# Measuring ECG signals

by samaaa
Tags: measuring, signals
 P: 48 Hi guys:I have a project to design ECG i want to calculate the six leads(I,II,III,aVR,aVL and aVF) by this method: i will measure the voltage of three probe or electrode(RA,LA and LL) RL as a reference by using DAC(digital to analog converter) the DAC connected to PC computer, and then calculate(I,II,III,aVR,aVL and aVF) mathematically(by using matlab) according to these equations: I=LA-RA,II=LL-RA,III=LL-LA,aVR=-((I+II)/2),aVL=I-(II/2) and aVF=II-(I/2) so,what's your opinion about this method versus classical method(other methods)???
Mentor
P: 40,935
 Quote by samaaa Hi guys:I have a project to design ECG i want to calculate the six leads(I,II,III,aVR,aVL and aVF) by this method: i will measure the voltage of three probe or electrode(RA,LA and LL) RL as a reference by using DAC(digital to analog converter) the DAC connected to PC computer, and then calculate(I,II,III,aVR,aVL and aVF) mathematically(by using matlab) according to these equations: I=LA-RA,II=LL-RA,III=LL-LA,aVR=-((I+II)/2),aVL=I-(II/2) and aVF=II-(I/2) so,what's your opinion about this method versus classical method(other methods)???
I believe you mean "ADC". A DAC is an output device, and an ADC is an input device.

You will need to do some preamplification between the ECG pads and the ADC inputs. Instrumentation amplifiers are traditionally used for this function...

You also need to consider patient safety in such a design. UL544 is the applicable standard in the US, for example:

http://ulstandardsinfonet.ul.com/toc...sp?fn=0544.toc

You have to be very careful not to let any bias voltages show up between pads on a patient -- they have very low impedance access to the patient's body.
 P: 661 I hope I didn't understand... You have already realized that only potential differences can be measured, have you? For instance, you can measure I, II and III. These voltages are tiny and must be shielded properly before careful preamplification.
P: 48
Measuring ECG signals

 Quote by berkeman I believe you mean "ADC". A DAC is an output device, and an ADC is an input device.

 Quote by berkeman You will need to do some preamplification between the ECG pads and the ADC inputs. Instrumentation amplifiers are traditionally used for this function...
i will use AD620a for preamplification .
P: 48
 Quote by Enthalpy I hope I didn't understand...
why

 Quote by Enthalpy You have already realized that only potential differences can be measured, have you?
i don't understand what you mean,can you explain more?
P: 384
 Quote by Enthalpy I hope I didn't understand...
You have already realized that only potential differences can be measured, have you?

samaaa, this was a trick question and you flunked.
 P: 208 he can just reference each to ground guys don't give him a hard time
 Sci Advisor PF Gold P: 2,080 Listen to Berkeman's advice. To protect the body against all faults, no matter how unlikely, it is common and prudent practice to run the instrumentation amplifier from a battery and couple it to the rest of the system through an opto-isolator. Then if your PC power supply shorts, or a meteor hits your computer and it melts down putting 110V onto your ECG amplifier, the human subject cannot be shocked. This is particularly important since ECG gel electrodes are designed to provide a low resistance connection to the body, thus vastly lowering the electrocution threshold.
 P: 48 Carl Pugh , my english is not strong enough to understand trick question trini and marcusl thank you for your advice
 P: 563 As I read this RL is the Reference - A Relatively neutral point on the Body - regarding Heart function. The calculation of I, II and III are highly dependent on these points being linear, between RA and LA for example. So this - while technically valid, is probably not very consistent patient to patient - which is important. In the US this is EKG - I have seen have 6 to 8 electrodes - MANY very smart people have tried to simplify this but the subject is too variable - Humans vary too much to make more assumptions).

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