Post by thread:[EE] Trying to understand the "right leg driver"

On page 15 of the AD620 datasheet ( http://www.analog.com/static/imported-files/data_sheets/AD620.pdf), there's a circuit at the bottom for an electrocardiograph. I'm trying to understand how the right leg driver circuit is working. 1. Why is it called the "right leg driver"? Why can't it be the left leg? 2. Why is the inverting input of that opamp hooked up to the gain resistors of the AD620? 3. How does this work

BCC: Dr K : Yes? > 1. Why is it called the "right leg driver"? Why can't it be the left leg? Tradition / Heart relative./ ~~= Neutral point. Some other reference points would do OK but this is what was chosen for various reasons and once chosen that's what's used. Waveforms seen at all points are relative to this reference and will be different when seen wrt any other. Experts are skilled at glancing at waveforms and learning what they tell them about the heart. Measure from somewhere else and all waveforms will change. Russel

V G wrote: > I'm trying to understand how the right leg driver circuit is working. It's basically introducing a signal in the right leg to null out the common mode signal from the electrodes near the heart. This actively cancels common mode noise before it gets into circuits where it could cause trouble and be perceived as differential signal, or cause other problems like swamping the front end so that it becomes non-linear and introduces manufactured noise signals or becomes deaf to the tiny differential signal. > 1. Why is it called the "right leg driver"? Most peoples hearts are on the left side. The right leg is therefore the spot on the body farthest fron the heart and a signal there will be more common mode than one introduces elsewhere. In practise, the left leg probably works about as well. ******************************************************************** Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products (978) 742-9014. Gold level PIC consultants since 2000

On Sat, Jan 29, 2011 at 3:28 PM, Olin Lathrop <spam_OUTolin_piclistTakeThisOuTembedinc.com>wrote: > V G wrote: > > I'm trying to understand how the right leg driver circuit is working. > > It's basically introducing a signal in the right leg to null out the common > mode signal from the electrodes near the heart. This actively cancels > common mode noise before it gets into circuits where it could cause trouble > and be perceived as differential signal, or cause other problems like > swamping the front end so that it becomes non-linear and introduces > manufactured noise signals or becomes deaf to the tiny differential signal. > 1. Where is the input signal to the opamp coming from? I don't understand why the inverting input is connected to the gain selection resistors. 2. Is it critical what opamp I use for the right leg driver? Can I use an LM324

On Sat, Jan 29, 2011 at 10:28 PM, Olin Lathrop <.....olin_piclistKILLspam@spam@embedinc.com>wrote: > V G wrote: > > I'm trying to understand how the right leg driver circuit is working. > > It's basically introducing a signal in the right leg to null out the common > mode signal from the electrodes near the heart. This actively cancels > common mode noise before it gets into circuits where it could cause trouble > and be perceived as differential signal, or cause other problems like > swamping the front end so that it becomes non-linear and introduces > manufactured noise signals or becomes deaf to the tiny differential signal. > > > 1. Why is it called the "right leg driver"? > > Most peoples hearts are on the left side. The right leg is therefore the > spot on the body farthest fron the heart and a signal there will be more > common mode than one introduces elsewhere. In practise, the left leg > probably works about as well. > Not quite right. It is not about the "distance" of reference point to the heart, it's about the impedance seen between the heart and the reference point. If you are talking about inhomogeneous and isotropic structures, using geometrical distance is like taking pi as 2 (not even 3). If it's EKG what you are trying to perform, closest surface to semimembranosus or right earlobe will do the same

:: Why is it called the "right leg driver"? Because it is attached to the right leg of the subject. The right side signal is negative to the left arm signal - in other words the left arm sensor is the + connection, the right arm sensor is a - connection as is the right leg. In a full ECG machine there are normally either 12 or 15 leads. An ECG machine for the above set up takes an average of the signal between the two negative connections and the positive left wrist. Over time the positions of the lead placements has become standardised to allow proper comparisons of data obtained. Colin -- cdb, colinKILLspambtech-online.co.uk on 30/01/2011 Web presence: http://www.btech-online.co.uk Hosted by: http://www.justhost.com.au

V G wrote: > 2. Is it critical what opamp I use for the right leg driver? Can I > use an LM324? That should be fine. Strictly speaking, you don't need a right leg driver at all. However, especially when your diff amp common mode rejection is suspect, its a good idea. It's a low frequency signal that mostly just has to drive the body capacitance, so just about anything, including a LM324 should be fine, assuming it can drive with enough range. ******************************************************************** Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products (978) 742-9014. Gold level PIC consultants since 2000

On Sat, Jan 29, 2011 at 4:07 PM, Olin Lathrop <.....olin_piclistKILLspam.....embedinc.com>wrote: > V G wrote: > > 2. Is it critical what opamp I use for the right leg driver? Can I > > use an LM324? > > That should be fine. Strictly speaking, you don't need a right leg driver > at all. However, especially when your diff amp common mode rejection is > suspect, its a good idea. It's a low frequency signal that mostly just has > to drive the body capacitance, so just about anything, including a LM324 > should be fine, assuming it can drive with enough range. > Thank you. I'll make a simple circuit, test it with an oscilloscope as soon as I can get my hands on one and report back

On Sat, Jan 29, 2011 at 4:07 PM, Olin Lathrop <EraseMEolin_piclistspam_OUTTakeThisOuTembedinc.com>wrote: > V G wrote: > > 2. Is it critical what opamp I use for the right leg driver? Can I > > use an LM324? > > That should be fine. Strictly speaking, you don't need a right leg driver > at all. However, especially when your diff amp common mode rejection is > suspect, its a good idea. It's a low frequency signal that mostly just has > to drive the body capacitance, so just about anything, including a LM324 > should be fine, assuming it can drive with enough range. > > Can I also use the LM324 for the output amplifier? That is, to amplify the output signal from the AD620