SINGLE SUPPLY OP AMP OFFSET PLUS
(In fact, the actual voltage is the offset voltage of the auxiliary amplifier-or, if we are to be really meticulous, this offset plus the voltage drop in the 100-kΩ resistor due to the auxiliary amplifier’s bias current-but this is close enough to ground to be unimportant, particularly as the changes in this point’s voltage during measurements are unlikely to exceed a few microvolts). Negative feedback forces the output of the DUT to ground potential.
SINGLE SUPPLY OP AMP OFFSET FULL
This means that the dc voltage at the output of the DUT is amplified by the full gain of the auxiliary amplifier and applied, via a 1000:1 attenuator, to the noninverting input of the DUT. The auxiliary amplifier, as an integrator, is configured to be open-loop (full gain) at dc, but its input resistor and feedback capacitor limit its bandwidth to a few Hz. Symmetrical supplies are used, even with "single supply" op amps with this circuit, as the system ground reference is the midpoint of the supplies. 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.
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