In DC to low-frequency sensor signal conditioning applications, relying solely on the common-mode rejection ratio (CMRR) of an instrumentation amplifier is not enough to ensure effective noise suppression in harsh industrial environments. To prevent unwanted noise from propagating into the system, it's essential that the components in the low-pass filter at the input of the instrumentation amplifier are properly matched and adjusted. This allows for internal EMI/RFI filtering and enhances CMRR, ensuring that the noise is reduced to an acceptable signal-to-noise ratio (SNR).
As an example, consider the low-pass filter shown in Figure 1. The resistive sensor is connected to a high-impedance instrumentation amplifier through a network consisting of RSX and CCM. Ideally, if the CCM capacitors on each input pin are perfectly matched, the common-mode noise will be significantly reduced before reaching the INA input.
When the common-mode filter capacitor (Ccm) is perfectly matched, the noise is almost completely attenuated. Figure 2 illustrates this result from a TINA SPICE simulation where a 100mVpp, 100kHz common-mode error signal is injected into the INA333 input.
However, off-the-shelf capacitors typically have a tolerance of 5% to 10%. If the Ccm values on each pin do not match, the differential tolerance can reach as high as 20%. Figure 3 clearly shows the impact of capacitor mismatch, highlighting how the common-mode noise (eN) at the output of the resistive sensor affects the system.
This input mismatch (ΔC) creates a cutoff frequency error, allowing common-mode noise eN to differentially enter the INA input and eventually become an error voltage at the output. Equations 1 through 3 illustrate the amount of common-mode noise that reaches the input.
Assuming the sensor signal frequency (Vsensor) is much lower than the noise-cut frequency of all common-mode filters (i.e., fC ≥ 100 × fsensor), and that RS1 = RS2, the differential noise becomes a common-mode noise signal (eN) at VIN, as shown in Equation 4.
Equation 4 further demonstrates that when a 100mVpp, 100kHz common-mode error signal is applied to the INA333, and there is a 10% RC mismatch at a 1.6kHz cutoff frequency, the resulting errors can be significant.
Figure 5 presents a more efficient and commonly used input filtering method, which involves adding a differential capacitor (Cdiff) between the instrumentation amplifier inputs. Although this approach doesn't fully resolve the issue, Cdiff must be carefully selected based on two key criteria:
1. The cutoff frequency should be high enough to avoid the signal bandwidth and ensure stable filtering.
2. The differential cutoff frequency must be low enough to reduce common-mode noise to an acceptable level, allowing the instrumentation amplifier’s CMRR to handle residual noise and achieve an acceptable SNR.
Equation 5 outlines the general principle for adjusting Cdiff. Figure 6 compares the VinP and VinN plots with and without Cdiff. It's clear that without differential capacitance, the output of the INA333 varies, and this difference is amplified, contributing to noise that reduces the SNR. When Cdiff = 1 μF, the difference between VinP and VinN is minimized.
Figure 7 highlights the overall improvement in noise performance of the INA333 output when Cdiff = 1 μF.
In summary, the low-pass filter placed at the front end of the instrumentation amplifier should include a differential capacitor that is at least 10 times larger than the common-mode capacitor. This helps reduce the impact of Ccm mismatches, converting common-mode noise into differential noise and significantly improving filter efficiency.
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