Dedicated scan generator IC vs DACs for scanning electron microscope

[Nerdbert]

Hi everybody,

for my work at university I'm basically building a scanning electron microscope. For that I'm in need of 2 phase coupled triangle or sawtooth waves to drive the deflecting elements, ideally with a sync pulse when the wave reaches the maximum. That could however be done afterwards with a comparator.

The maximum needed frequency would be in the 100 kHz range. An adjustable amplitude and offset would be helpful, but that could also be done with an op-amp circuit afterwards, I guess.

Finally there is no need for high voltage since we are going to use high voltage amplifiers.

The question is, are there any dedicated ICs that do that sort of thing? Maybe something that was used in CRTs?
Or would I be better off just using some ADCs?

Thank you very much for your help.

[dmills]

An opamp, cap and resistor will get you a ramp, maybe use the micro to generate the reset pulse to discharge the integrator capacitors (Or go old school and do that with a 556!).

Of course the nice thing about a DAC is that you can use lookup tables to produce non linear sweeps to correct for (some) issues in your electron optics, that right there might make it worthwhile.

Regards, Dan.

 

[Berni]

You can look into CRT oscilloscope time base ramp generators. They are often built out of transistors and passives and can work at MHz rates.

The downside of them is that while very simple they could be difficult to digitally set the speed over a wide range of ramp speeds. In oscilloscopes they use big rotary switches to swap the passives out as you select different timebases.

Digital DAC methods do provide flexibility advantages where you can easily set the speed accurately over a very wide range of sweep rates. And yes being digital you can also use a lookup table to fix any potential linearity issues in the amplifiers or the electron deflection system. You can run a DAC at a few MHz sample rate using SPI and a MCU. Adding some anti aliasing filtering on top and you get your nice smooth ramp.

[Marco]

You could just use one of the low cost Analog Devices DDS ICs, or one of the many modules thereof.