Synchronizing Multiple Devices; Delta-Sigma Adcs And The Oversample Clock - National Instruments NI 447 Series User Manual

Chapter 3 Device Overview and Theory of Operation

Synchronizing Multiple Devices

Delta-Sigma ADCs and the Oversample Clock

NI 447X User Manual
This section provides low-level background information about the
electrical signals needed to tightly synchronize NI 447X modules. This
section describes three signals: the oversample clock, the SYNC pulse,
and the acquisition start trigger.
The 24-bit ADCs employed on the NI 447X belong to a class of
components called delta-sigma (or ∆Σ) ADCs. The advantages of
delta-sigma components as compared to other digitizers include high
dynamic range, excellent linearity, and digital filtering to remove aliased
frequency components from the data.
Most ADCs, including the successive approximation ADCs used in many
DAQ devices, are timed by a
pulse train that drives the acquisition. In most cases, a rising edge on the
sample clock signal starts a conversion. When an ADC is timed by a sample
clock, the acquisition rate is exactly equal to the frequency of the sample
clock. For example, a 10 kHz sample clock produces a 10 kS/s acquisition
rate.
One distinguishing feature of delta-sigma converters, including those on
the NI 447X, is that they use an
As the name implies, the physical frequency of the oversample clock signal
is greater than the sample rate. When a single NI 447X acquires data, the
high-frequency oversample clock is locally generated by a Direct Digital
Synthesis (DDS) chip on the device. If two or more NI 447X modules are
synchronized, they must share the oversample clock to ensure tight
synchronization of the acquisition.
♦
NI PXI-4472
The NI PXI-4472 can use either its internal DDS timebase or a timebase
received from another NI PXI-4472 over the PXI backplane. If you
configure the NI PXI-4472 to use the internal timebase and place the
NI PXI-4472 in Slot 2, you can program the device to drive its internal
timebase over the PXI backplane to another NI PXI-4472 that you program
to receive this timebase signal. The default configuration at startup is to use
the internal timebase without driving the PXI backplane timebase signal.
This timebase is software selectable.
sample clock. This clock is simply a digital
oversample clock
3-14
to drive the conversion.
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