The available range of operational amplifiers is extremely diverse. Usually, the initial selection for a specific purpose is based on technical specifications — for example, on the websites of semiconductor distributors. If several candidates then remain on the shortlist, you may have to carry out further measurements yourself, as it is not always possible to ascertain all the relevant parameters under the planned operating conditions from data sheets. This is exactly what the opamp tester presented here is for.

Once you’ve made a selection of possible candidates, you often have no choice but to order samples and then put these opamps through their paces. Harmonic distortion, in particular, depends on the input level, the gain, the output level, the load, and the frequency. In data sheets, these values are often only found for certain parameters that do not cover all applications and are also not always truly comparable between manufacturers. The following small test board was thus developed in order to be able to carry out the necessary tests on different opamps under the same boundary conditions. This opamp tester can also be used to select ICs easily for equality or to find the best specimens.

Principle

The simplicity of the circuit makes a block diagram unnecessary. The circuit in Figure 1 shows two dual opamps in a DIL housing. The circuit of IC1 is designed for non-inverting operation and that of IC2 for inverting operation. The inputs of IC1A and IC1B are brought to a defined input impedance of around 50 kΩ using R1 and R6 respectively, as the non-inverting inputs of the two opamps have a high impedance. R3 is not populated for IC1A. For this reason, the gain here is +1. For IC1B, on the other hand, the gain has a value of +10 due to the ratio of (R8 + R9) / R8. However, the respective gains can easily be adapted to your own requirements by changing the values or adding R3.

Opamp tester circuit
Figure 1: The circuit of the opamp tester is fairly simple. Two dual opamps plus a few passive components are all you need.

For IC2A, the gain is exactly −1 due to the ratio of R12 / R13. The values of R16 and R17 result in a gain of −10 for IC2B. Therefore, in this test circuit with two dual opamps, you get four test circuits with inverting and non-inverting and single and tenfold gain. The values shown in the circuit in Figure 1 are suitable for typical audio applications. It should be noted that the inputs of the inverting circuits have a significantly lower impedance of around 3 kΩ for IC2A and around 1 kΩ for IC2B than for IC1A and IC1B.

Each opamp output is connected to two plug connectors. The measuring device is connected to the two-pole connectors via a 220 Ω resistor. These series resistors are intended to prevent overshoot with capacitive loads (e.g. due to the typical capacitance of ≥30 pF at the input of an oscilloscope). The exact value is not critical and should be in the range of 50…1,000 Ω — essentially it is supposed to prevent the opamp from oscillating under capacitive loads. Loads can be connected to the four-pole plug connections, allowing different operating conditions to be achieved. This sufficiently describes the circuit.

Opamp Tester: Additional Notes

The power supply is symmetrical via JP13. Two electrolytic capacitors and two 100 nF capacitors per IC are used for decoupling. Voltage regulators were deliberately omitted so that the behavior of the opamps can be tested with different supply voltages.

Layout opamp tester
Figure 2: The layout files for the circuit board can be downloaded from the Elektor webpage here.

The layout files for my circuit board can be downloaded from the Elektor web page for this article (Figure 2). High-quality machined DIL sockets are provided for the opamps so that different ICs can be changed quickly and without soldering. SMD ICs require what are known as BoBs (breakout boards), which you can make yourself or buy. Figure 3 shows an assembled board with such SMD BoBs. In my experiments, I could not find any measurable influence from these adapter boards.

The assembled board
Figure 3: The assembled board with IC1 in the SOIC8 housing on an adapter board. IC2 is designed as a
“large” DIL version. A load is connected to output JP12.

If you want to test opamps for audio applications, a low-noise design is recommended. All resistors must therefore be metal film versions. At the bottom-right of Figure 3, you can see a plug-in load in the form of a resistor soldered to a four-pole SIL socket strip. This makes it easy to create different pluggable loads that are fitted with different resistors and/or capacitances.

To measure small distortions, you need appropriate measuring devices. Over the years, I have published many articles on the subject of measurement technology in Elektor. My review of the QA403 from QuantAsylum describes an interesting audio analyzer. Furthermore, certain filters offer the option of extending the measurement range in the direction of the smallest distortions. An example can be found here.

Editor's note: This article appears in Elektor November/December 2024.

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