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.

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% (Reference 1). 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.