Re: electrical problems with Galvanic Skin Response circuit ( GSR SCL )

Hi Stefan,

after 14 days of waiting, finally the new AD623 arrived here.
But unfortunately the new IC shows the same behaviour.
To exclude all possible sources of error, i built the circuit alone with 
only the necessary parts, once on a breadboard and another time i 
soldered the components directly together.
The result was the same, see the attachment. The gain is to little and 
changes with the input voltage, what should not happen.
Furthermore i couldn't see any oscillation in HF-range on my Oscillocope.
By the way, why can't i use an ordinary OPamp instead of the AD623, only 
because of the rail to rail feature?
I am at a loss, what else can i do?

regards Steffen


> Steffen,
>
> Rail-to-rail opamps cannot reach quite to the rails, I would have expected
> perhaps within 100mV from the top rail, but I checked a few datasheets and
> some devices come surprisingly close, really only a few mV.
>
> The AD623 is supposed to be a rail-to-rail INA, but I see it performs
> rather poorly in reaching the rails, the datasheet gives a worst case of
> 500mV below Vcc.
>
> Your opamp output swing up to 4.7V sounds a bit low, but I would not worry
> too much about that.  It might be attributable to measuring errors,
> rounding off errors or lower than anticipated VCC.  For instance, the
> LM78xx type of regulators have a poor output voltage tolerance.  You might
> use a LM7805 which you assume gives VCC = 5.00V and if you measure, find
> the voltage only to be 4.82V.  Obviously your opamp would then swing
> proportionally lower than what you could expect from a true 5.0V supply.
>
> I can't think of anything that could have your INA misbehave in the way you
> describe.  Do you have decent decoupling capacitors over your opamp and
> INA?  If you don't, there is always a possibility that a component might
> oscillate at a high frequency. A newbie might possibly miss this even when
> using a scope.  If an IC oscillates then you typically get these sort of
> non-sense measurements.
>
> Perhaps you should try to replace the INA with a new part.  Ideally from a
> different batch.  The incidence of grey, copycat, etc. components is ever
> present.  At work we had an incidence where an IC just didn't work.  After
> desoldering from the circuit, it was found that all pins where
> open-circuit.  The IC was cut open and it was established that it didn't
> have the actual chip inside, it was simply an empty plastic package (!)
> Thank you China-man.  I once received some Texas Instruments INA128
> instrumentation amplifiers and each and every one in the packet of about 20
> was faulty.   With the INA128s they where obviously faulty but sometimes
> components seem to work - sort off.  So you start suspecting all sorts of
> other components and debugging becomes a nightmare.
>
> Electrostatic damage due to incorrect component handling is also a
> possibility.  That too often means not total component failure but often a
> limping component that just doesn't seem to perform right.
>
> In mixed signal circuitry it is common to call the supply rail for the
> analog circuitry 5VA, the A meaning analog.  The microprocessor on the same
> PCB might also run from 5V and then is often run from a separate supply to
> minimise noise.  So the micro's supply you would then call 5VD, the D
> indicating that it is for the digital sub-system.  In a similar vain one
> often calls a supply +VHT for some high voltage line (HT = high tension) or
> you might give a label such as 5VRF for your RF tuner module supply,
> 3V3_LED for your display supply, etc.
>
> The ISO is indeed an electrically isolated line from a DC/DC converter.
>
> Regards
> Stefan
>
>
>
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