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Picolog ADC-20 conversion time

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Picolog ADC-20 conversion time

Postby Anthony Anaton » Mon May 21, 2012 8:36 am

Hi there
we are using our ADC-20 for some measurements, and we have some questions regarding the conversion time and resolution of the logger
1) The conversion time is per channel so if it's set at 60ms , there will be 16 measurements for 1 channel, 8 for 2 channels etc. This also means that for conversion time set at 660ms for two channels , the time interval should be greater than 2x660ms. Right?
Now.. what happens if it's a differential channel and not a single ended one? How many measurements are taken for the differential channel? For example... in 60ms.. will there be 16, or 8 measurements per second on one differential channel?

2) What is the resolution of the logger in V and not in bits. For a conversion time of 340ms or 660 ( 20 bits)
the actual resolution I seem to get is not the one I calculate by the bits either on the scale of 1250mv or 2500.

Does the documented resolution in the manual refer to single ended channels thus it's different with differentials?

Thanks in advance
Anthony Anaton
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Re: Picolog ADC-20 conversion time

Postby ziko » Mon May 28, 2012 2:41 pm

Hi,
1)
The ADC20 has 4 differential channels or 8 single ended channels (the ADC24 has double this)
The conversion time for 1 channel can vary between 60ms to 660ms depending on how much resolution you are using. Look at section 4.4 in the user manual:

http://www.picotech.com/document/pdf/adc20.en-3.pdf

For example using Ch 1 in single ended mode, and a resolution of 20bits will give you a conversion time of 660ms for the ADC, now if you drop the resolution to 18 bits the conversion time for the ADC will drop to 60ms. So you are sacrificing resolution for conversion time. (Correction) The conversion time for the differential mode will be the same as the ADC takes this into account.

There is one ADC on the ADC20 (and indeed the ADC24) and there is a multiplexer that switches from channel to channel. So the ADC will read the value from the selected channel, after the specified time is set to read the data from the channel, the multiplexor then switches to another channel and so on.

2) The resolution is 20 bits, this is 2 to the power of 20 which equals 1048576. So if you had a voltage range of +/-2500 mV the resolution would be the peak to peak voltage divided by the number of points, i.e. 5000mV / 1048576 = 0.004768 mV which is approximately 4.768 micro Volts. The resolution will be the same for single ended or differential.

Let me know if you require any further clarification!

Hope this helps
Ziko

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