help choosing op-amps - Electronic Circuits Projects Diagrams Free

raviram87 · 15th February 2008, 03:54 PM

i am building a project that deals with the extraction of EOG signals (which are biological signals caused due to eye movements)....

These are extremely small signals with an amplitude range between 0.3mV to 1mV and frequency range of 0 to 25 hz....

so i took these signals (with electrodes), fed them to an instrumentation amplifier (AD620 set to a gain of 1000) and subsequenctly filtered the resulting signal using 4th order active low pass filters.... but still couldnt get appraciable outputs....

i used OP177 type op-amps for building the filters.... wat could be the problem??? can anyone pls help me..... wat type of op-amps can give good results???

thanks in advance... :-)

raviram87 · 15th February 2008, 04:01 PM

and i am attching the datasheet of OP177 for reference for anyone who needs further information....

Expecting ur help soon.... :-)

RadioRon · 15th February 2008, 07:41 PM

There is nothing wrong with the choices you have made for the amplifiers. The instrumentation amp is not noisy, the amps have plenty of gain, and your bandwidth is modest, so I see no problem with the amp choices.

Have you confirmed that the amplifiers are working by substituting an alternate input signal? If so, can you adjust your alternate source down to a very low level to confirm the gain of your amplifiers?

If the system works with an alternate source of signal, then you must suspect your electrodes or the physiology.

KMoffett · 15th February 2008, 10:00 PM

raviram87,

As a thought...have you attached the common in the instrumentation amp circuit to a quiet reference point, like the ear lobe, on the patient? Or are you just using the two differential inputs?

Ken

raviram87 · 16th February 2008, 03:40 AM

@Radioron...

thanks so much for ur suggestions.... let me just check ur ideas and get back to u as soon as possible...

@KMoffet...
I have attached a picture that shows u where we will be placinf the electrodes on the user.... the black coloured ground electrode has been connected to the reference input of the AD620....

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15th February 2008, 03:54 PM	(permalink)
raviram87	help choosing op-amps i am building a project that deals with the extraction of EOG signals (which are biological signals caused due to eye movements).... These are extremely small signals with an amplitude range between 0.3mV to 1mV and frequency range of 0 to 25 hz.... so i took these signals (with electrodes), fed them to an instrumentation amplifier (AD620 set to a gain of 1000) and subsequenctly filtered the resulting signal using 4th order active low pass filters.... but still couldnt get appraciable outputs.... i used OP177 type op-amps for building the filters.... wat could be the problem??? can anyone pls help me..... wat type of op-amps can give good results??? thanks in advance... :-)

15th February 2008, 07:41 PM	(permalink)
RadioRon	There is nothing wrong with the choices you have made for the amplifiers. The instrumentation amp is not noisy, the amps have plenty of gain, and your bandwidth is modest, so I see no problem with the amp choices. Have you confirmed that the amplifiers are working by substituting an alternate input signal? If so, can you adjust your alternate source down to a very low level to confirm the gain of your amplifiers? If the system works with an alternate source of signal, then you must suspect your electrodes or the physiology. __________________ RadioRon

15th February 2008, 10:00 PM	(permalink)
KMoffett	raviram87, As a thought...have you attached the common in the instrumentation amp circuit to a quiet reference point, like the ear lobe, on the patient? Or are you just using the two differential inputs? Ken __________________ "To invent, you need a good imagination and a pile of junk." Thomas A. Edison (1847 - 1931)