right leg driving circuit is used for common mode voltage rejection, and represent medium measured voltage which is much bigger than ECG signals and must be canceled (this could saturate the 1'st stage of input amplifier and depend of electrode potential, electrode contact and environment noise). Human body act as a big antenna...
It's used as a reference point (common or 'ground') for the ECG signals at other points on the body. My colleage told me that 'end of right leg' is used as reference because it's the farthest point from heart. May be someone could confirm why 'end of right leg' is used as reference.
The body´s displacement current flows not to ground but rather to the op-amp output circuit.This reduces the pickup as far the ECG amplifier is concerned and effectively the patient.
Its the body displacement current caused by (60Hz) EM interference on the leads. The induced current flows into the body since the body's impedance is lower than that of the inputs of the Diff-op-amp. This current is what they refer to as the displacement current.
So I've been trying to get a copy of Websters "Driven-right-leg circuit design" paper and his "60Hz interference in electrocardiography" papers to no avail. I have yet to find an adequate rational for the design of a DRL circuit. I understand what they do and all, however no one seems to care to coherently explain the selection of component values. Webster's "medical instrumentation" glosses over a lot. Might anyone be so kind as to throw me a good link or even email me the pdfs of those articles. Thanks!
- B
Hello out there.
I know it is a very late reply but I am studying articles for this topic and by this I found this thread.
And for everyone who is interested. There are several articles on IEEE (https://ieeexplore.ieee.org) where the right leg drive is described. Simply search for either "driven right leg" or "right leg drive". The only thing necessary is an account for the IEEE database otherwise you have to pay for each article. Accounts for the database are usually found at the library of universaties.
The mentioned book (Medical Instrumentation, John G. Webster [Editor]) is worth buying if you plan to develope circuits in this area.
Good luck!
I hav read tat right leg driver is designed by adding up the two leads and amplify it before giving it to the right leg ie the reference.... Can any one help me in designing tat???? In some circuit i have seen a parallel capasitor along wit the feedback resistance... Why is it so????
You guessed it almost right. It is not sum of two but normally three leads, what is called as the wilson central terminal (WCT). It is common in ECG circuits to use inverted, amplified version of WCT for driving the right leg. It doesn't matter how big a gain you give here as long as it doesn't saturate the output of your RLD amplifier.
Another use of RLD is biasing the body. Some circuits operates in bipolar supply configuration where as some other does it in unipolar way. Here the non-inverting input of the RLD amplifier should be connected to mid supply (Vcc - Vss) / 2 so that the ECG signals are always within the supply range of your other amplifiers.
Check out reference designs by TI for their delta-zigma ADC ADS1258 / ADS1298 with respect to ECG.
The capacitor in parallel to the feedback resistor makes the circuit a low pass filter, possibly to prevent the feedback from making rest of the circuit sections oscillate.
right leg driving circuit is used for common mode voltage rejection, and represent medium measured voltage which is much bigger than ECG signals and must be canceled (this could saturate the 1'st stage of input amplifier and depend of electrode potential, electrode contact and environment noise). Human body act as a big antenna...