An instrumentation amplifier offers precise gain without feedback resistors, and, at any value of gain, it provides high input impedances at its noninverting and inverting inputs. In a typical IC instrumentation amplifier, a single resistor that connects across two gain-adjustment pins determines the circuit's overall gain. Integrated versions of most instrumentation amplifiers allow the pins to remain open for unity gain but require finite-value gain-setting resistors for gains exceeding one. Although the gain-adjustment resistor might comprise a tiny surface-mounted device, its electrodes and internal resistive layer extend the conductive surface connected to the IC's gain-adjustment pins. The extended surface acts as an antenna and thus makes the amplifier more susceptible to stray external electromagnetic fields.
Figure 1 shows an instrumentation amplifier that offers a gain of two without using any external resistors. The circuit comprises a cascade of asymmetrical, differential-output amplifier, formed by two channels of IC1; an Analog Devices AD8222 instrumentation amplifier; and a difference amplifier comprising one half of IC2, a second AD8222. All three instrumentation-amplifier sections in the circuit provide a stand-alone gain of one. Because the differential outputs of the first stage have opposite signs, their difference is twice that of the difference of the input signals.
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| Figure 1. | Based on two dual-section instrumentation amplifiers, this composite instrumentation amplifier offers a gain of two with an error margin of less than 0.06% and requires no gain-setting resistors. |
The circuit's worst-case gain error does not exceed the value of δ2 = 3δ1, where, at a gain of one, δ1 represents the maximum gain error of one section of the AD8222. For B-grade ICs, you calculate the value of δ2 as δ2 ≤ 0.06% (see AD8222 datasheet). Typically, the value of δ2 rarely reaches its maximum value. Given the reasonable assumptions that all three amplifiers' gain errors are independent and obey a gaussian distribution, the probability of occurrence of δ2 = 3δ1 is about 1/20 the probability of encountering a single amplifier that has a maximum gain error of δ1.
