From: Joerg Hansmann (info_at_jhansmann.de)
Date: 2002-01-07 18:51:50
Hi Andreas,
----- Original Message -----
From: sleeper75se <sleeper75se_at_yahoo.se>
To: <buildcheapeeg_at_yahoogroups.com>
Sent: Monday, January 07, 2002 9:25 AM
Subject: [buildcheapeeg] Re: Input protection
...
> But... :-)
>
> You've only measured DC-impedance,
Yes. I have had trouble setting up the AC-analysis with
SWCAD3... (The GUI is quite different from MICROSIM)
> and unfortunately we don't get
> those fabulous numbers for AC.
You are right. But the results are nevertheless
very usable:
About 25.6 MegOhms impedance at 50Hz with C1=100pF and
about 130 MegOhms impedance at 50Hz with C1=0pF.
As you can see the impedance depends largely on C1.
That is the price for HF suppression (562kHz -3dB BW)
What is missing in this simulation is the effect of the
DRL. It will enhance the common mode impedance by
its gain (e.g. 1000, giving 12.8 GOhms impedance
( (25.6 Meg || 25.6 Meg ) * DRL_gain)
).
Differential mode impedance will only be twice in
the final circuit (impedance sum of + and - input circuit)
ca. 50MegOhms
> So in the specifications we'll have to write:
>
> Input impedance: It depends.
Yes... On C1 and the DC offset...
> It will probably work quite nicely anyway, but perhaps not with
> INA114
I think it is ideal for INA114.
Unlike high ohmic protection resistors the new protection circuit
is not very susceptible for the INA114 input noise current.
> - it would be too noisy.
Why ?
I do not understand the reason.
Regards,
Joerg
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