PSN-L Email List Message
Subject: Advantages of more ADC bits
From: rsparks rsparks@..........
Date: Fri, 06 Feb 2009 19:57:55 -0800
I have been considering what advantage the amateur seismologist might
see if making an upgrade to more bits in the A/D converter.
First, some background. If any of this is incorrect, please offer
A/D converters are also known as ADC devices. Their function is to
convert an analog voltage to a digital number. Each device will have a
basic range such as +/- 10 volts. That range will be divided into a
number of intervals depending upon the number of bits available to the
device. Each voltage interval will be assigned a digital number.
Bits are a basic division of computer technology and are expressed as
powers of two (2^xx). Thus, an 8 bit device will have 2^8 = 256
divisions. Not too accurate if 256 divisions are used to find a point
between +/- 10 volts. Also not very sensitive as the each
division/count is 20/256 = 0.078125V.
Here is a list of devices/seismometers and the sensitivity/accuracy in bits:
Volksmeter 24 bit
PSN/Cochrane 16 bit
Saum Infiltec 16 bit
AS1 12 bit
Dataq 8/10/12 bit
As I wonder about this information, I get to wondering "Why bother to
upgrade?", at least for amateur purposes.
Two reasons come to mind. You will see more quakes with a more
sensitive instrument. You can not really compensate with more
amplification because the dynamic range is smaller with the low bit
count devices. Yes, you can raise the amplification level so that an 8
bit device will respond to the same voltage signal that the 24 bit
device will see, but the 8 bit device will be saturated after only 256
counts while the 24 bit device would have only recorded 256 counts out
of 16.7 million possible counts.
The second reason is to have better fidelity or accuracy. Any FFT taken
of a trace will be more accurate if each individual reading is better
placed in the digital data table. An expanded trace of low count
signals will look more "sine wave" like. An easier way to make the
wave more "sine like" might be increased sample rate. Ultimately, both
accuracy and sample rate must be increased for superior results.
It is fun to consider upgrades, harder to make them.
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