Abstract
This article presents the analysis and design guidelines for the conventional common-mode/differential- mode passive EMI filter typically implemented in ECG and bioimpedance (BioZ) AFE circuits. Details are illustrated of how an unbalanced EMI filter facilitates common-mode noise bleed into the differential signal path, thereby reducing SNR performance. This is referred to as common-mode to differential-mode conver- sion (CM-to-DM conversion). Through the judicious choice of components, a designer can mitigate related SNR degradation while providing appropriate signal filtering for ECG and BioZ AFEs.
What You’ll Learn
- Learn how to analyze the transfer function of a CM-to-DM filter.
- Identify noise sources that can potentially corrupt perfor- mance in unbalanced filter circuits.
- Gain understanding of common-mode to differential-mode conversion.
- Learn how to set the common-mode filter bandwidth and the differential-mode filter bandwidth.
- Recommended filter settings for use with the MAX3000x ECG and BioZ AFE devices.
Introduction
This article presents an analysis and discussion of the performance limitations due to imbalances in the conventional common-mode (CM)-to-differential-mode (DM) passive filter. Figure 1 shows the schematic diagram of a typical circuit implementation of the MAX30001 electrocardiogram (ECG) analog front end (AFE). The two external electromagnetic interference (EMI) filters (one highlighted in blue) shown in Figure 1 are conventional CM-to-DM filter circuits.
The referenced external EMI filters (implemented with conventional CM-to-DM filter circuits) offer both common-mode and differential-mode bandwidth limiting. Moreover, by judiciously choosing just one component value (the differential mode capacitor), a designer can mitigate signal-to-noise ratio (SNR) degradation due to the imbalance of the common-mode signal path. Not bad for just five passive components!
Before diving into this circuit, let’s briefly discuss what exter- nal EMI sources can be anticipated. EMI is a circuit disturbance associated with external sources of electromagnetic induc- tion (such as magnetic coupling), electrostatic coupling (such as capacitive coupling), or conduction. Fundamentally, EMI can couple into circuits via radiation and/or conduction. Figure 2 shows the frequency spectra with examples of several common sources of EMI.
Read the full article here.
