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Número de pieza

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RHF310

Rad-hard 400 µA high-speed operational amplifier

STMicroelectronics 

RHF310

Intermodulation distortion product

5

Intermodulation distortion product

The non-ideal output of the amplifier can be described by the following series of equations.

Vout = C0 + C1Vin + C2V2in + … + CnVnin

where the input is Vin = Asinωt, C0 is the DC component, C1(Vin) is the fundamental and Cn

is the amplitude of the harmonics of the output signal Vout.

A one-frequency (one-tone) input signal contributes to harmonic distortion. A two-tone input

signal contributes to harmonic distortion and to the intermodulation product.

The study of the intermodulation and distortion for a two-tone input signal is the first step in

characterizing the driving capability of multi-tone input signals.

In this case:

Vin = A sin ω1t + A sin ω2t

therefore:

Vout = C0 + C1(A sin ω1t + A sin ω2t) + C2(A sin ω1t + A sin ω2t)2… + Cn(A sin ω1t + A sin ω2t)n

From this expression, we can extract the distortion terms and the intermodulation terms

from a single sine wave.

● Second-order intermodulation terms IM2 by the frequencies (ω1-ω2) and (ω1+ω2) with

an amplitude of C2A2.

● Third-order intermodulation terms IM3 by the frequencies (2ω1-ω2), (2ω1+ω2), (−

ω1+2ω2) and (ω1+2ω2) with an amplitude of (3/4)C3A3.

The intermodulation product of the driver is measured by using the driver as a mixer in a

summing amplifier configuration (Figure 25). In this way, the non-linearity problem of an

external mixing device is avoided.

Doc ID 15577 Rev 3

15/22

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