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I have 1 board that reads data from a sensor, while another board wants to get data from that sensor.

Is it better to have an ADC on the first board, then send that data via i2c to the 2nd board? Or is it better to send the analog signal from the first board directly to the ADC of the 2nd board?

My assumption is that sending the analog data across 2 boards is going to have a lot of noise.

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  • \$\begingroup\$ Is this for a CubeSat? (Your avatar looks like a CubeSat.) What's the nature of the noise sources that are giving you pause for concern? \$\endgroup\$ Dec 24, 2015 at 0:54
  • \$\begingroup\$ You are correct, this is for a CubeSat. There are different modules and components between our payload board and the command and data handling board. I'm concerned of the noise it would pickup from having to pass information across such a distance. \$\endgroup\$
    – Olivier
    Dec 24, 2015 at 1:17

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Either approach might be alright: analog point-to-point connection (down to, say, 10 or 12 bits of resolution), or I2C. CubeSats are small. They operate far from external sources of EMI. If the switch-mode power supplies in the EPS (electric power subsystem) are properly designed, they should not generate much EMI.

Is it likely that the payload will generate EMI?

From systems engineering standpoint, both point-to-point and bus have their pros and cons.
The pro of point-to-point is that it's simple (in the k.i.s.s. way). The con is that it doesn't scale as well as I2C.
The pro of the I2C is that it scales well, and it's convenient. The con is that complexity increases. The bus can also be a single point of failure: I2C bus can get stuck.

If you have more than one sensor on that board, and they lend themselves to an A/D connected via I2C. Or, if you anticipate that there may be more sensors on that board in the future. Then I2C might be convenient.

p.s. In school, I have designed a C&DH board for the MAST CubeSat. It had an extensive I2C bus, which worked without a hitch.
Later, I have designed and then had to grapple with an overgrown [because of lack of a better upfront judgement] I2C bus in an industrial instrument.

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    \$\begingroup\$ operate far from external sources of EMI - I'm surprised, I thought space was quite a hostile EMI environment. \$\endgroup\$
    – pjc50
    Dec 24, 2015 at 10:26
  • \$\begingroup\$ It's certainly a hostile radiation environment. EMI is 'magnified' though in the sense that your noise floor for say communications is so low that even something that would pass FCC on Earth can render your radio useless. \$\endgroup\$ Dec 24, 2015 at 17:46
  • \$\begingroup\$ @pjc50 Report on EMI in Earth orbit. (Other than EMI, there are unpleasant phenomena like radiation and surface charging.) \$\endgroup\$ Dec 24, 2015 at 17:58
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Sending the digital signal is the better option, as it will be less susceptible to interference, or distortion as it travels between the two points.

Along the way not only can the sensor pick up noise from various sources, but losses along the sense lines can distort the reading.

Sometimes sensors need to be placed far from the ADC due to environmental or other conditions (high heat, etc.), in which case those distortions are taken into account during the design of the system.

Place the ADC on the sensor board, and send the digital signal out from there.

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Digitize it. What ground is your analog signal referenced at? Chances are low your intra-board harnessing has dedicated signal grounds vs. power grounds. The long run will have a greater chance of susceptibility to noise and it is also additional harnessing which I can tell you from experience is the last thing you want in a satellite.

Additionally, what reference voltage will it be sampled against, and where is that voltage located? Digitizing the signal as early as possible is IMO in your best interest.

Now, if this signal has say a 1Hz filter on it (some kind of power rail), the discussion becomes different as its so slow as to be virtually DC

Finally, I'm not sure what your system bus is, but an I2C link can be extended to carry additional data with no extra penalty of wires.

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