In a message dated 09/01/2007 16:16:40 GMT Standard Time, royb1@...........
writes:
With the usual 14 or 16 bit A/D's, is that dynamic range necessary?
Bob
Hi Bob,
A 16 bit ADC with no noise has +/-1/2 bit uncertainty. This is about 96
dB. Each factor of 2 gives 6 dB change. With +/-10V input 1 bit = 0.305 mV
Quiet opamps may give 0.1 to 10 Hz input noise levels well below 1 micro
volt; true chopper amplifiers may be much less than this. The CAZ opamps
tend to give 1 to 2 micro volts, but these may give OK results for long period
signals, when 1/f drifts become large. It is a good principle to use odd
orders of low pass filter with a capacitor across the feedback resistor of the
first opamp. Never amplify high frequency signals. This limits the effect of
intermodulation distortion and subharmonics
Digital filters tend to give performance in the mid 70 dBs, or less, but
also suffer from switch transient feed through - 5 mV? - which may need
additional analogue filtering before putting it into an ADC.
In seismometry, we are seeking very low noise levels at very low
frequencies. You just can't afford to throw away +/-4 bits signal through a poor
choice of filter, or +/-3 bits by failing to average out the internal ADC noise.
While you can increase the amplifier gain to display small signals over
internal noise, a reduction of the range by a factor of 8 or more is very
undesirable.
Your available dynamic range is usually far less than the maximum range
of signals that you can receive.
If you use a Lehman or similar long period sensor, you should set your
background microseism signal to maybe 200 counts. If you don't do this, you
may not be able to sense the long period low amplitude signals masked by the
microseism background.
Regards,
Chris Chapman
In a message dated 09/01/2007 16:16:40 GMT Standard Time, royb1@comcast=
..net=20
writes:
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>With the=20
usual 14 or 16 bit A/D's, is that dynamic range=20
necessary?
Bob
Hi Bob,
A 16 bit ADC with no noise has +/-1/2 bit=20
uncertainty. This is about 96 dB. Each factor of 2 gives 6 dB change. With=20
+/-10V input 1 bit =3D 0.305 mV
Quiet opamps may give 0.1 to 10 Hz input noise=20
levels well below 1 micro volt; true chopper amplifiers may be much less tha=
n=20
this. The CAZ opamps tend to give 1 to 2 micro volts, but these may give OK=20
results for long period signals, when 1/f drifts become large. It is a good=20
principle to use odd orders of low pass filter with a capacitor across the=20
feedback resistor of the first opamp. Never amplify high frequency signals.=20=
This=20
limits the effect of intermodulation distortion and subharmonics
Digital filters tend to give performance in the=
mid=20
70 dBs, or less, but also suffer from switch transient feed through - 5 mV?=20=
-=20
which may need additional analogue filtering before putting it into an=20
ADC.
In seismometry, we are seeking very low noise=20
levels at very low frequencies. You just can't afford to throw away +/-4 bit=
s=20
signal through a poor choice of filter, or +/-3 bits by failing to average o=
ut=20
the internal ADC noise. While you can increase the amplifier gain to=20
display small signals over internal noise, a reduction of the range by a fac=
tor=20
of 8 or more is very undesirable.
Your available dynamic range is usually far=20=
less=20
than the maximum range of signals that you can receive.
If you use a Lehman or similar long period sens=
or,=20
you should set your background microseism signal to maybe 200 counts. If you=
=20
don't do this, you may not be able to sense the long period low amplitude=20
signals masked by the microseism background.
Regards,
Chris Chapman