This question is in reference to my previous question where some suggestions were made that went somewhat off-topic.

Basically I have a built a system with an FPGA and very sensitive ADC (sensitive to picoamp currents) which is working well, but now it has to be separated into two PCBs.

Major questions are

  1. How to properly share GND between the two boards in order to minimize ADC noise/error at all costs?
  2. How to properly buffer the CLK and serial signals in and out of the ADC?

My proposed solution: enter image description here

Concerns I'm having:

  • I know the ribbon should have GND in between each signal (since some are 40MHz) but should GND be connected to both planes on either side?
  • It was suggested to buffer the ADC I/O so that the ADC doesn't have to drive a lot of current, but where do the buffers get their power and ground from?
  • Here I have signals traveling over the edges of ground planes, which I've heard defeats the purpose of having a ground plane.
  • Since the PCBs are already getting a GND connection at the power supply, doesn't also having GND in the ribbon create a loop?
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    \$\begingroup\$ If you have buffers already why not keep the ADC isolated? You can bring in a ground and supply for the regulator, and use isolators to buffer the input/output. There would be a local ground plane and supply currents return to the mainboard through the primary side of the isolated regulator. \$\endgroup\$ – crasic Jan 27 '16 at 22:47
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    \$\begingroup\$ My concern with optoisolators is that maybe it's overkill, and preventing me from learning how to handle grounding issues the "right" way. I'm already using them to isolate this system from USB for instance. \$\endgroup\$ – Jay Keegan Jan 27 '16 at 22:56
  • \$\begingroup\$ What kind of load do optoisolators present to the ADC outputs? The ADC doesn't specify it's drive current capability other than listing 100Kohm as test condition under Timing. The point of the buffer was to prevent the ADC from having to push a lot of current IIRC. Also they have to work at 40 MHz. \$\endgroup\$ – Jay Keegan Jan 27 '16 at 23:48
  • \$\begingroup\$ Do you need to write to the ADC at all? \$\endgroup\$ – ThreePhaseEel Jan 28 '16 at 1:14
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    \$\begingroup\$ @Jay , there are a number of isolator chips that are capable of fast buffering (i've used this ) . The best way to think about is to keep a neatly isolated layout for the ADC board up to the isolator (local ground tied at the isolated supply). The MCU side of the isolator is conceptually a seperate component, it needs an MCU ground connection, but this is only used for 1. The MCU side of the isolator IC's, and 2. The ground for the upstream side of the isolated supply \$\endgroup\$ – crasic Jan 28 '16 at 3:00

Using isolators is the most straight forward way to couple boards like this while avoiding grounding and noise headaches.

For an isolated design to function, you need isolating buffers for all the digital input and outputs as well as an isolated power supply, although the later can be ignored if the power ground acts as the "star point" for your daughter board and you do not require isolation on the sensor part (e.g. a 'floating adc').

Conceptually this can be imagined by the following diagram


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Thanks. I've ordered a few digital isolators for testing including your suggested from Si Labs. I'm going to keep the question open because I'd still like to know the best way to solve this problem without galvanic isolation also. Seems like this should be a widely applicable question... grounding and signaling in a multi-board system. \$\endgroup\$ – Jay Keegan Jan 28 '16 at 15:42
  • \$\begingroup\$ Do you know whether capacitance between the adjacent (isolated) ground planes at the digital isolator could be an issue? I'm talking like 100 max pF-level stray capacitance between pcb planes. \$\endgroup\$ – Jay Keegan Feb 11 '16 at 4:41

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