[Openeeg-list] LT1168 as a INA114 Replacement

[Stefan J. <bio...@gm...>]

Mike

The transistor clamps should have protected your INA amp from static,
I use clamping diodes in my design and have never really analysed the
transistor design.  I'm sure Joerg will on your email as well.

As far as INA replacements are concerned you can look at the following devices:

INA114  (modEEG)
INA118  (monolithEEG)
INA128
INA129
AD620
LT1168

INA = BurrBrown/TI, AD = Analog Devices, LT = Linear Technology

LT is normally the most expensive (I am talking generally speaking,
not specifically instrumentation amps),  while TI has an excellent
free sampling service. On my list the LT1168 is $7.10 while a INA129
is $3.05!  But this might not be an absolute fair price comparison as
there might be date and quantity discount differences.

ModEEG is NOT (normally) battery operated AND it has all it's stages
AC coupled.  This means low drift and low bias currents is not quite
as important as on a DC coupled amplifier.  Current consumption is
also unimportant.

INA114 is for low drift but is rather more noisy.  INA118 is for low
voltage and/or low power consumption. I would go for INA129 or INA128
because these are cheaper with good noise performance.  When using
INA128 you will have a slight gain error, small enough to either
ignore, or you could compensate with one of the gain setting resistors
further down the circuit, or indeed the 2x gain setting resistors (the
latter if you can obtain E96 range resistors).  Other than gain,
INA129 = INA128.

AD620 = INA129.

It is not easy to compare INAs from different manufacturers, as the
critical noise figures are normally expressed differently so it's like
comparing apples with pears.  From what I can remember, the AD620 had
the slight edge iro noise performance. You need to look at the noise
graphs in the datasheet and you need to consider the gain of the INA,
I think it's 12.5 for modEEG.

Talking about noise, theoretically it would have made sense to
increase the gain of the INA stage to something closer to x100.  But I
did tests recently and (in practice) found no real difference.

Generally speaking, all these devices should give good results.  So
base your decision on price, availability and personal manufacture
preference.

Please note that I did not consider ESD protection in this comparison.

Let's see what Joerg has to say...

Regards
Stefan