Hi all,
in my last message, i proposed a few DSO areas that could be improved. As i think input stage was the most difficult (and interesting one) of the list, i started with that.
My basic design goals:
-Cheap enough (<$5)
-Input impedance 1 MOhm for the whole input range (x10 probe friendly)
-AC+DC capable
-More accurate & less noisy
-Easy to calibrate (Offset + gain)
Other less important goals:
-Small size
-Single supply (while conserving ground for reference)
-Easily extendable to dual channel
The image below shows the designed circuit. As depicted is dual-channel, but can be easily ported to single channel. The integrated circuits used don’t have to be those exactly. Input stage AD8615 (single op-amp) can be replaced by a cheaper dual op-amp version (AD8616) or by less capable version (like AD8602). Both op-amps are from Analog Devices, but any other that fits input noise, input current, Vos & GBW parameters can also be valid.
For the programmable gain amplifier a LTC6912 is used. It’s digitally controllable, dual channel, low noise and has an integrated midsupply. Other alternatives exist and can also be used with minor modifications/additions (MAX9939, PGA113, AD8231…).
In terms of cost, TL082, SP3232, HC51 are not needed anymore so althought initial IC cost is near the $5 for the dual channel version, the final cost will be lower.
As i said in a previous message, this schematic is only a schematic (a very first draft of it). Althought i made some minor simulation of it, it’s necessary to build it to check if everything works as expected. I expect to build it in a few days to validate it. In the meantime, all comments/suggestions/complaints are welcome.
Slimfish
