# newbie question about bit depth

• posted

Hi everyone,

This is a very newbie question from an equally newbie electronic guy...

I have a requirement where I need to measure something to accuracy of

0.5 temperature units. How many bits would be needed for digitization of data? What information do I need to be able to calculate the bit depth of the acquisition card?

Cheers, D

• posted

read a few of the datasheets from here

martin

• posted

I think you need to express in terms of volts.

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• posted

Hello,

Looking into your perfil, you are a math/software person. Before you can decide on the A/D converter or data acquisition card, you need to know the characteristics of your sensor and electronics.

For this you need to know the lowest and higher voltage that your circuit / sensor combination will generate and the lowest dV/dT if the sensor / electronics combination isn't linear (as is the case with, for example, NTC type sensors).

For example: your sensor outputs minimum 1V and maximum 4V, and dV/dT is minimum 20mV/degree. This means you must be able to detect a 10mV step. Assuming a 0 to 5V input ADC, the amount of codewords required must be > 5/0.005 = 1000. That requires at least 10 bits. Maybe you want more resolution (for example 0.1 degree readout), this also requires more bits (13 bits).

But there is more. Your sensor has inaccuracy; your ADC will introduce errors (like offset, gain error, non-linear error, etc). Also you conversion formula from resistance/voltage/current to temperature will introduce some error. As a result, a 10 bit ADC from the datasheet will not be sufficient.

Probably you would like to calibrate the sensor / ADC combination. During calibration, you measure also with the system to be calibrated; this introduces an error also. Your standard (controlled temperature bath) will also have some error.

So guaranteeing a (worst case) accuracy of 0.5 degree requires a full analysis of all components. You can be lucky. Probably the speed is no issue. There is a reasonable choice in AD conversion products with 20 bits or more, with low offset, gain error en extremely good linearity. You will probably have more problems with the analog electronics.

You might consider buying a complete temperature-measuring instrument with digital output.

Best regards,

Wim PA3DJS

• posted

None at all, if the temperature is constant. An infinite number, if the temperature can range over an infinite span.

You need to specify the minimum resolution (0.5 temperature units, I presume), and a range. So if you wanted to measure degrees C from 0 to

100, in 1/10th degree increments, you'd need at least 1000 steps, and probably 2000. 2^10 is 1024, so your 1000 steps require ten bits; 2000 steps would require 11.

Note that you'll also have other error sources in there -- your ADC is just one of the sources you need to take into account.

```--
Tim Wescott
Control systems and communications consulting```

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