Higher Resolution, Smarter Sampling - NEW ACQ132CPCI DSP Firmware
The D-TACQ ACQ132-32-02C digitizer features 32 simultaneous analog inputs, each sampling continuously at 2MHz. The card is typically used in transient capture applications where the data is fed at full rate to the local 1GB DDRAM memory; after the shot the data is read out either over the backplane PCI interface, or, most commonly out the front panel gigabit Ethernet interface. The sampling resolution is 14 bit.
The same card may be fitted with 32MHz converters, and the additional data from these devices is used in a number of innovative ways:
- Oversampling Digital Filter
First, a digital oversampling filter may be configured in on-board FPGA logic. A typical setting configures an input sample rate of 32MSPS, accumulation over 16 samples, output sample rate 2MSPS. The oversampling increases the effective resolution of the system so that the card acts as a 32 channel 16 bit 2MHz unit, while greatly increasing the alias rejection of the system. The analog input channels are divided into four groups, and each group may be run with a different decimation factor, enabling multi-rate sampling. Further, input channels can be selectively disabled or "masked", allowing the remaining channels to operate at a higher rate, using the bus bandwidth normally assigned to the masked channels.
e.g.: The default configuration is 32 x 2MSPS, but other possibilities include
16x1MSPS + 2 x 16MSPS, 16 x 1MSPS + 4 x 4MSPS, 16 x 1 MSPS + 8 x 4MSPS etc.
for continuous application, the overall data rate should be kept less than or equal to 128MB/s
- Programmable Pattern Generator
The card implements a programmable 4-bit digital pattern generator. The patterns may be infinitely long, and one of the outputs may be assigned as the Gate in Repeating Gate Mode, above. The digital patterns may also be distributed to other cards in the CPCI crate using backplane PXI signaling lines, or out of the crate via a rear-side connector.
The sampling system captures data gated by an external pulse. The pulses may be generated on-board using the Programmable Pattern Generator. This reduces the overall amount of data to focus on externally defined "times of interest". If the gate pulses are short enough that all the samples in the pulse are held in local FPGA memory, much higher output sample rates may be used (eg 32 x 32MS/s). The gate pulses are delivered to memory accompanied by a timestamp to assist with post-shot data reconstruction.
A typical application for this device is in the field of fusion energy research serving a reciprocating Langmuir probe diagostic. Langmuir probes are used to measure the electric field in plasma fusion energy research. The probe comprises an array of sensors that have to be sampled simultaneously. The probe typically takes a number of plunges into the plasma. The digital pattern generator controls the plunges, and limits the overall amount of data by limiting the duration of sampling. The increased signal to noise ratio achieved by oversampling is a key factor.
The sampling system is sufficiently flexible for use in other basic energy science research designs, as well as aerospace test and sonar applications.