# Measuring ECG signals

1. Nov 6, 2012

### 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)

2. Nov 6, 2012

### Staff: Mentor

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/tocs/tocs.asp?fn=0544.toc [Broken]

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.

Last edited by a moderator: May 6, 2017
3. Nov 6, 2012

### Enthalpy

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.

4. Nov 7, 2012

### samaaa

i will use AD620a for preamplification .

5. Nov 7, 2012

### samaaa

why

i don't understand what you mean,can you explain more?

6. Nov 7, 2012

### Carl Pugh

You have already realized that only potential differences can be measured, have you?

samaaa, this was a trick question and you flunked.

Last edited by a moderator: Nov 7, 2012
7. Nov 7, 2012

### trini

he can just reference each to ground guys don't give him a hard time

8. Nov 7, 2012

### marcusl

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.

9. Nov 7, 2012

### samaaa

Carl Pugh , my english is not strong enough to understand trick question