Intro: I'm designing a makeshift oscilloscope and now need an input cascade for it (it is more fun project for me, rather than industrial-grade tool).
The matter is I want to get something faster compared to popular 200 kSps things based on arduino (atmega). My current approach utilizes TLC5540 adc with stm32f030 controller, giving 8 MSps - quite enough for now.
The problem is: that ADC seem to have input not well suited for using directly:
- it is low-impedance, hundreds Ohms, if I got it correctly
- it is about 2V range.
Thus I want input cascade with higher input impedance (at least 10-100 kOhms) and either unity gain (with resistor divider) or adjustable 0.5-0.2 gain.
OpAmp voltage follower - usually I use something like LM358 for such case - but it won't suit the intended frequency; there are others, like TL084 - but they seem to need more than 5V power supply... I can resort to this option if no better available... MC3317x and MC3407 may suit marginally...
NPN voltage follower - no problem with frequency, but it won't work linearly below ~0.7V input as I understand.
I think, I can manage with some shift and gain - e.g. if input
0..5 V range would be mapped to
0.8..2.2 V - but the mapping needs to be linear. E.g. stupid and straightforward approach could look like this:
As about accuracy - it is not an extreme-precision tool of course, so 3-5% error is not critical...
So what better options may I miss? - probably some JFET/MOSFET follower? Thank you in advance for suggestions!