Typical precision operational amplifiers (op amps) may have to 1MHz gain-bandwidth product. In theory, users may expect GHz RF signal attenuation levels to very low levels, because they go far beyond the bandwidth of the amplifier. However, the reality is not the case. In fact, it included in the amplifier electrostatic discharge (ESD) diodes, input structure, and other non-linear elements in the input of the amplifier to the RF signal "rectified." In a practical sense, RF signal is converted into a direct current (DC) offset voltage,the DC offset voltage of the amplifier input offset voltage added.

Users may ask: "For a given RF signal generated by the DC offset voltage, how do I determine its magnitude?" In fact, the sensitivity of the amplifier to the RF interference depends on whether the amplifier is used in design and technology. For example, many modern amplifiers have built-in RF filters, can minimize the chances of this problem. The filter for low-gain bandwidth op amp is the most effective because the cut-off frequency of the filter can be set to a lower frequency, which can provide higher RF signal attenuation coefficients. In addition, a number of technology products have a stronger internal RF immunity. For example, compared to bipolar devices, most of the complementary metal oxide semiconductor (CMOS) devices have a stronger anti-RF interference. The input stage design, etc. Other factors may also affect the RF immunity.

Taking into account all these factors, the board and system level design how to choose the amplifier it should answer is:? Depends on electromagnetic interference suppression ratio (EMIRR). The technology index is similar PSRR and CMRR, because of the impact it at the input of the amplifier converts the RF interference DC offset voltage. As an example, Figure 1 shows the EMIRR curve OPA333. It may be noted from the graph, when the frequency is 1000MHz when the op amp has 120dB of EMIRR. This is a very high level of inhibition, so that the curve of the curve directly be compared with other devices becomes possible.

When using OPA333 EMIRR IN + and frequency compared to the example

EMIRR curve shows the op amp is conducted against RF signal (the signal is applied to the non-inverting input) interfere with the ability of the measured value. The term "conduction" means that the RF signal is applied directly to the use of an impedance matching type printed circuit board (PCB) of the op amp input. In addition, the reflection amplifier input were characterized and described.

Finally, with a digital multimeter to measure the RF signal generated by the DC offset voltage. Please note that between the amplifier and the multimeter using low-pass filter to prevent potential errors caused by residual RF signals pass through the amplifier caused. Figure 2 shows the test circuit used to characterize EMIRR of.

Equation (1) and (2) gives the mathematical definition EMIRR. Two equations for each other reset version. Equation (1) shows the relationship with the RF signal and offset voltage change between. Note that the offset voltage variation caused by the square of the RF signal used. This means that the increase in the amplitude of the incoming RF signal is small may result in a significant increase in the offset voltage. Please also note that the role of the term is to reduce the impact of EMIRR RF signals; in other words, the larger EMIRR (dB) can significantly reduce the offset voltage. Equation (2) is used to calculate the form EMIRR (dB) of the equation in the characterization of the process.

among them

EMIRR (dB) - electromagnetic interference measurement is conducted from the RF signal rejection ratio (in dB) is applied to the non-inverting input of the amplifier

| △ Vos | - is the determination of the offset voltage (caused by RF interference) changes

VRF_PEAK - is applied to the peak RF interference non-inverting input of the amplifier

Finally, note that many other factors, such as PCB layout and shielding, may also affect the RF immunity user's system. However, once the user to optimize the design of these factors, the use of the amplifier can be achieved with good EMIRR the best performance. Also, users do not need to make any complicated calculations. Only EMIRR curve

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