PSN-L Email List Message

Subject: Advantages of more ADC bits
From: rsparks rsparks@..........
Date: Fri, 06 Feb 2009 19:57:55 -0800

Hello All,

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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