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Digital Storage Oscilloscope (DSO)

A cheap and simple analog input stage

You may know my schematics for a PC-based Digital Storage Oscilloscope, logic analyzer and data acquisition system:

Often people want to build a simplified, less expensive device. Maybe with a cheap 8 bit ADC and a fast micro-controller or small CPLD/FPGA. For this we need an analog input stage. OK, here is my suggestion: It consists of two cheap reed relays, two fast, current feedback operational amplifiers and a video multiplexer. The high impedance input stage is made by a JFET pair as seen in old school analog oscilloscopes, see "Art of Electronics" second edition, page 135. This amplifier is designed for an input range of +-18, +-10, +-5, +-2, +-1, +-0.5 and +-0.25 V (full scale) to feed an ADC with 2Vp-p single-ended input range, i.e. ADS830 of TI. I have done no tests, but I assume a (more or less) flat frequency response up to 100 MHz. Here is the schematics in PDF format. Its drawn with gschem, the free, open source schematics editor of the gEDA suite. Let me know if you are interested in the layout data, I will make it available here. But of course it's not too much work to redraw it with your favorite tool.