Re: [Openeeg-list] Gain line up

[Joerg H. <in...@jh...>]

Hi,

so...@pk..., 25.06.2006 23:51 :
> I have breadboarded an EEG system similar to ModularEEG, but with a
> switched cap filter replacing the op-amp "Besselworth". Since my filter
> has a gain of 1, if I don't want to add another stage, I need to put the
> extra gain in the inst-amp and op-amp gain stage. Using round numbers, if
> I want an adjustable gain of 1000-20000, I could use:
> 
> instrumentation amp 10, op-amp 100-2000

This one should be preferred.

> 
> or
> 
> instrumentation amp 100, op-amp 10-200
> 
> Is there a suggestion of one distribution over the other (or somewhere in
> between)? From the INA114 datasheet, I don't see a spec problem going with
> a gain of 100. The CMRR is better. The slew rate is longer, but not a
> problem at EEG frequencies. The biggest problem with using a higher gain
> at the inst-amp that I can figure is that it would take a smaller dc 
> offset on the input to make the ouput go to the rails.

Yes. This could make the input stage quite useless.

The brainmaster BM2E has a tricky method to have high gain in the input stage 
and block DC before it can saturate the amplifier:

Instead of a simple gain setting resistor a R-C series combination is used that 
lets the instrumentation amplifier input side act as a highpass.

>>From the INA114 datasheet, I see an alternate way to ac couple the output:
> drive the reference input with an integrator fed by the output. Has anyone
> experience with configuring an instrumentation amp like this?

Yes.

See MonolithEEG
http://openeeg.sourceforge.net/doc/hw/#othersites

and my analysis of reference integrator performance

http://www.jhansmann.de/eeg/simulations/simulations.html
INA118 with active highpass (Reference Integrator)

In short:

The reference integrator performance is often over estimated, because it has 
only effect on the output amplifier. It can not avoid saturation of the input 
stages.


> Would it be
> helpfull in reducing problems with dc offsets on the inputs? 

No.

> Right now I
> have the op-amp stage ac coupled as with ModularEEG (1uF,1M) so that all
> dc problems are pretty much confined to the inst-amp stage.

Yes.

Regards,

	Joerg