PCIe-24DSI32
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4-2
4.2 Analog Inputs
The 32 analog input channels are arranged in four channel groups, with each group containing
one-fourth of the channels present on the board. Group 0 through Group 3 can be designated
as either active or inactive, with only active groups sending input data to the data buffer.
Analog-to-digital conversions can be performed on signals from any of several sources, which
are selected by the input configuration switches shown in Figure 4.1. During normal operation,
the analog input signal passes through a differential amplifier which removes any common
mode voltage that might be present. During selftest and autocalibration operations, the input
configuration switches can be used to replace the system input signal with either a precision
zero (ground) or positive full-scale reference voltage.
A range control attenuator adjusts the maximum level of the signal to the full-scale input range
of 2.0 VRMS required by the ADC. High frequency noise and digital filter images are
attenuated by a 2-pole Butterworth lowpass filter, the cutoff frequency of which can be selected
as either of two fixed frequencies.
The final conditioned, scaled and filtered input signal is digitized by each ADC into a 24-BIt
serial data word that is deserialized into parallel format by the local controller. The local
controller then attaches a 5-bit channel tag to the data word, applies offset and gain correction
factors, and finally transfers the corrected data to the input data buffer.
Antialias filtering is provided by each ADC in the form of a lowpass digital filter with a sharp
cutoff frequency at approximately 40-50 percent of the sampling frequency. The digital filter
has no filtering effect at multiples of the sampling clock, which is a multiple (x32-128) of the
sampling frequency. To prevent extraneous signal frequency components within these "filter
images" from appearing in the passband, the hardware image filters shown in Figure 4.1 can be
configured to provide filtering within the digital filter images.
4.3 Autocalibration
Autocalibration is an embedded firmware utility that calibrates all input channels to a single
internal voltage reference. The utility can be invoked at any time by the control software.
The offset error of each channel is determined first, by selecting a zero input reference level
and storing the values reported from all channels. A precision internal voltage reference then is
used to calculate the gain error, and both offset and gain correction values are stored in static
RAM for retrieval during data acquisition.
The internal voltage reference is adjusted to equal 99.000 percent of the selected input voltage
range. This in-range value ensures that the ADC's will provide the nonsaturated, or in-range,
responses that are necessary during the calibration adjustment process.
4.4 Sampling Clocks
An internal sample rate generator provides a frequency range of 20-55 MHz, which is divided
down by a software-specified integer to provide sample rates from 2.0 KSPS to 200 KSPS.
