Inventory of radio frequency interference rectification error circuits in amplifiers

In real-world applications, an increasing amount of radio frequency interference (RFI) must be dealt with, especially where signal transmission lines are long and signal strength is low, as is the case with typical applications of instrumentation amplifiers due to their inherent common-mode rejection Ability to extract weaker differential signals from stronger common-mode noise and interference. But there is a potential problem that is often overlooked: RF rectification in instrumentation amplifiers. When strong RF interference is present, the integrated circuit may rectify the interference, which then manifests itself as a DC output offset error. Common-mode signals at the input of an instrumentation amplifier are usually attenuated by its common-mode rejection. RF rectification still occurs because even the best instrumentation amplifiers cannot actually reject common-mode noise at signal frequencies above 20 kHz. The amplifier's input stage may rectify strong RF signals, which then manifest themselves as DC offset errors. Once rectified, a low-pass filter at the instrumentation amplifier output will not be able to eliminate this error. If RFI is intermittent, it can cause undetectable measurement errors.

Designing Practical RFI Filters

The most practical solution to this problem is to use a differential low-pass filter before the instrumentation amplifier to attenuate the RF signal. This filter has three purposes: to remove as much RF energy from the input lines as possible; to balance the AC signals between each line and ground (common); and to maintain a high enough input impedance across the entire measurement bandwidth to Avoid loading the signal source.

Figure 1 is a basic block diagram of various differential RFI filters. The component values ​​shown in the figure are all selected for the AD8221. The typical -3dB bandwidth value of the AD8221 is:

Figure 1 Low-pass filter circuit used to prevent radio frequency interference rectification errors

1MHz, typical voltage noise level is 7 nVQQ screenshot 20141204111321.jpg. In addition to suppressing radio frequency interference, this filter also has an input overload protection function. Because resistors R1a and R1b help isolate the instrumentation amplifier input circuit from external signal sources.

Figure 2 is a simplified diagram of this anti-RF interference circuit.

As can be seen from the figure, the filter forms a bridge circuit with its output connected across the input pins of the instrumentation amplifier. Given this connection method, any mismatch between the two time constants of C1a/R1a and C1b/R1b will cause the bridge to be unbalanced, thereby reducing the high-frequency common-mode rejection performance. Therefore, resistors R1a and R1b and capacitors C1a and C1b should always be equal.

Figure 2 Capacitor C2 forms a bypass for C1a/C1b and can effectively reduce the AC common mode suppression error caused by component mismatch.