The right leg drive amplifier or reference amplifier within the ADAS1000 is used as part of a feedback loop to force the patient’s common-mode voltage close to the internal 1.3 V reference level (VCM_REF) of the device. By using the RLD feature, it effectively centres the electrode input voltages relative to the input span, providing maximum input dynamic range.
A common question around the RLD function is related to what happens if I don't use the RLD?
The significant benefit of the RLD feature is to improve CMRR performance, thereby rejecting noise and interference from external sources such as fluorescent lights or other patient-connected instruments, and absorbs the dc or ac lead-off currents injected on the ECG electrodes.
Take a look at the response of the ADAS1000 during ECG capture from a patient simulator. This shows a 3-lead configuration of Lead I/II/III.
As you can see, when the RLD is not connected, there is significant contribution of mains noise onto the ECG signal of interest.
With RLD connected:
Without RLD connected:
More about the RLD function itself:
Within the ADAS1000, the RLD amplifier can be used in a variety of ways. Looking at the block diagram below, the area within the thick black line encompasses what's inside the ADAS1000. A number of external passive components are used outside the ADAS1000 device to configure the gain. These are left external to ensure flexibility for the customer.
One of the components, Rin, is optional. It can be provided externally, alternatively, the internal 10k series resistors shown in each electrode path can be used as this "Rin" resistor (any combination of these switches, SW1 to SW6 can be closed) with the Rin resistance value changing depending on how many are closed.
The input of the RLD amplifier comes from the CM_OUT signal, either externally via the CM_OUT pin or internally.
The DC gain of the RLD amplifier is set by the ratio of the external RFB to the effective RIN value (internal or external).
The dynamics and stability of the RLD loop depend on the chosen DC gain and the resistance and capatiance of the patient cabling. Typically loop compensation using external component will be required. In some cases, lead or lag compensation may be required.
Patient protection resistor in the RLD path
Note that the RLD amplifiers short circuit current capability exceeds the acceptable current that can be driven onto a patient (set by the various medical standards). A patient protection resistor is required in series with the RLD_OUT pin to the patient cable to achieve compliance with the medical standards.
We are using RLD function in our battery operated PCB design. But see huge baseline wander where the waveform seems to be riding on a low frequency waveform and frequently oscillates at saturation levels. We are using recommended external components in the RLD path.
Any idea what could be going wrong?