Instrumentation amplifiers are analog subsystems that amplify low-level signals in the presence of high common mode noise. These differential amplifiers are optimized for DC signals and are typically characterized by high gain, high input impedance, and high common mode rejection ratio (CMRR). The most commonly used instrumentation amplifiers include two or three operational amplifiers and several precisely matched resistors. The use of multiple operational amplifiers improves both gain linearity and accuracy. In these devices, CMRR is approximately equal to half the resistor mismatch plus the gain. The output or common mode error is fixed by the resistor mismatch and independent of both the gain and the output signal. Instrumentation amplifiers that include only single operational amplifiers are also available, but do not achieve the low drift and low bias currents of more advanced devices.
Device specifications for instrumentation amplifiers include the number of analog channels and the number of differential channels. When single-ended outputs are available, suppliers often specify the maximum number of analog channel outputs as twice the number of differential outputs. Differential channels, which have two inputs, define the voltage as the signal to process between the two inputs. Other device specifications for instrumentation amplifiers include maximum output voltage, gain range, and input impedance. Gain, the factor by which input signals are multiplied, is frequently greater than unity; however, gain can be fractional when a reduction (attenuation) of signal amplitude is desired. Impedance, the resistance to alternating signal flow, is a result of the resistance, capacitance, and inductance of a device’s circuitry. For instrumentation amplifiers, input impedance is typically specified as much greater than the impedance of the devices whose signals are amplified.
Additional specifications for instrumentation amplifiers include AC and DC voltage inputs and integral filters that allow some signals to pass while blocking others. Instrumentation amplifiers also vary in terms of common mode rejection ratio, a measurement that is expressed mathematically as CMRR = 20 log (differential gain / common mode range). An amplifier’s ability to obtain the difference between two inputs while rejecting the signal common to both is defined by both the CMRR and the common mode range.
Instrumentation amplifiers are available in a variety of form factors. Some devices mount on integrated circuits (ICs), standard DIN rails, or printed circuit boards (PCBs) that attach to enclosures or plug into computer backplanes. Others bolt into walls, cabinets, enclosures, or panels. Rack-mounted units fit inside a standard 19” telecommunications rack. Modular styles include stackable units that dock in bays, slots, or boxes. Benchtop or freestanding instrumentation amplifiers often feature full casings or cabinets and integral interfaces.
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