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