Mail*Link(r) SMTP FWD>Analog guard rings
Hi David,
Technically you not want to bridge the electrical spacing between the guard
rings and input terminals; and depending on how critical the design
requirements, you also should avoid doing it on the internal layers.
If you use solder resist (in lieu of a good conformal coating) it will absorb
moisture and collect surface contaminants that will bridge over the guard
rings and provide an alternative path for leakage currents/voltages between
the input terminals and adjacent conductive patterns/components.
The following is more information that expands on the design considerations
for guard rings that I did in Chapter 13, of the 4'th edition of the Printed
Circuits Handbook, edited by Coombs.
You will also need to consider adding guard rings around the input terminals
in the power/ground layers if you are using PTH/IMT (plated-through hole /
insertion mounted technology). Even with SMT, you may need to consider
"isolating" the power/ground plane directly underneath and adjacent to the SMT
lands and electrically connecting it to you guard rings. Effectively, what
you are trying to do is to form a "Faraday Cage" around the critical input
circuitry to isolate the contents for all undesired voltages.
As you are aware in using guard rings, it cannot be overemphasized about using
a low-impedance voltage source for the guard ring(s) (or Faraday Cage) that
follows the input voltage and minimizes the differential voltage between the
guard ring and op-amp's input signal. As we know from Ohm's Law electrical
theory, if there is no voltage difference, there is no current - reguardless
of resistance -- and that's the goal of guard rings, follow the input voltage
at the op-amp terminals (which for non-inverting (buffer) amplifiers may
include additional components that are at the same voltage as the input
signal), especially with differential input instrumentation amplifier signals
.
If the "bulk" resistance of the base material is a concern, then include a
"ring" of PTH's around the input terminals (in the guard rings) between all
internal conductive patterns (layers). This controls the leakage
currents/voltages between PTH barrels and other internal conductive patterns.
Check to see if the op-amp has an alternative "package" with a pair of un-used
(no-connection) terminals adjacent to the input terminals. If they do, try to
use it because this enhances the use and addition of guard rings by providing
additional space around the critical input terminals.
One of the other areas of concern for "serious" printed circuit designs are
the balance terminals for "zeroing out" the input off-set voltage. In many
op-amps these terminals are internally connected to the load/source
resistors/circuitry of the op-amps input transistors/FET's as such, this can
be another source of "stray" voltage/currents that may effect functional
operation.
Hope this helps,
Ralph Hersey
[log in to unmask]
--------------------------------------
Date: 8/21/96 12:03 PM
From: David M Fulmer
Topic: Surface moisture affecting operational amplifier output
drift on a FR-4 multi-layer PCB.
(board layup) - 1). component layer, 2). ground plane, 3).
power plane, 4). solder side layer
Question: When shielding the input pins of an operational
amplifier with guard rings, is it recommended to
expose the guard traces (solder over tin reflow), rather than
covering them with solder mask ?
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