From: sleeper75se (sleeper75se_at_yahoo.se)
Date: 2002-01-23 11:56:48
Hi Joerg,
I've been looking at the schematics (version 1h) for your input stage
and I have a couple of thoughts...
* The output from the instrumentation amplifier is biased by your
connecting 2V to the reference input. Internally, there are 2 x
25kohm resistors so the input resistance is 50k.
* The resistor in the HP-filter is 1Mohm.
* The ground point used in the second gain stage is connected to the
amplifier via a 10k resistor.
* The DRL provides "ground" driving for the user, and has "very high"
input resistance.
So, the total resistance that the virtual ground amplifier sees,
excluding the filter, is somewhere around (50k || 10k || 1M || 330k
|| 360k) = 7.9kohms, but this can easily be doubled if the second
gain stage is modified.
Now, this leaves us with the filter. What happens if the filter
capacitors are connected to the negative rail (or "real" ground)
instead of virtual ground? There is never any DC current passing
through the capacitor, so the DC level on the ground-side is quite
uninteresting, right?
My question: If the filter caps are connected to the negative
rail/real ground how much decoupling is really necessary for the
rest? Could not a regular opamp handle this all by itself without a
47 uF decoupling?
Regards,
Andreas
This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:37 BST