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I have some digital sensors with ADC build in and the problem is this type of sensor have lots of control pin. It is hard to separate this sensors from my mcu board. Analog sensors are much simple but susceptible to noise.

If I want to transmit few channels of analog signal with bandwidth of 3200Hz over a short distance, am I going to face any problem? How to overcome them?

Edit: Good accuracy is one of my biggest concern. The data is used for scientific research purpose they want to minimize any error. Short distance means about 1 to 2 Meters.

pstan
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    "Good" accuracy is no spec at all. Neither is "short" distance. – Olin Lathrop May 30 '12 at 13:10
  • How many channels? Really how much accuracy (1% or 0.1% or 0.01%)? Is your precision requirement different from your accuracy requirement? Any latency requirement? – The Photon May 30 '12 at 18:08
  • And, are the sensors all located together on one board or in one box or do you have a separate board/box for each sensor? – The Photon May 30 '12 at 18:09

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3.2 kHz is relatively low. You are basically wanting to send audio signals around. This can be done just like it would be with audio, which is usually with a shielded coax cable. You don't say what accuracy you need, but a bigger problem may be offset caused by the power current in the ground line. There are ways to overcome that too, but things get complicated with analog as you want higher accuracy or signal to noise ratio.

Another possibility to consider is to put a small microcontroller on the remote boards right with the sensor. This can do whatever it has to to talk to the sensor. It then sends the data back digitally. Another advantage is that this micro can sample the sensor much more often than the minimum 6.4 kHz Nyquist rate, then do dessimation and send on the data close to the Nyquist rate. This allows some range for your hardware anti-aliasing filter.

Let's say you send a new sample at 10 kHz rate and it takes 2 bytes. Using a ordinary UART this would require a 20 bit times at 10 kHz rate, for a minimum baud rate of 200 kHz. You aren't stuck with a standard baud rate when you own both ends. Run it at 500 kBaud and you have plenty of overhead. 500 kBaud is quite possible over the "short distance" you specify. Now you only need 3 wires in the cable, power, ground, and digital signal.

Olin Lathrop
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  • Thanks! I need to operate in battery and accuracy is important. Do I need a differential signal for the UART? – pstan May 30 '12 at 12:59
  • @pstan: Differential transmitter and receiver over a twisted pair would be the best, but at only 500 kBaud and a few meters probably unnecessary. – Olin Lathrop May 30 '12 at 13:11
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If you can move your sensors together onto one other board, you might use one of the smaller micro-controllers to manage the sensors and transmit data packets back to the main board.

JRobert
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summary

Sending analog audio signals a few meters over shielded coax cable is a solved problem.

If your signals are limited to 3200Hz and you need 8 or 10 bits of precision per sample, then I would be pretty comfortable using standard audio coax cable to send the raw analog signals. That might be the lowest-cost, lowest-battery-power way to handle things.

If you require DC-accurate readings at 3200Hz and 20 or 24 bits of precision per sample, shipping analog signals over even 2 meters of cable is basically impossible at any price. If you need that precision, you are forced to digitize the signals right at the source, and ship them over the cables in some digital format.

details

Transmitting in digital format generally requires one to spend a little more money on electronics at each sensor, but it allows you to save a little money on lower-cost UTP cables and low-cost connectors. In a few cases, transmitting in a digital format lets you use fewer cables -- a single daisy-chain through each sensor ending at the host, where each sensor forwards data from the "upstream" sensors "downstream" towards the host computer, as well as sending its own data "downstream" towards the host computer on the same cable. With an analog system, you are pretty much forced to run an independent wire to each sensor -- analog multiplexing techniques end up costing more than digital multiplexing techniques.

As the bandwidth goes up, or the desired precision of the signals goes up, or both, analog cables need more and more shielding (i.e., get more expensive) to block outside interference and cross-interference.

Eventually you reach a point where it's basically impossible to put enough shielding on a cable to get the desired bandwidth and precision.

Suggestion

Post a new question something like "I have a bunch of (insert name here) digital sensors that I want to distribute over a large area of a few meters, but I don't want to run dozens of digital control wires per sensor back to my host MCU. What's a reasonably low-cost, low-energy digital circuit I can put to reduce the number of wires I need to run to each sensor? I might be willing to run a full 4-pair CAT5 cable to each sensor to carry power + data, but no more! Ideally much less -- is it possible to share one 4-pair CAT5 cable among 2 or 3 sensors to carry power + data?"

If you are willing to spend a few extra bucks on digital chips in order to avoid the hassle of programming a MCU at each remote location, please specify "without a MCU" (like How to decode morse code with digital logic ).

It's possible the resulting circuit may give you the full precision available from your (insert name here) digital sensors, but have a net cost less than an analog-signal system, when you balance the extra digital electronics and the low-cost unshielded cables and connectors vs. the lower analog electronics cost and the higher shielded cables and connectors.

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davidcary
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