Input filter prevents instrumentation-amp RF-rectification errors
Instrumentation amplifiers serve in a variety of applications that need to extract a weak differential signal from large common-mode noise or interference. However, designers often overlook the potential problem of RF rectification inside the instrumentation amplifier. The amplifier's common-mode rejection normally greatly reduces common-mode signals at an instrumentation amplifier's input. Unfortunately, RF rectification still occurs, because even the best instrumentation amplifiers have virtually no common-mode rejection at frequencies higher than 20 kHz. The amplifier's input stage may rectify a strong RF signal and then appear as a dc-offset error. Once the input stage rectifies the signal, no amount of lowpass-filtering at the instrumentation amplifier's output can remove the error. Finally, if the RF interference is intermittent, measurement errors may go undetected. The best practical solution to this problem is to provide RF attenuation ahead of the instrumentation amplifier by using a differential lowpass filter. The filter needs to remove as much RF energy as possible from the input lines, preserve the ac signal's "balance" between each line and ground (common), and maintain a high enough input impedance over the measurement bandwidth to avoid loading the signal source. Figure 1 provides a basic building block for a wide range of differential RFI filters.
The component values are typical of those for the latest generation of instrumentation amplifiers, such as the AD8221, which has a typical –3-dB bandwidth of 1 MHz and a typical voltage noise level of 7 nV/. In addition to RFI suppression, the filter also provides additional input-overload protection; resistors R1A and R1B help isolate the instrumentation amplifier's input circuitry from the external signal source. Figure 2 shows a simplified version of the RFI circuit. It reveals that the filter forms a bridge circuit whose output appears across the instrumentation amplifier's input pins. Because of this connection, any mismatch between the time constants of C1A/R1A and C1B/R1B unbalance the bridge and reduce high-frequency common-mode rejection. Therefore, resistors R1A and R1B and capacitors C1A and C1B should always be equal. C2 connects across the "bridge output" so that C2 is effectively in parallel with the series combination of C1A and C1B. Thus connected, C2 effectively reduces any ac common-mode-rejection errors from mismatching. For example, making C2 10 times larger than C1 provides a 20-times reduction in common-mode-rejection errors arising from C1A/C1B mismatch. Note that the filter does not affect dc common-mode rejection.
The RFI filter has differential and common-mode bandwidths. The differential bandwidth defines the frequency response of the filter with a differential input signal applied between the circuit's two inputs, +IN and –IN. The sum of the two equal-value input resistors, R1A and R1B, and the differential capacitance, which is C2 in parallel with the series combination of C1A and C1B, establish this RC time constant. The –3-dB differential bandwidth of this filter is equal to BWDIFF=[1/(2πR(2C2+C1))]. The common-mode bandwidth defines what a common-mode RF signal "sees" between the two inputs tied together and ground. C2 does not affect the bandwidth of the common-mode RF signal, because this capacitor connects between the two inputs, helping to keep them at the same RF-signal level. Therefore, the parallel impedance of the two RC networks (R1A/C1A and R1B/C1B) to ground sets common-mode bandwidth. The –3-dB common-mode bandwidth is equal to BWCM=1/(2πR1C1).
Using the circuit of Figure 1, with a C2 value of 0.01 µF, the –3-dB differential-signal bandwidth is approximately 1900 Hz. When operating at a gain of 5, the circuit has measured dc-offset shift over a frequency range of 10 Hz to 20 MHz of less than 6 µV referred to the input. At unity gain, there is no measurable dc-offset shift. Some instrumentation amplifiers are more prone to RF rectification than others and may need a more robust filter. A micropower instrumentation amplifier, such as the AD627, with its low-input-stage operating current, is a good example. The simple expedient of increasing the value of the two input resistors, R1A/R1B, that of capacitor C2, or both can provide further RF attenuation at the expense of reduced signal bandwidth. Some steps for selecting RFI-filter component values follow:
1. Decide on the value of the two series resistors and ensure that the previous circuitry can adequately drive this impedance. With typical values of 2 to 10 kΩ, these resistors should not contribute more noise than that of the instrumentation amplifier itself. Using a pair of 2-kΩ resistors adds Johnson noise of 8 nV/. This figure increases to 11 nV/ with 4-kΩ resistors and 18 nV/ with 10-kΩ resistors.
2. Select an appropriate value for capacitor C2, which sets the filter's differential (signal) bandwidth. Set this value as low as possible without attenuating the input signal. A differential bandwidth of 10 times the highest signal frequency is usually adequate.
3. Select values for capacitors C1A and C1B, which set the common-mode bandwidth. For decent ac common-mode rejection, these capacitors should have values 10% or lower of the value of C2. The common-mode bandwidth should always be less than 10% of the instrumentation amplifier's bandwidth at unity gain.
You should build the RFI filter using a pc board with a ground plane on both sides. All component leads should be as short as possible. Resistors R1 and R2 can be common 1% metal-film units. However, all three capacitors need to be reasonably high-Q, low-loss components. Capacitors C1A and C1B need to be ±5%-tolerance devices to avoid degrading the circuit's common-mode rejection. Good choices are the traditional 5% silver micas, miniature micas, or the new Panasonic ±2% PPS film capacitors (Digi-key part number PS1H102G-ND).