Typical precision operations (op amps) can have 1 MHz gain bandwidth. In theory, users may expect Gigabole levels of RF signals to very low levels because they far beyond the bandwidth of the amplifier. However, this is not the case.
In fact, the electrostatic discharge (ESD) diode, input structure, and other nonlinear components included in the amplifier will "rectify" in the input of the amplifier. In the actual sense, the RF signal is converted into a DC (DC) offset voltage, which adds an amplifier input offset voltage.
The user may ask: "For the DC offset voltage generated by a given RF signal, how do I determine its amplitude?" In fact, the sensitivity of the amplifier to RF interference depends on the design and techniques used in the amplifier.
For example, many modern amplifiers have built-in RF filters that minimize the probability of this problem. This filter is most effective for the low gain bandwidth transport because the cutoff frequency of the filter can be set to a lower frequency, which provides a higher RF signal attenuation coefficient.
In addition, some technical products have stronger internal anti-RF interference capabilities. For example, most complementary metal oxide semiconductor (CMOS) devices have stronger anti-RF interference capabilities compared to bipolar devices. Other factors such as input stage design can also affect anti-RF interference capabilities.
Considering all these factors, how should board and system-level designers choose the amplifier? The answer is: To watch electromagnetic interference suppression ratio (Emirr). This technical indicator is similar to the power suppression ratio and common mode suppression ratio because it converts the effect of RF interference into a DC offset voltage in the input of the amplifier.
As an example, Figure 1 shows the EmIRR curve of the OPA333. It can be noted from the curve that the op amp has a 120 dB of EmIR when the frequency is 1000 MHz. This is a very high inhibition level, so that the curve is possible to compare the curve of other devices.
Figure 1, an example of the Emirr IN + and frequency when using OPA333
The Emirr curve demonstrates the measurement value of the anti-RF signal that is transmitted (this signal is applied to the non-inverting input) interference capability. The term "transmitted" means that the RF signal is directly applied to an op amp input terminal using an impedance matching type printed circuit board (PCB). In addition, reflection of the amplifier input is characterized and explained.
Finally, the DC offset voltage generated by the RF signal is measured with a digital multimeter. Note that a low-pass filter is used between amplifiers and multimeters to prevent potential errors caused by residual RF signals through the amplifier. Figure 2 shows a test circuit for characterizing EmIRR.
Figure 2, test circuit used to characterize EmIRR
Equation (1) and (2) give the mathematical definition of Emirr. Two equations are mutually reset each other. Equation (1) shows the relationship between the RF signals used and the shift voltage.
Note the migration voltage change caused by the square of the RF signal used. This means that the increase in incident RF signals can result in a significant increase in the offset voltage. Please also note that the role of the term Emirr is to reduce the effect of the RF signal; in other words, larger Emirr (dB) makes the change in the offset voltage have greatly reduced. Equation (2) is in the form of an equation for calculating Emirr (DB) during characterization.
in
Emirr (DB) - - The electromagnetic interference suppression ratio (in DB) measured at the conducted RF signal is applied to the input of a non-inverter amplifier;
| △ VOS | - - is a variation of the measured offset voltage (caused by RF interference);
VRF_peak - - is a peak RF interference applied to the amplifier non-inverting input;
Finally, please pay attention to many other factors, such as PCB layout and shielding, can also affect the anti-RF interference capability of the user system. However, once these factors are optimized in the user's design, the optimal performance can be achieved using an amplifier with good Emirr. Moreover, users do not need any complex calculations. Only the Emirr curves of different amplifiers can be selected to select the device that is best for the user. The author wants the user to optimize the ability of the user system anti-RF signal interference with the Emirr specification.
Our other product:
