Difference between revisions of "StereoscanElectronics"
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Revision as of 11:21, 5 April 2014
Scanning, e-beam lens correction, and image acquisition functions may need to be taken over by software and/or rapidly prototyped analog hardware. Complex synchronous waveforms may need to be synthesized at high-speed in response to other high-speed events. Probably the most demanding task is to correct "wiggle" on one axis while sweeping the beam across another. Internal voltages may range from 1uV-30kV.
- ~100kHz Analog I/O Bandwidth
- 209.5dB Dynamic Range (not necessarily continuous, though it would be nice)
- At least 18 input and 18 output channels.
- In-phase acquisition and response.
- One-shot sample-and-hold behavior may be acceptable.
Maximum Speed, Maximum Dynamic Range ADC/DACs
Modern audio devices can achieve approximately 120dB dynamic range at 192kHz sampling rates. The remaining 89.5dB (1V-30kV) dynamic range may be achieved through rarely switched gain programming resistors, or a second ADC.
Usually these devices operate over I2S. Unfortunately, the I2S protocol requires ~4.6MHz per 24bit 192kS/s channel. Reading or driving 18 channels would require ~83MHz.
In-phase data acquisition response requires precise synchronization and low-latency (<100 microseconds). This probably necessitates a shared clock, and host software may need to implement one-shot functionality rather than real-time response. If synchronization between inputs and outputs is not feasible, a timing signal may be fed from an analog output to an analog input.
Candidate devices include:
- ES9018 (Claims to be a 32-bit DAC.)
Maximum Resolution ADC
LTC2440 is ATMega32U4/Arduino compatible, and most appropriate for independent low-speed applications (ie. vacuum gauge).
High Performance Analog Amplifier Chips
Same as used in the biosignal amplifier should work here, both for low and high-impedance measurement as well as fully differential (using an instrumentation amplifier).
I2S port seems to be derived from HDMI, which is output only.
I2S port speed doesn't seem to be specified, though one individual reports the use of a software clock to generate 19.2MHz, which some problems.
USB input, I2S output mode only. Unknown I2S port speed.
ATMega32U4 chips can quickly provide USB connectivity and manage ADC/DAC devices. Unfortunately, these systems are slow.
Probably the only way we can efficiently interface with a massive load of I2S devices. Need to get something easy to program, and compatible with important OpenCores like OpenRISC and I2S.
Platforms under consideration:
- spartan3 (HacDC may already have some.)
- Mojo v3 FPGA Development Board
- Altera DE0 Board
Some USRP devices may be directly capable of real-time synchronized operation at high speed. Cost is high, dynamic range is only 16 bit, and many ferrite common-mode chokes would be needed to minimize noise.