In a former post
https://www.eevblog.com/forum/projects/usb-logic-analyzerscope-with-sigrok-apprentice-project/ I have been looking for a cheap and simple MSO project, but after many interesting ideas, this came down (due to apprentice level complexity, budget and effort limits) to a USBee AX-pro compatible version. I could simply buy this (e.g. LHT00SU1 clone, see PCB picture here
https://www.eevblog.com/forum/projects/usb-logic-analyzerscope-with-sigrok-apprentice-project/msg4943815/#msg4943815, remember volume I am interested in is around 20-50 pcs), however its "front-end" is pretty poor.
So here I am looking for a front-end feeding an 8 bit ADC with 12 MS/s max. sampling rate. I want it isolated (current plan is to use digital isolators for the 8 ADC data and 1 clock lines), in addition we have lots of pulsed signals and little "true" analog, so I'd go for a somewhat higher bandwidth and am not worried about aliasing. 10 MHz sounds adequate. From the voltage range, must is 50 V max. fullscale range, going down to maybe (wish) 5 mV fullscale, which would be a 1:10000 range. Besides AC/DC coupling I'd like an adjustable offset so you could zoom into a 10 mV signal sitting on a 1.5 V baseline.
Unless I find someone who modifies sigrok to control front end parameters (attenuation, coupling, offset) (or do it myself), changing all those settings has to be manual, in its most primitive form by jumpers and a potentiometer for offset. Better of course programmable in case someone finds the time to implement that in sigrok.
Oscilloscope frontends have been discussed here a number of times, although usually for higher bandwiths. Old Tek or Hameg (see here
https://cdn.rohde-schwarz.com/hameg-archive/HM205-3_deutsch.pdf on page 37 for a HM205-3 input amplifier schematic) designs are not really helpful as they are designed around custom multi-level cam switches, which are difficult to emulate electronically.
Better starting points seem
https://hobby-research.at.ua/NS3/NS_3.0_rev_3.0_.pdf (mentioned somewhere in this forum) or the Hantek 6022BE
https://www.eevblog.com/forum/testgear/hantek-6022be-20mhz-usb-dso/msg2554422/#msg2554422 frontend.
Naively I thought "TI makes those nice PGAs, just use one of those"... best I came up with was PGA113 with "scope gains" 1 2 5 10 20 50 100 200, however its 10 MHz x1 bandwitdh drops to 230 kHz at x200.
So I think direction is one or two prescalers (1:100, 1:10) and one or two selectable gain stages (1 up to 50 as shown in the schematic above). Stanford Research SR560 might also be an inspiration, but this is very low noise and bandwidth 1 MHz only.
Final consideration would be about probe connection - adding a 1/10x passive probe would increase dynamic range "for free", but those I have encountered were crap quality and simply adding a passive probe triples the bulk of the whole setup. I find the tiny size of the Saleae logic 8 really nice - needs about as much space as your mouse. So I'd put the analog input just besides the digital ones on the 0.1" connector for adding "dupont" style mini grabbers. At 10 MHz bandwith and 10 cm cable I think I could get away with that.
Any ideas?