Designing a High-Density EMG Acquisition System Using AD620 Instrumentation Amplifiers

A Comprehensive Guide for Reliable, High-Fidelity Bioelectric Signal Capture

Key Takeaways

Optimal Biasing and REF Configuration: Correctly biasing the AD620 inputs and configuring the REF pin are crucial for maximizing signal integrity in both single and dual supply operations.
Robust Circuit and PCB Design: Implementing effective grounding, shielding, and decoupling strategies minimizes noise and crosstalk, ensuring high-fidelity signal acquisition.
Comprehensive Safety and Filtering: Incorporating safety measures and precise filtering techniques safeguards human subjects and ensures clean, usable EMG signals for microcontroller processing.

1. Single-Supply vs. Dual-Supply Operation

1.1 Single Supply (3.3 V or 5 V)

Input/Output Biasing

When operating the AD620 with a single supply, it is essential to establish a mid-supply reference voltage to center the input and output signals within the ADC's readable range. For a 3.3 V system, a mid-supply reference of 1.65 V is ideal.

Connect the AD620's REF pin to a mid-supply voltage (e.g., 1.65 V for 3.3 V systems) using a voltage divider and buffer.
Attach 1 MΩ resistors from both the positive (+IN) and negative (–IN) inputs to the mid-supply reference to maintain the common-mode voltage within the acceptable range.

Gain Resistor

Setting the appropriate gain is critical for amplifying the small EMG signals. A 47 Ω resistor between Pins 1 and 8 of the AD620 sets the gain to approximately 1000×, which is suitable for most EMG applications.

Saturation & Swing Limits

In single-supply operation, the AD620's output will swing around the mid-supply voltage. Expect the output to vary typically within ±1.2 V around the 1.65 V reference, depending on the load and gain settings.

1.2 Dual Supply (e.g., ±5 V, ±9 V, or ±12 V)

REF Pin to Ground

Connecting the REF pin directly to ground allows the AD620 to output signals that swing symmetrically around 0 V, utilizing the full dynamic range provided by the dual supplies.

No Need for Mid-Supply Bias

With dual supplies, the AD620 can handle inputs that swing both positive and negative relative to ground, eliminating the need for a mid-supply bias. This configuration maximizes the input common-mode range and output swing.

Arduino Interface Considerations

Since Arduino Nano 33 BLE operates with a single-supply ADC (0–3.3 V), directly interfacing a dual-supply AD620 output is not feasible. To address this:

Implement a level-shifting or offset circuit using operational amplifiers to translate the dual-supply output into the single-supply range.
Alternatively, adjust the microcontroller's reference point in software to accommodate the shifted signals.

2. Pinout and Component Selection

2.1 AD620 Pin Configuration

Pin	Connection Details
1 (RG)	Connect a 47 Ω resistor between Pin 1 and Pin 8 to set the gain to ~1000×.
2 (–IN)	Series with a 1 kΩ resistor to the negative electrode. Connect a 1 MΩ resistor to mid-supply (single-supply) or ground (dual-supply).
3 (+IN)	Series with a 1 kΩ resistor to the positive electrode. Connect a 1 MΩ resistor to mid-supply (single-supply) or ground (dual-supply).
4 (–VS)	Connect to ground (single-supply) or to the negative rail (dual-supply).
5 (REF)	Single-supply: Connect to mid-supply (e.g., 1.65 V) with a 0.1 µF capacitor to ground. Dual-supply: Connect directly to ground.
6 (OUTPUT)	Connect to the analog input on the microcontroller (e.g., A0). Optionally add a low-pass filter consisting of a 100 Ω resistor in series and a 10 nF capacitor to ground.
7 (+VS)	Single-supply: Connect to +3.3 V or +5 V with a 0.1 µF ceramic and a 10 µF electrolytic capacitor to ground. Dual-supply: Connect to the positive rail.
8 (RG)	Connect to the other side of the 47 Ω gain resistor from Pin 1.

2.2 Reference Voltage Generation

Generating a stable mid-supply reference is vital for single-supply operations. This can be achieved using a voltage divider and buffering.

Use two 47 kΩ resistors in a voltage divider configuration from +3.3 V to GND.
Buffer the mid-point with a voltage follower op-amp to stabilize the reference voltage.
Add a 10 µF capacitor to the buffered reference to filter out any noise.

2.3 Protection Strategies

Protecting the amplifier inputs from potential damage due to excessive voltages or electrostatic discharge (ESD) is essential for reliable operation.

