PCB Layout Advice

Thread Starter

A.Macdonald

Joined Jun 7, 2020
2
Hi All,

I'm a reasonably experienced PCB designer, but my latest project has a few unique aspects that I haven't had to deal with before, and so I have a few questions for the forum crowd.

At its core, my project is a 32 channel source meter. This means that there is a lot of analog circuitry and not everything can be as close to the source as possible, and I'm not sure for which are most cruicial to keep close to the source.

There are basically three stages in the design: the current driver, the current sense, and the voltage sense. Due to the space confines that having 32 channels poses, all the ananlog circuitry will basically follow a line from one end of the board to the other. I'm currently trying to fit everything on a 6 layer board.

I put the current sense resistor and the BJTs that drive the current as close as possible to the connector. This is pretty obvious since it's the only part of the circuitry that will see a high current (up to 125 mA each), and I definitely wanted to keep those traces short.

By this point, the "leading edge" (remember all the circuitry is basically following a line) is nearly 3 inches away from the sense resistor. I've run the Voltage Sense trace, the two current sense traces, and a trace for the relay coils (not as important) for all channels along the edge of the board on inner layers. I'll note that these lines are going to be fairly low speed, close to DC.

At this point I'm going to just toss out whole bunch of questions, flow of consciousness style.

1) Is there a problem with running them for long distance, albeit on an inner layer and in a mostly straight line?
2) Should I put the differential pair (the pair across the current sense resistor) on the same layer, or different layers?
3) Which stage should go next? They're all analog, but they all necessarily cannot be close. My gut is telling me to finish off the analog circuitry that drives the BJTs.
4) Should I separate the DAC and ADC from the analog circuitry, or keep them close by? It's easier to run 4 digital serial lines than 32 analog ones, and distance isn't as much of a concern for digital signals.
5) The traces may be 6 inches long before they reach the last stage. Is that problematic? Which stage would your recommend being last?

Thank you!

-Alexander
 

DickCappels

Joined Aug 21, 2008
6,593
Though your description may be through in some respects, it does not talk much about the frequencies involved, the frequency response of various parts of the circuit, or the voltages and current involved.

I think these things are important to finding the best answers to your questions. Can you post a schematic?
 

Thread Starter

A.Macdonald

Joined Jun 7, 2020
2
Though your description may be through in some respects, it does not talk much about the frequencies involved, the frequency response of various parts of the circuit, or the voltages and current involved.

I think these things are important to finding the best answers to your questions. Can you post a schematic?
Hi Dick,

Unfortunately I cannot post a schematic, but I can answer your questions. The analog portion of the design is low frequency, pretty well close to DC, for all intents and purposes. The highest/lowest voltage on the analog side is +/- 12 V. The maximum load current is 125 mA per channel. I've designed it such that Vin can be up to +/- 10 V, although we're probably going to end up using it closer to 1.2 V.

Thank you,

-Alexander
 

DickCappels

Joined Aug 21, 2008
6,593
1) Is there a problem with running them for long distance, albeit on an inner layer and in a mostly straight line?
If it is almost DC then crosstalk should not be a problem except where ground return currents might cause voltage drops across analog ground paths.

2) Should I put the differential pair (the pair across the current sense resistor) on the same layer, or different layers?
Should not matter as long as the grounding is clean.

3) Which stage should go next? They're all analog, but they all necessarily cannot be close. My gut is telling me to finish off the analog circuitry that drives the BJTs.

I'll leave this for somebody else.

4) Should I separate the DAC and ADC from the analog circuitry, or keep them close by? It's easier to run 4 digital serial lines than 32 analog ones, and distance isn't as much of a concern for digital signals.


Good thinking. The analog and digital grounds should only meet at one point -where they go back to the power supply.

5) The traces may be 6 inches long before they reach the last stage. Is that problematic? Which stage would your recommend being last?

According to your description, no problem at all as long as the IR drop along the trace is too large.
 

ChetanShah

Joined Sep 19, 2020
5
before you decide to simply make your printed circuit board larger and spread out your components, try some of these PCB layout tips and see if routing becomes more easy.
  1. Make important nodes accessible
  2. Give space between components
  3. Place components with the same orientation
  4. Print the layout to see if components' sizes match
  5. Exchange wiring directions between layers
  6. Select the width of lines depending on current
  7. Know the specifications of the manufacturer
  8. Avoid 90º angles with traces
  9. Use the silk layer
  10. Use the schematic vs. layout comparison
  11. Create a ground plane
  12. Place bypass capacitors
  13. Route the differential signal traces in parallel
  14. Consider spots of heat
  15. Make parallel footprints for hard to find components
 

MrChips

Joined Oct 2, 2009
21,890
As Dick says, at DC the design is not critical.

1) Is there a problem with running them for long distance, albeit on an inner layer and in a mostly straight line?
Keep it short in straight lines away from high current traces. The important parameter here is impedance. Low impedance signal lines are less likely to pickup stray interference. Inner layer does not improve signal integrity so much. You need at least two signal layers and one ground plane. Add a power layer for higher currents. That makes it 4 layers. Add two more layers only if it simplifies the layout of signal lines.

2) Should I put the differential pair (the pair across the current sense resistor) on the same layer, or different layers?
Differential pair should be paired, running side by side on the same layer.

3) Which stage should go next? They're all analog, but they all necessarily cannot be close. My gut is telling me to finish off the analog circuitry that drives the BJTs.
Go sequentially according to the flow diagram.

4) Should I separate the DAC and ADC from the analog circuitry, or keep them close by? It's easier to run 4 digital serial lines than 32 analog ones, and distance isn't as much of a concern for digital signals.
Keep ADC away from DAC. ADC has sensitive inputs. DAC is output. ADC and DAC are mixed signal components, i.e. analog and digital. Keep digital lines away from analog signals. Use separate ground planes for analog and digital. Use "star" ground connection.

5) The traces may be 6 inches long before they reach the last stage. Is that problematic? Which stage would your recommend being last?
Can't tell until we see the signal flow diagram and related components. Can you draw a block diagram?
 
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