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Multi-channel DAQ board uses switched-cap over-sampling A/Ds

Posted: 19 Jan 2006 ?? ?Print Version ?Bookmark and Share

Keywords:Alex Mendelsohn? VME? data-acquisition card? Interactive Circuits and Systems? ICS?

VME bus data-acquisition card

This 16-channel forced-air cooled 6U-sized VME bus data-acquisition card is actually an Interactive Circuits and Systems Ltd (ICS) product. ICS is now part of the Radstone Group.

Priced starting at about $10,600, Radstone Embedded Computing's board is replete with analog signal conversion and conditioning based on a pair of Analog Devices AD9260 data converter chips. The AD9260 is a switched-capacitor CMOS over-sampling A/D (analog-to-digital) converter chip.

The AD9260 helps the ICS-8145 board simultaneously sample on its sixteen channels. These ICs operate at speeds to 20MS/s per channel (the actual data output rate depends on the over-sampling ratio selected; it can be either 8 or 4).

The AD9260 chip also contributes to high dynamic range, especially when running at its 8X over-sampling ratio. That's done on-chip using a technique that combines sigma-delta and flash pipeline converters. The IC also includes an on-chip decimation filter and a series of three half-band FIR (finite impulse response) filter stages.

Sync for multiple boards
Back to the ICS-8145 board itself. Not mentioned in Radstone's release notes (on the left) is the fact that the '8145's sampling clock and trigger can be synchronized for use with as many as sixteen boards. Triggering can be internal (software controlled) or external. The trigger is internally synchronized to the sampling clock by the ICS-8145.

Radstone's triggering scheme gives you the ability to configure high channel-count systems, such as those used in phased array radar beam-forming applications. This syncing is done by means of a synchronizing cable or backplane overlay.

Master-slave operation
To achieve multi-board sync, one board is designated as master. It then provides clock and trigger signals to slave boards. Both internal or external clock and trigger signals can be used with multiple board configurations. In the case of external clock and/or trigger signal, you supply external signals to the master; it, in turn, distributes the clock and trigger to the slaves.

As indicated, the ICS-8145 provides a number of clock and trigger options. An internal sampling clock based on a programmable oscillator gives a resolution of less than 2.3kHz at the output rate, over a range of 57kHz to 20MHz.

Or, you can use an external sampling clock of twice the required over-sampling frequency. So, if a sample output rate of 2.5MHz/channel is required, using 8x over-sampling, the input sample rate would be 20MHz and the clock frequency would be 40MHz.

An FPDP II port
The board's VME64 interface also provides control and data access, but you can use the system's FPDP II (front-panel data port) as an output, permitting multiple-card high-bandwidth systems to be assembled without interrupt latencies. The FPDP II interface is programmable from 10MHz to 50MHz, giving you up to 400MBps bandwidth. The VMEbus supports DMA (direct memory access) master/slave interfacing. Both VME and FPFP can be used simultaneously.

Although the ICS-8145's FPDP II interface provides a path for rapid transfer of A/D data, this interface will also revert to ANSI/VITA 17 FPDP operation when connected to interfaces that don't support FPDP II. That runs at 160MBps.

In high channel-count applications, where high data-rate data is read across the VME bus, system interrupt latency can also be a problem. Latency can result in loss of data, especially if a non-realtime operating system is used.

To overcome that, the ICS-8145 packs 2MB of on-board storage, and, as the press release notes, it can be expanded to 8MB. The board is also designed to support transient capture, where the large memory buffer also comes into play.

Three capture modes
In operation, data is acquired in either a continuous mode or in a capture mode. For capture mode, a programmable number of samples are at each application of a trigger. The ICS-8145 can also be operated in a circular data-buffering mode to collect data before and after a trigger. You can operate in a capture mode without pre-trigger data or in a capture mode with pre-trigger data.

The board accepts differential signals in four Input voltage ranges. These include 10V p-p, 1V p-p, 100mV p-p, and 10mV p-p. These ranges are software-selected, and gain is programmed in groups of four channels each. Each channel has a high-Z input of greater than a M?.

- Alex Mendelsohn

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