Place 1 kΩ resistors in series with each input electrode close to the AD620 to limit current in case of voltage spikes.
Use TVS diodes or Schottky diodes on the input lines to clamp any transient voltages and provide ESD protection.
Implement shielded cables for electrodes to minimize electromagnetic interference (EMI).

3. Circuit Layout for 16 AD620 Modules

3.1 Modular Approach

Designing a scalable and maintainable system involves creating a modular schematic that can be replicated for each AD620 channel.

Create a single “channel” schematic comprising all necessary components and connections for one AD620 module.
Replicate the channel schematic 16 times, ensuring uniformity across all modules.
Distribute the mid-supply reference voltage to each AD620’s REF pin and input bias resistors uniformly.

3.2 Noise Reduction

Minimizing noise and crosstalk between channels is critical for high-fidelity signal acquisition.

Physically separate each amplifier block on the PCB to prevent electromagnetic coupling.
Route signal traces short and away from high-frequency digital lines or power rails.
Utilize ground segmentation or star grounding to isolate sensitive analog signals from noisy digital grounds.

3.3 Grounding and Shielding

Implementing effective grounding schemes enhances the system's immunity to external noise.

Adopt star grounding where all ground connections converge at a single point to avoid ground loops.
Use a large, continuous ground plane on the PCB to provide a low-impedance path for return currents.
Enclose sensitive analog sections within shielded areas or use ground traces between signal layers for additional protection.

4. PCB Design Guidance

4.1 2-Layer vs. 4-Layer PCB

Choosing the appropriate PCB layer configuration significantly impacts noise performance and signal integrity.

2-Layer PCB: Suitable for simpler designs but poses challenges in maintaining clean ground planes and minimizing interference.
4-Layer PCB (Recommended):
- Top Layer: Signal routing and placement of components.
- Inner Layer 1: Dedicated ground plane to provide a shield and return path.
- Inner Layer 2: Power plane (e.g., +3.3 V or dual supplies) to ensure stable voltage distribution.
- Bottom Layer: Additional signal routing or low-noise analog traces.

4.2 Decoupling

Proper decoupling reduces power supply noise and enhances overall system stability.

Place 0.1 µF ceramic capacitors as close as possible to each AD620's power pins to filter out high-frequency noise.
Implement bulk capacitors (10 µF–47 µF) at the board-level power entry to stabilize voltage levels during transient conditions.

4.3 Via Stitching

Via stitching connects multiple ground planes through vias to reduce impedance and improve noise immunity.

Utilize multiple vias around the perimeter of the PCB and near sensitive components to ensure a robust ground connection.
Place ground vias near each AD620 module to provide a low-impedance path for return currents.

5. Safety Considerations

5.1 Isolation

Ensuring the safety of human subjects is paramount in bioelectric signal acquisition systems.

Incorporate isolation amplifiers or optocouplers to electrically separate the EMG acquisition circuit from the microcontroller and power sources.
Use isolated power supplies or DC-DC converters to prevent ground loops and potential electrical hazards.

5.2 Medical-Grade Supplies

Utilizing medical-grade components and power supplies minimizes risks associated with bio-signal measurements.

Opt for power supplies certified for medical use with minimal leakage currents to ensure safe operation.
Ensure all components in contact with the human body, such as electrodes, comply with medical safety standards.

5.3 Current Limiting

Limiting current prevents excessive current flow that could harm both the user and the equipment.

Implement 1 kΩ series resistors on each input to limit fault currents in case of inadvertent short circuits or high-voltage transients.
Use high-value pull-up resistors (e.g., 1 MΩ) to further restrict current flow during abnormal conditions.

6. Filtering & Signal Conditioning

6.1 Low-Pass Filtering

Low-pass filters attenuate high-frequency noise, ensuring that only relevant EMG signals are processed.

At the AD620 output, add an RC low-pass filter with a 100 Ω resistor in series and a 10 nF capacitor to ground. This configuration targets frequencies above 500 Hz for attenuation.
Consider implementing active low-pass filters using operational amplifiers for more precise control over the cutoff frequency.

6.2 Anti-Aliasing

Preventing aliasing is crucial for accurate digital representation of bioelectric signals.

Implement an anti-aliasing filter with a cutoff frequency slightly below half the sampling rate. For a sampling rate of 2 kHz, set the cutoff around 1 kHz.
Ensure that the ADC's input range accommodates the filtered signal without clipping.

