedance, usually made on field-effect transistors [1-3]. The use of such a probe significantly reduces the amount of capacitance introduced into the device. However, the disadvantages of some of them are the low gain or the presence of a level shift at the output, which makes it difficult to measure the DC voltage. In addition, they have a narrow operating frequency range (up to 5 MHz), which also limits their use and requires short connecting cables. The probe described in [2] has somewhat better parameters. It should be noted that all these probes can work effectively with oscilloscopes that have a high input impedance.

Currently, broadband oscilloscopes with an operating frequency range of up to 100 MHz and higher, with a low input impedance of 50 ohms, are becoming more common, so their connection to a custom device often becomes almost impossible. Not all of them are equipped with active probes, and the use of resistive dividers leads to a noticeable decrease in sensitivity.

The active probe, the description of which is offered to the attention of readers, is free from these shortcomings. It works with various oscilloscopes, the input impedance of which can be low-resistance - 50 Ohm or high-resistance - up to 1 MΩ, has an operating frequency range of 0 ... 80 MHz and a fairly high input impedance at low frequencies - 100 kOhm. Its transmission factor is 1 or 10, i.e. it not only does not weaken, but also strengthens the signal. The advantages of the probe include its small dimensions.

Such parameters were achieved through the use of a modern high-speed op-amp from Analog Devices. In particular, this probe uses the AD812AN op-amp, which has the following main characteristics:

Upper operating frequency - not less than 100 MHz; input resistance - 15 MΩ with an input capacitance of 1,7 pF; input voltage - up to +13,5 V, and the output voltage slew rate - 1600 V / μs; output current (with an output resistance of 15 ohms) - up to 50 mA; current consumption in the absence of an input signal - 6 mA.

In addition, the op amp has low harmonics (-90 dB at 1 MHz and a load of 1 kΩ) and low noise (3,5 nV / ^ Hz), protection against K3 (current limited to 100 mA), power dissipation in a small package large enough - 1 W. It should be added to this that the price of a microcircuit containing two op-amps with such parameters is relatively low ($3...4).

The scheme of the active probe is shown in fig. 1. Basically, it corresponds to the standard op-amp switching circuit. The transfer coefficient KU is changed by switching SA1 of the elements of the feedback circuit and has two values: 1 and 10. Switch SA2 selects the operating mode: with a "closed" input, when the capacitor C1 is turned on at the input and the DC component of the voltage does not pass to the input, or with "open "inlet as she passes.

The frequency response of the probe when operating on a load with a resistance of 50 ohms for different transmission coefficients is somewhat different. For Ku=1, it has a slight rise (up to 20...25%) at frequencies of 20...45 MHz and decreases to a level of 0,7 at frequencies of 70...80 MHz and to a level of 0,3 at 100 MHz. For Ku=10, the frequency response is flat up to 20 MHz and smoothly drops to 7 at a frequency of 40 MHz, and decreases to 100 at a frequency of 3 MHz.

When the probe is connected to an oscilloscope or frequency meter with a large input impedance (usually Rin = 1 MΩ) through a high-frequency cable 1 m long, the amplitude of the maximum output voltage of the op-amp reaches 12 V (with Upit = +15 V) at frequencies up to 10 ... 15 MHz and smoothly decreases to 3 V at frequencies of 30 ... 40 MHz. When the probe is loaded on the low-resistance input (Rin = 50 Ohm) of the oscilloscope, the maximum output voltage is 4 V at frequencies up to 1 MHz and decreases to 0,5 V at frequencies of 30 ... 40 MHz. It should be especially noted that the presence of the amplification mode allows you to observe input signals with an amplitude of 10 ... 200 μV on the oscilloscope screen with a sensitivity of 300 mV per division!

A relatively small resistance R3 (100 kOhm) is installed at the amplifier input. This is done because the input current of the op-amp is fractions of µA and the offset of the DC voltage level at the output in this case is approximately 50 mV at KU = 1 or 500 mV at Ku = 10. An increase in this resistance will lead to a corresponding increase in bias. As the practice of measuring broadband signals shows, an input impedance of the probe of the order of 100 kOhm is quite enough. It is possible to increase it up to 1 MΩ by changing R3 accordingly, but this will lead to the above consequences. At high frequencies, the input resistance is lower and is mostly capacitive, but this does not affect the measurement procedure, since high-resistance circuits are rare at high frequencies.

About the design. Most of the probe parts are placed on a printed circuit board made of double-sided foil fiberglass, a sketch of which is shown in fig. 2. On one side of it, an op-amp and all resistors are placed, on the second - capacitors C2-C5. Connections between mounting sides are made by conductors through holes in the board. The switches are installed on the probe body, and the capacitor C1 is installed directly on SA1.

The probe body (Fig. 3) consists of a plastic tube 1 (from a felt-tip pen with a diameter of about 18 mm), which is inserted into a metal casing 2. Board 3 is placed inside the tube, switches SA1 and SA2 (4 and 5) are fixed on it. Through the bottom of the tube, the connecting and supply wires are brought out - 6. The common wire of the board is connected to the casing, and through the hole in it the wire for the metal pin X1 - 7 is brought out. All internal connections must be made with a wire of minimum length, and external - power and signal circuits - respectively shielded and HF cable.

Since one of the two op-amps is not used in the microcircuit, its inputs (pins 5 and 6) are connected to a common wire.

Setting up the device comes down to setting the required gain, which, when the probe is working with an oscilloscope with a high input impedance, is set to 10 at a frequency of 10 MHz by selecting the resistor R1 (with SA1 closed). If the probe is used with an oscilloscope with a low-impedance input, part of the output signal is quenched by terminating resistor R5. Therefore, a resistor R6 is introduced into the circuit, and by selecting its resistance (when SA1 is open), the transfer coefficient is set to 1. When SA1 is closed (high sensitivity mode), the gain equal to 10 is set by selecting resistor R1.

Resistors MLT, C2-10, C2-33, R1-12, capacitors C1-C3 of the KM series or other small-sized ones (K10-17, K10-47), C4, C5 - K52 groups or similar are applicable in the device. You can use wideband AD812AR or AD817AN, AD818AN op amps from the same company, which are cheaper due to the lower bandwidth (50 MHz), but their use will also lead to a reduction in the operating frequency band.

To power the probe, a bipolar stabilized power supply with an output voltage of % 12 ... 15 V is required. It should be noted that the current consumed in the absence of a signal is 10 ... 15 mA, when operating on a low-resistance load when a signal is applied, the current can increase up to 100 mA .

Literature

1. Grishin A. Active probe for an oscilloscope. - Radio, 1988, No. 12, p. 45.
2. Ivanov B. Oscilloscope - your assistant (active probe). - Radio, 1989, No. 11, p. 80.
3. Turchinsky D. Active probe to the oscilloscope. - Radio, 1998, No. 6, p. 38.

Author: I. Nechaev, Kursk

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