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With
the multitude of Analog to Digital Converters (ADCs)
on the market, one can become somewhat confused about
which one is most suitable for data conversion and acqusition
for use with Senis Transducers.
We offer here a brief summary of the features that are
most important to consider when choosing an ADC for
this task.
Input type:
Differential or Single-Ended. Most ADCs offer the option
of either input type, selected with jumpers, switches,
or through software control. The Senis Hall Transducers
are designed with a differential output, so you should
choose an ADC with a differential input. If for some
reason this is not possible, you may use a single-ended
input device, but performance may be degraded depending
upon how the transducer is connected.
Resolution:
This is likely the most important item to consider when
selecting an ADC. The resolution is the number of 'steps'
that the ADC uses to acheive a 'full-scale' reading.
For general use with the Senis transducers, a 16-bit
ADC will suffice, but if you are using a low noise transducer,
a 20-bit ADC is recommended.
Scaling:
Many ADCs will offer input scaling. This feature allows
the user to divide or multiply the input voltage to
more closely match the full scale range of the ADC,
effectively enhancing the measurement.
Sample Rate:
This would be perhaps the second most important item
to consider. The sample rate must be at least twice
the frequency the you are measuring, but 5 times is
much better. So if you are looking for frequencies of
120Hz, a sample rate of at least 240Hz will suffice,
but a rate of 600Hz will provide better data.
Channel Scan Rate:
Since we are dealing with a three axis transducer, we
must use more than one channel. The channel scan rate
is the maximum rate that the ADC can select a new channel
and make a measurement. This is an important consideration,
since many ADCs actually have a relatively slow scan
rate (when compared to the sample rate.) To acheive
a sample rate of 600Hz on three channels, you will need
a channel scan rate of at least 1.8kHz.
Michael E. Duffy, October 2000
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