As
successful as the LCHEAPO instrument is, it relys on a 16-bit
analog to
digital converter (ADC), while industry and community
demand is for 24-bit
systems. One overwhelming advantage of an
oversampling 24-bit system is
that post-sensor amplification is
often not required, and the need to
carefully match filtering
and amplification to the local noise environment
is avoided. Further,
with the use of a 24-bit system, the expansion of the
limited
resolution of a 16-bit ADC through complex engineering
solutions
like gain ranging is no longer needed. Finally, the
extraordinarily
large frequency dependence of seafloor noise coupled with
the
large range in amplitudes of acoustic signals demands 24-bit
conversion
for high fidelity recording.
We have chosen to use a chip set for the 24-bit system manufactured by the Crystal Semiconductor Corporation. In particular, we use the CS5322 24-bit Variable Bandwidth A/D Converter and the CS5321 High Dynanmic Range Delta-Sigma Modulator. This is the same chip set used by RefTek for the IRIS PASSCAL instruments. The dynamic range specifications for the converter system depend upon sample rate:
| Sample Rate | 31.25 | 62.5 | 125 | 250 | 500 | 1000 | 2000 |
| Dynamic Range (dB) | 133 | 132 | 130 | 127 | 125 | 121 | 99 |
The maximum voltage input to the system (it is possible to exceed this by 5%) is +/- 4.5V which yields a decimal output between -5,242,879 and +5,242,880. The system least count (lc) is, thus, 0.858 microVolt. For a sample rate of 125 Hz, the anticipated rms noise is 2.62 counts and the anticipated spectral level is 0.11 counts**2/Hz. Based on the spectrum below, obtained from the 24-bit L-Cheapo, the instrument meets the advertised specifications.