Guest
On Sep 25, 6:00 pm, Glen Walpert <nos...@null.void> wrote:
4th edition I have.
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Yep, I've seen some engineering flops because they had no idea how to
lay out a circuit. These books are a great help.
One thing interesting about star grounds when a shield is
involved is it seems to work better if there is also a lot of copper
at the node where the sheild is connected. And don't use the sheild as
a bridge to other grounds, the solder cracks from temp changes and
poof.
Its too bad you can't buff the signal right on the glass
electrode. Perhaps mount a pc board on the electrode and hermetically
seal it. The connecting wires can probably carry the supply supplies
and the output signal. It might solve a lot of problems.
Have fun
Alan
haven't seen this 5th edition, but it is probably better than theOn Wed, 24 Sep 2008 07:12:01 -0700 (PDT), wolti_At <wo...@sil.at
wrote:
On 19 Sep., 10:43, Comcast1...@gmail.com wrote:
On Sep 17, 3:38 pm, wolti_At <wo...@sil.at> wrote:
Hello,
I am currently working on a design where I need to amplify the signal
generated from a glass electrode and a reference electrode. Right now
I have a basic input amplifier with an instrumentation amplifier from
analog devices working. The challenging task on these designs is the
very high output resistance of the glass electrodes (up to 200Meg) and
the noise immunity. The input stage is very simple and all input
traces have a guard ring which is biased to the common mode voltage
(to reduce leakage). The inamp currents are handled by a connection to
the media which the probe is placed in.
Since this is my first design with electrodes I heard a bit around on
the net and some people seem to have a different input stage which is
based on a capacitor and an integrator. I wonder if someone has seen
such other circuits because it would be interesting for me to evaluate
other options.
Thanks for any input,
Christian Walter
Christian, Guard rings work great for preventing leakage and they work
best if the guard ring voltage tracks to voltage of the source. Let's
use a peak detector as an example, tie the cap to the + of an op amp
configured as a unity gain buffer and tie the output of the op amp to
the guard ring. The guard ring voltage tracks the cap voltage. Does
this help you?- Zitierten Text ausblenden -
- Zitierten Text anzeigen -
Hello,
One more question regarding the guard rings and driving the shield.
1) For the shield I can drive the shield with the same voltage as the
inner conductor. I would do this by an simple opamp because I can not
directly connect to the gain input of the inamp (would reduce common
mode performance) and a small series resistors for damping. In this
case I would reduce leakage but I think there could be stability
problems (multiple opamps and the shield also couples to the inner
conductor by a capacitance).
Indeed, stability needs to be considered with any amplifier. Nothing
special about this case as far as I can see.
2) I could drive both shields with the common mode voltage of the two
inputs. I have seen this at least in one application sheet. This will
give me some leakage currents because the potential is not the same
for the conductor or the shield but maybe this is better in some way.
What is the better solution? The same applies for the guard rings but
for the guard rings I would drive them with the same voltage.
Since you have 2 separate cables to your 2 separate electrodes it
might be best for each shield to track its own signal. But you would
be much better off reading the grounding and shielding book I
recommended and doing the analysis yourself, considering the details
of your specific situation, than accepting my guess.
http://www.amazon.com/Grounding-Shielding-Circuits-Interference-Techn...
I haven't seen this 5th edition, but it is probably better than the
4th edition I have.- Hide quoted text -
- Show quoted text -
4th edition I have.
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
Yep, I've seen some engineering flops because they had no idea how to
lay out a circuit. These books are a great help.
One thing interesting about star grounds when a shield is
involved is it seems to work better if there is also a lot of copper
at the node where the sheild is connected. And don't use the sheild as
a bridge to other grounds, the solder cracks from temp changes and
poof.
Its too bad you can't buff the signal right on the glass
electrode. Perhaps mount a pc board on the electrode and hermetically
seal it. The connecting wires can probably carry the supply supplies
and the output signal. It might solve a lot of problems.
Have fun
Alan