In a message dated 07/02/2009, rsparks@.......... writes:

I have  been considering what advantage the amateur seismologist might 
see if  making an upgrade to more bits in the A/D converter. 

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, whereas  a 16 bit device will have 65,536 divisions.  
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

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. Ultimately, both accuracy and 
sample rate must be increased  for superior results.
Hi Roger,
 
    Another important question to ask is 'What is  the chip background noise 
level?' The depends on the integration interval  and whether a series of 
signals are averaged. Also ask 'What is your  seismic background noise level?'
 
    The older 16 bit ADCs used to have 3 to 4 bits of  internal digital noise 
on them. But with a 20 micro second conversion rate you  could take and 
average 64 sequential readings. You need to take 4 readings  to average out 1 bit 
of digital noise, 16 to average out 2 bits etc. You  can push signal averaging 
to about 18 bit accuracy on a 16 bit ADC for  single channel low sample rates.
 
    24 bit ADCs may only give about 19 / 20 bits true  resolution, or even 
less if you require a rapid sample rate. The 16 bit Sigma  Delta ADCs can give 
16 bits true resolution for 20 samples / second, but may be  less for greater 
sample rates. Some 24 bit systems only have +/-2 volts input  range, which 
requires you to use very low noise / costly  amplifiers. 
 
    You also need to check the amplifier noise level  and the local seismic 
noise levels that you detect. The local noise may be  mostly due to ~6 second 
period microseisems. The noise level for P and S waves  at 0.5 to 5 Hz and for 
surface waves at periods over 12 seconds may be  much lower, so the amplifier 
self noise is important. Assuming that you  have a 16 bit true ADC with a 
+/-10 V input range, 1 bit represents 0.305 milli  V. You definitely need the 
amplifier to have a lower noise level than  this.
 
    12 bits resolution only gives you +/-2048 signal  steps, so even if there 
is no ADC noise, your practical dynamic range is limited  to about 3 
Magnitudes. 16 bits gives a more practicable +/-32,768 step  range. Remember that 
Earthquake Magnitudes have a logarithmic  scale. Dynamic range may be a serious 
problem if you live close to say, the  New Madrid earthquake zone and experience 
local quakes. For quake amplitude /  distance motions see 
_http://jclahr.com/science/psn/magnitude.html_ (http://jclahr.com/science/psn/magnitude.html) 
 
    Amplifiers suffer from several sorts of noise,  including resistive, 
input voltage and current and 1/f noise. The 1/f noise  is serious and may be 
dominant at periods over ~10 seconds. Chopper amplifiers  do not suffer from 1/f 
noise, but they are more expensive to construct. The very  low noise levels 
seen in commercial seismic systems is due to the use of  capacitative distance 
transducers and chopper detection systems. Velocity  measuring systems are 
limited by noise at longer periods.
 
    Regards,
 
    Chris Chapman   








In a message dated 07/02/2009, rsparks@.......... writes:
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  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>I have=20
  been considering what advantage the amateur seismologist might 
see if=20
  making an upgrade to more bits in the A/D converter. 

Bits are a ba=
sic=20
  division of computer technology and are expressed as 
powers of two=20
  (2^xx).  Thus, an 8 bit device will have 2^8 =3D 256 
divisions, w=
hereas=20
  a 16 bit device will have 65,536 divisions.  
Here is a list of=20
  devices/seismometers and the sensitivity/accuracy in=20
  bits:

Volksmeter         24=20
  bit
PSN/Cochrane   16 bit
Saum Infiltec     =
; 16=20
  bit
AS1                 &n=
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  12 bit
Dataq               &nbs=
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  8/10/12 bit

 I get to wondering "Why bother to upgrade?", at l=
east=20
  for amateur purposes. 

Two reasons come to mind.  You will see=
=20
  more quakes with a more 
sensitive instrument.  You can not really=
=20
  compensate with more 
amplification because the dynamic range is smalle=
r=20
  with the low bit 
count devices.  Yes, you can raise the amplifica=
tion=20
  level so that an 8 
bit device will respond to the same voltage signal=20=
that=20
  the 24 bit 
device will see,  but the 8 bit device will be saturat=
ed=20
  after only 256 
counts while the 24 bit device would have only recorded=
 256=20
  counts out 
of 16.7 million possible counts.

The second reason i=
s to=20
  have better fidelity or accuracy.  Any FFT taken 
of a trace will=20=
be=20
  more accurate if each individual reading is better 
placed in the digit=
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  data table. Ultimately, both accuracy and 
sample rate must be increase=
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  for superior results.
Hi Roger,
 
    Another important question to ask is 'What=
 is=20
the chip background noise level?' The depends on the integration interv=
al=20
and whether a series of signals are averaged. Also ask 'What is your=20
seismic background noise level?'
 
    The older 16 bit ADCs used to have 3 to 4 bits=20=
of=20
internal digital noise on them. But with a 20 micro second conversion rate y=
ou=20
could take and average 64 sequential readings. You need to take 4 readi=
ngs=20
to average out 1 bit of digital noise, 16 to average out 2 bits etc. Yo=
u=20
can push signal averaging to about 18 bit accuracy on a 16 bit ADC for=20
single channel low sample rates.
 
    24 bit ADCs may only give about 19 / 20 bits tr=
ue=20
resolution, or even less if you require a rapid sample rate. The 16 bit Sigm=
a=20
Delta ADCs can give 16 bits true resolution for 20 samples / second, but may=
 be=20
less for greater sample rates. Some 24 bit systems only have +/-2 volts inpu=
t=20
range, which requires you to use very low noise / costly=20
amplifiers. 
 
    You also need to check the amplifier noise leve=
l=20
and the local seismic noise levels that you detect. The local noise may be=20
mostly due to ~6 second period microseisems. The noise level for P and S wav=
es=20
at 0.5 to 5 Hz and for surface waves at periods over 12 seconds ma=
y be=20
much lower, so the amplifier self noise is important. Assuming that you=
=20
have a 16 bit true ADC with a +/-10 V input range, 1 bit represents 0.305 mi=
lli=20
V. You definitely need the amplifier to have a lower noise level than=20
this.
 
    12 bits resolution only gives you +/-2048 signa=
l=20
steps, so even if there is no ADC noise, your practical dynamic range is lim=
ited=20
to about 3 Magnitudes. 16 bits gives a more practicable +/-32,768 step=20
range. Remember that Earthquake Magnitudes have a logarithmic=20
scale. Dynamic range may be a serious problem if you live close to say,=
 the=20
New Madrid earthquake zone and experience local quakes. For quake amplitude=20=
/=20
distance motions see http://jclahr.com/scie=
nce/psn/magnitude.html
 
    Amplifiers suffer from several sorts of noise,=20
including resistive, input voltage and current and 1/f noise. The 1/f n=
oise=20
is serious and may be dominant at periods over ~10 seconds. Chopper amplifie=
rs=20
do not suffer from 1/f noise, but they are more expensive to construct. The=20=
very=20
low noise levels seen in commercial seismic systems is due to the use of=20
capacitative distance transducers and chopper detection systems. Velocity=20
measuring systems are limited by noise at longer periods.
 
    Regards,
 
    Chris Chapman