SBAA188B July   2011  – October 2026 ADS1191 , ADS1192 , ADS1194 , ADS1196 , ADS1198 , ADS1291 , ADS1292 , ADS1292R , ADS1294 , ADS1294R , ADS1296 , ADS1296R , ADS1298 , ADS1298R , ADS1299

 

  1.   1
  2.   Improving Common‐Mode Rejection Using the Right‐Leg Drive Amplifier
  3.   Trademarks
  4. 1Introduction
  5. 2Common‐Mode to Differential Signal Conversion
  6. 3Techniques to Improve Common-Mode Rejection
  7. 4Closed-Loop RLD system
  8. 5The Effect of Lead‐Off Disconnection in an RLD System
  9. 6Improving CMR Using the RLD Amplifier with the ADS1298
  10. 7Conclusion
  11. 8References
  12. 9Revision History

6 Improving CMR Using the RLD Amplifier with the ADS1298

The test setup used to measure the CMR of the ADS1298-based ECG subsystem discussed in the ADS1298ECGFE-PDK User's Guide (see Ref. 3) is shown in Figure 6-1. A snapshot of the measurement setup is also provided; see Figure 6-2. A 20-V peak‐to‐peak sine wave was used as the excitation signal. The value of coupling capacitors C2 and CT (from the ac main) are equal to 100 pF. 220 pF is value of capacitor CB that connects the ac ground and the EVM board ground. Electrodes are modeled using resistor R and capacitor C connected in parallel as illustrated. Values of R and C are 51 kΩ and 47 nF, respectively.

Test Setup with ADS1298EVM with Protection Circuitry Elements Figure 6-1 Test Setup with ADS1298EVM with Protection Circuitry Elements

The standard ECG cable is used in this experiment. This cable includes a 10-kΩ protective resistance. The cable is followed by the onboard protection RC filter. The values of the components used to implement the filter are presented. All measurement results are noted with S1 = open and S2 = closed. This configuration corresponds to complete mismatch in the signal differential path. In accordance with the AAMI and IEC standards (see Ref. 4 and Ref. 5), the system CMR in this complete mismatch condition should be at least 92 dB.

Table 6-1 compares the expected and measured CMR values for different values of the protection resistor RP with and without the RLD feedback loop. The value of CP is chosen such that zero as a result of RP and CP is approximately 60 Hz. Expected values are obtained by TINA SPICE using Spice modeling of the PGA and RLD amplifier in the ADS1298.

Table 6-1 Summary of Measurement Results
No RLD Feedback
RP Measured VPP Measured CMR
1 MΩ 5.9 mVPP 70.6
100 kΩ 1.7 mVPP 81.1
75 kΩ 1.6 mVPP 82.0
50 kΩ 1.36 mVPP 83.3
With RLD Feedback (1 MΩ || 1.5 nF)
1 MΩ N/A N/A
100 kΩ 0.43 mVPP 93.3
75 kΩ 0.35 mVPP 94.9
50 kΩ 0.316 mVPP 96.0

 

 

Measurement Setup Using ADS1298EVM Figure 6-2 Measurement Setup Using ADS1298EVM

Figure 6-3 presents the CMR of the ECG subsystem at three different conditions:

  • (a) RP = 1 MΩ
  • (b) RP = 100 kΩ
  • (c) RP = 100 kΩ with RLD feedback
Output Spectrum Comparison Figure 6-3 Output Spectrum Comparison

An inverting gain configuration is achieved using the RLD amplifier available in the ADS1298. In this experiment, the values of RF and CF used are 1 MΩ and 1.5 nF, respectively. The closed-loop gain of the RLD loop depends on the number of channels that are used to detect the common-mode voltage. In this experiment, only one channel is used to sense the common potential of the input. For the given feedback resistor, as the number of channels used to measure the common-mode potential increases, the gain also increases accordingly. The nominal value of RCM used in the ADS1298 is equal to 220 kΩ. The closed‐loop bandwidth is approximately 106 Hz.

The ADS1298 provides the option to monitor the value of electrode potential through the lead-off feature. It also supports digital control, which can be used to turn on or turn off averaging of the PGA outputs. These two options can be used efficiently to ensure that whenever there is a loose contact or disconnection of the electrode, the RLD feedback is opened to avoid the railing of the bias potential at the patient’s body.