6.3 EMG Frequency Range Considerations

EMG signals typically range from 20 Hz to 500 Hz. Designing filters and selecting sampling rates within this range ensures accurate signal capture.

Choose a sampling rate of at least 1–2 kHz to provide sufficient temporal resolution for EMG analysis.
Focus filtering efforts on the 20–500 Hz band to isolate meaningful muscle activity signals.

7. Scalability & Testing

7.1 Expanding to 16 Amplifiers

Scaling the design to accommodate 16 AD620 amplifiers and 32 electrodes requires careful planning to maintain signal integrity and manage power distribution.

Use hierarchical sheets in PCB design software like KiCad to replicate the channel schematic efficiently.
Ensure that the power supply can handle the cumulative current draw of all 16 amplifiers. For example, verify that the 3.3 V supply can deliver sufficient current without significant voltage drops.
Maintain consistent component placement and routing practices across all modules to simplify troubleshooting and assembly.

7.2 Testing Procedures

Validating the performance of the EMG acquisition system before human application ensures reliability and safety.

Begin with a prototype containing 2–4 channels to verify noise levels, layout clarity, and signal integrity.
Use an oscilloscope to monitor each channel for signs of crosstalk, oscillations, or unexpected noise patterns.
Perform signal quality tests by simulating EMG signals to assess the amplifier's response and filtering effectiveness.

7.3 Firmware and Code References

Implementing efficient firmware on the Arduino microcontroller is essential for accurate EMG data acquisition and processing.

Develop firmware that utilizes the Arduino's ADC with appropriate oversampling or hardware filtering to minimize noise.
Implement data buffering and transmission protocols compatible with BLE to ensure seamless data flow.
Reference existing open-source EMG processing libraries for guidance on signal conditioning and analysis.

8. Additional Research-Level Optimizations

8.1 Driven Right Leg (DRL) Circuit

Incorporating a DRL circuit enhances common-mode rejection, further improving signal quality.

The DRL feedback actively cancels out common-mode noise by driving the right leg electrode with an inverted version of the common-mode signal.
This technique reduces interference from power line noise and other external electromagnetic sources.

8.2 High-Resolution ADC

Utilizing high-resolution ADCs increases the signal-to-noise ratio (SNR), enabling more precise EMG signal capture.

External 16-bit or 24-bit ADC modules offer better performance compared to the built-in ADCs of microcontrollers like the Arduino Nano 33 BLE.
Connecting these ADCs via SPI or I²C interfaces allows for high-speed, high-accuracy data acquisition suitable for detailed EMG analysis.

8.3 Advanced Enclosure & Shielding

Protecting the entire system from electromagnetic interference (EMI) ensures consistent performance.

Enclose the PCB within a metal casing grounded at a single point to shield against external EMI sources.
Use shielded connectors and cables for all external connections, including electrodes and power inputs.

8.4 Active Low-Pass Filter

Adding an active low-pass filter stage allows for dynamic tuning of the filtering characteristics based on application needs.

Implement an operational amplifier-based active filter following the AD620 output to provide sharper cutoff frequencies and steeper roll-offs.
Adjust filter parameters to adapt to varying EMG signal requirements without compromising the fidelity of the captured data.

Conclusion

Designing a high-density EMG acquisition system using AD620 instrumentation amplifiers involves meticulous attention to biasing, component selection, circuit layout, and safety considerations. By implementing a single or dual-supply configuration appropriately, selecting precise resistor and capacitor values, and adopting robust PCB design practices, a reliable and high-fidelity EMG system can be achieved. Additionally, integrating advanced optimizations like DRL circuits and high-resolution ADCs further enhances signal quality and system performance. Rigorous testing and scalability planning ensure that the design remains robust, adaptable, and safe for human application, culminating in a professional-grade EMG acquisition solution suitable for microcontroller-based data acquisition and BLE transmission.

References

AD620 Datasheet - Analog Devices
Standards of Instrumentation of EMG - ScienceDirect
Designing a Low-noise, High-resolution, and Portable Four Channel EMG System - PMC
AD620 Product Info - Analog Devices
High-Density Surface EMG: Techniques and Applications - ScienceDirect
Design of a Flexible High-Density Surface EMG System - IEEE Xplore
Design of Heart Rate Detection System Based on AD620 Chip
Real Time Multichannel EMG Acquisition System (IJSTE)
AD620 Datasheet, Pinout, User Guide
AD620 Gain Calculation and Circuit Guidelines (Chegg)