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The total supply voltage is, of course, 2 × V. The supply voltages, +V and –V, of the DUT are of equal magnitude and opposite sign. It is helpful if it has dc open-loop gain of one million or more if the offset of the device under test (DUT) is likely to exceed a few mV, the auxiliary op amp should be operated from ☑5-V supplies (and if the DUT’s input offset can exceed 10 mV, the 99.9-kΩ resistor, R3, will need to be reduced). The additional "auxiliary" op amp does not need better performance than the op amp being measured. The circuit of Figure 1 minimizes most of the measurement errors and permits accurate measurements of a large number of dc-and a few ac-parameters. The switches facilitate performance of the various tests described in the simplified illustrations that follow. Figure 1 shows a versatile circuit that employs this principle, employing an auxiliary op amp as an integrator to establish a stable loop with very high dc open-loop gain. The measurement process can be greatly simplified by using a servo loop to force a null at the amplifier input, thus allowing the amplifier under test to essentially measure its own errors. But in open-loop measurements their high open-loop gain, which may be as great as 10 7 or more, makes it very hard to avoid errors from very small voltages at the amplifier input due to pickup, stray currents, or the Seebeck (thermocouple) effect. They are often used in high precision analog circuits, so it is important to measure their performance accurately. Op amps are very high gain amplifiers with differential inputs and single-ended outputs.