From Analog to DSP
Comments on converting Analog layouts to Digital
Written by Dan Morgan
Excerpted from Communication Systems Design April 1996
With an increasing number of companies designing DSP into their products, the divisions between different specialities are falling away. Engineers and designers now have to have some knowledge of analog and digital designing, software engineering and mathematics as well.
DSP must operate a very high speeds - in many cases better than 20 ns per instruction.
Traces must be shorter, and power and ground planing must be more exact. If not, there is a risk
of noise and distortion of the signal being sampled as well as falsely triggering digital devices.
Even after the design has been finalized and the components sourced, subtle problems on the
boards themselves can keep the product from working properly or be the cause malfunctions that
cannot be explained or quantified. To assist you to successfully transfer an analog design to
digital, here are a few points to keep in mind when still in the design stage.
Keep oscillator circuits and buffers as far from analog circuitry as possible. Isolate power supplies to such circuits and user ferrite beads or other methods for squelching noise.
Use the lowest power and slowest speed logic you can get away with. It is the signal transition that will create the worst harmonics, the faster the signal transition, the greater the amplitude.
Traces from op amps to data converters need to be kept as short as possible, because the impedance of the trace is directly proportional to the width. Keep traces as wide as possible.
Separate analog and digital ground plane systems. They should occupy separate planes; the greater the expanse of the ground plane, the lower the impedance. Separate the ground planes by at least ". Currents for each signal group need to return to the source, and, if they do connect, should only do so at one point ( where the power source is at its lowest impedance) on the current's own trace. This is true of all high-current and switching circuits.
Use sufficient, high-quality decoupling caps. Ceramics are good for small values and high frequencies, while tantalums are good for high values. Place these as close to the pins of the device as possible. All filtering caps in the signal path should be NPO/COG dielectric. Use BX/X7R dielectric for dc voltages.
Use inductors and beads to clean up power supply traces. RC networks on the power pins also filter noise and lack the danger of resonance.
Completely decouple any power entry or exit to the card. Use a 22 microfarad capacitor and .1 microfarad capacitor pair at each point.
Partition the board to see that all analog components are collected in one area, with the digital in another. This applies to traces as well. Analog traces should be over the analog ground plane and digital traces over the digital ground. If digital traces must penetrate into analog territory, they should be static ( slow voltage changes) and as short as possible.
Mixed signal components should straddle the split power/ground planes.
All resistors in the signal path for analog circuitry should be metal film. Use carbon for dc and power supply circuitry.
Use as few clocks as possible, because it is often possible to synchronize the entire circuit to the A/D clock. Use dividers for the rest, if necessary.
Be aware of extra clock chips that may be operating at a low level. The dc-dc choppers are not synchronized to the rest of the circuitry and chopper amplifiers.
Guard band analog signal traces.
Terminate unused op amps and digital circuitry. The "+" input of the op amp should be connected to ground and the "-" input connected to the output of the amplifier.
Beware of analog multiplexers, PLDs, gate arrays and opto-couplers that can add jitter to a clock line.