The gain is expressed by this equation:Ī v = V out/(V2 – V1 )= (1 + (2R1 / R gain)) (R3/R2) The third is a conventional differential amplifier. Of the three op-amps, one buffers each input. The InAmp is often explained using a model consisting of three op-amps with seven resistors providing negative feedback. Additionally, the measuring instrument employing an InAmp input typically has low dc offset, low drift, low noise, high open-loop gain, and a high common-mode rejection ratio. The purpose of the buffer amps is to eliminate the need for an impedance match between the input of the amp and the DUT or whatever is generating the input signal. An InAmp consists of a differential amp with a buffer amplifier on each input. The InAmp, based around the differential amp, gets around this problem. Most differential amps are based on op-amps these days, and this feedback may be implemented within the op-amp IC or, where temperature rise may be a problem, using a discrete component.Ī differential amplifier, to achieve high bandwidth at RF frequencies, must have a single-ended input that matches the source impedance, generally 50 Ω. It is realized by feeding the amplifier output through an appropriate resistance into the negative input. To eliminate signals (noise, bias voltages, etc.) that appear at both inputs, a low common-mode gain is desirable. Where A c is what’s called the amplifier’s common-mode gain. V out = A d (V +in – V -in) + A c ((V +in + V -in)/2) Taking this into account the equation for output voltage becomes: Accordingly, even with inputs that are equal, common-mode rejection is not absolute, and the amp would have an output that is non-zero. In reality, the two inputs on a differential amp have unequal gains. Where A d is the differential gain and V +in and V -in are the two input voltages. Ideally, differential amplifier output conforms to this equation: It can be helpful to understand how these two devices differ.Ī differential amplifier ideally amplifies the difference two input voltages but suppresses any voltage common to its two inputs. This kind of amplifier, sometimes abbreviated as InAmp, is itself based on a more fundamental amplifier setup, the differential amplifier. Many of the test instruments used in industry today have inputs based on instrumentation amplifiers.