Please help, AD624 based ECG design - I'm stuck.

[mirkoslav]

Hello!

I'm trying to make a simple Android powered single channel portable ECG monitor. I was trying to use Analog Devices AD624 Instrumentation Amplifier as analog front-end, a high-Q 50/100Hz notch filter and PIC 18f4550 for USB connection/power to Android smartphone for remote monitoring. I would open source the entire project after it's done.

The problem is, I'm not a great analog circuit designer and I need some help. I'm stuck with the AD624 front end.

This is the schematic I used, the problem is the output is either railing to +9V, -9V or producing nearly a square wave (clipped 50Hz) output signal. CMRR rejection is based on similar 50Hz signal present on + and - inputs, but in my case the signal on one lead has a larger amplitude than the one on the other. I'm thinking that's what causes the unwanted results.

Also I noticed it can be negative on average with respect to the ground and having the bottom halves of the 50hz noise slightly clipped (like diode rectified). I guess that's probably negative DC offset causing one of the diodes to rectify the lower half. What could be causing this offset and the general misbehavior of the circuit?

Please help, any advice would be highly appreciated!

EDIT: Pictures say a thousand words:

AD624 pin 1 (left arm input)

AD624 pin 2 (right arm input)

AD624 pin 9 (output)

 

[crutschow]

You circuit schematic post didn't work. In the "Advanced" post widow use the Attachments (paper clip) icon to post a bitmap or GIF file (JPEGs are usually too blurry for schematics).

 

[:Dark_Force]

Hi Mirkoslav

From your schematic I guess you connected the AD624 the wrong way. A short examination of the datasheet (Page 8) showed that the Gain-Pin (Pin 3) should only be connected to ONE of the gain-setting Pins. So you connect it EITHER to 11 or 13, but not both! Try this one...

 

[mirkoslav]

Hey, thanks for Your help and input!

I don't think that's my problem. Page 3 says - "FOR GAINS OF 1000 SHORT RG1 TO PIN 12 AND PINS 11 AND 13 TO RG2". Internally, they are precision resistors and you can jumper them to a few fixed gain settings without any external components. Check out the internal diagram on page 11.

Thank you!

 

[:Dark_Force]

Sorry, my bad =)
Check out some of these documents on biopotential measurements, I guess there is some valuable information for you:
**broken link removed**
**broken link removed**

Cheers...

 

[FvM]

The amplifier is simply overloaded by 50 Hz interferences, as clearly shown by the pin1/2 waveforms. Not a problem of the amplifier circuit itself, but related to unsuitable electrode configuration, power supply etc.