It is said that analog and digital ground lands should be connected together at just one loaction on a PCB.  THis makes eminent sends because it prevents AC currents in the digital ground land from flowing through the analog ground land..

[By the way, I call them lands rather than planes because they will be side by side – not one over the other.  In fact, sandwitching the analog and digital ground planes would undesireably cause digital ground plane AC voltages (yes, AC currents in the plane will cause some AC voltages)  to capacitively couple to the analog plane, and there could be magnetic coupling as well.]

For this separation of analog and digital ground lands to work, there must be no other ground loops.  FOr example if several boards plug into a backplane, it would create loops, if the board digital ground lands connect to a digital backplane ground and the analog board ground lands connect to an analog backplane ground.  this is because there is a connection between the analog and digital grounds on each board – thus many loops.

Also, I am always cautious when an interface chip (llike an A to D converter for example) has separate analog and digital ground pins.  These pins are suplsed be tied to the analog and digital board grounds respectively, but how much AC voltage can the chip tolerate between them?

To minimize all the above effects what I do is the following:

On a given board, I locate the interface chips in a small area between the analog and digital sections.  I use separate side-by-side analog and digital ground lands, with 1/4 inch b etween, and I connect the two ground lands together with a wide land (maybe an inch or two wide.  Assuming a 10″ x 10″ board.

This connecting land  goes under the interface chips, thus preventing significant AC voltages between the chips’ analog and digital ground pins.  Also I route all signals between the analog and digital sections over this connecting land.  this provides transmission line impedance continuity for single-ended signals, and also helps to avoid crosstalk even among differential pairs.

Where supply traces cross between the analog and digital sections, I either route them over the connecting land or I place series inductors in them, over the gap between analog and digital lands.

And very importantly, if I have a system with multiple boards in a backplane, I lay out the board connecting lands to be near the board edge that connects to the backplane.  This way the backplane hca have a single ground plane that connects to the connecting land rather than to wither the analog or digiral ground lands.  A simple way to avoid loops!

Larry Miller

Hybrid circuits are small, simple circuit boards fabricated on an insulating ceramic like aluminum oxide, or aluminum nitride (which has better heat conductivity).

The advantages of hybrids over the more common epoxy-glass or polyimide circuit boards include smaller boards and finer patterns (trace can routinely be made as thin as 0.0016″ and may be as thin as 0.004″).  This permits operation at higher multi-gigahertz frequencies withoug excess parasitic capacitance and inductance.  Also, especially using aluminum nitride, the hybrid has much greater heat-dissipation capacity than epoxy glass.

Hybrids are typically less thhan one inch on a side.  They are thus best used for relatively small designs.  They can be packages in sealed packages with emerging leads, and usaed on PCB’s as components.  They are built on a board about 3 or 4 inches square and cut into individual hybrids, which are then inspected and tested.

Assembly and packaging is usually done by another vendor from the hybrid fabricator.  Typical mounting techniques include surface mounting (for example 0201 and possibly 01005 components), and wire bonding of bare dice.  These techniques can be applied to the smallest components up to large devices such as power transistors.

Traces and lands consist of a metal stack that can include copper and often includes a gold plate.  Traces may be on one or both sides of the board.  The hybrid may contain vias, which are drilled with a laser and plated or filled.

Insulating areas can be placesd on top of the traces, allowing a second trace layer to bridge from one trace to another without shorting to intervening traces.

Resistors in teh range of a few ohms to a kilohm or more can be “printed” on the hybrid lithographically.  They can typically be laser-trimmed to +-5%.

Since the hybrid is relatively thick compared to the usual trace spacing, using the back surface as a ground plane by plating it solidly with metal, is less useful in the gigahertz range; the layout often should provide trace or lands as return paths adjacent to signal trace.

Initial cost and lead time for hybrids is higher than for epoxy-glass boards, but in quantity production, hybrids are cost effective.

Larry Miller