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APPENDIX B
71
is connected to HI and the IO pin is connected to the LO. TEDS cannot be read by the digitizer if
LO is connected to chassis ground.
Figure 4-24: IEPE with 1-wire TEDS Memory
The forward voltage of diode D1 and cable capacitance (C
sh
) and limit the maximum cable length
at which TEDS can be read. The IEPE sensor output is temporarily disabled by diode D2 when
reading TEDS because this involves turning OFF the IEPE current source and applying a positive
bias to the LO terminal while grounding the HI terminal.
Be careful when connecting a 1-wire memory IC directly to the BNC input. Enabling the IEPE
current source will damage the memory IC if diode D1 is not present.
A
NALOG TO
D
IGITAL
C
ONVERTER
(ADC
)
The DSA digitizers use 24-bit delta-sigma analog-to-digital converters. The ADC runs at a fixed
sampling rate so there is no need to change the corner of the analog anti-aliasing filter. Lower sample
rates are obtained by a decimating digital filter. Decimation by 2 and 5 are supported.
The table below shows the available data rates at which the ADC runs for each model.
Model
Oversampling Ratio ( f
CLK
/ f
DATA
)
Oversampling
Clock (f
CLK
)
32
64
128
256
Data Rate (f
DATA
) - Samples per Second (SPS)
EMX-4250
EMX-4251
131,072
65,536
32,768
N/A
4,194,304 Hz
204,800
102,400
51,200
N/A
6,553,600 Hz
EMX-4350
EMX-4380
409,600
204,800
102,400
51,200
13,107,200 Hz
524,288
262,144
131,072
65,536
16,777,216 Hz
625,000
312,500
156,250
78,125
20,000,000 Hz
Table 4-7: Available Data Rates
A
NTI
-A
LIAS
F
ILTER
The delta-sigma analog-to-digital converters have a built-in digital anti-aliasing low-pass filter that
eliminates any signals with frequencies above the Nyquist frequency and assure that no aliasing
occurs. The ADC digital low-pass filter is a linear phase digital filter exhibiting constant delay time
vs. frequency (constant group delay). The table below shows the key characteristics of the ADC
digital low-pass filter. Key characteristics of digital filters always track the selected data rate.
EMX-4350
EMX-4380
EMX-4250
EMX-4251
Passband (<±0.001 dB ripple) (Hz)
0.424 x f
DATA
0.417 x f
DATA
-3dB Corner Frequency (Hz)
0.488 x f
DATA
0.424 x f
DATA
Stopband (<-90 dB attenuation) (Hz)
0.576 x f
DATA
0.583 x f
DATA
Group Delay (seconds)
28 / f
DATA
39 / f
DATA
Table 4-8: ADC Digital Low-pass Filter Characteristics
1-Wire
TEDS
Memory
Gnd
IO
IEPE
Sensor
HI
LO
D1
D2
Csh
Cable
Capacitance
Sensor
Digitizer
