Hi Slimfish and all,
I have finally had some time to spend on the Programmable Gain Input stage. Slimfish, the PGA ICs you list and the LTC you feature in the schematic all have different characteristics. The LTC6912 has poor input characteristics (low impedance) so it requires the two op amp buffers you added. Another issue is there is No way to adjust the offset on a large scale. This is likely used to adjust the vertical position of the trace by the U10 pin PB11 ST uC.
After having evaluated the parts you list and many more, the TI PGA113 will provide the High Input Impedance, Low Capacitance, Low Offset & Drift, Self Calibration, and a Reference Input which can be used to move the trace vertically. It also has two separate power supply pins, one for the amplifier and one for the output buffer to match the ADC in the ST uC. The PGA113 has a two channel multiplexer, a high constant impedance and the bandwidth, noise, distortion and drift are more than adequate for the DSO Nano.
It runs off the existing +5 and +3 V power supplies (does not require an +/- supply) and does not need a separate input buffer. So this eliminates the U4, U5, and U7 ICs. Note that U7 also is the output buffer for the test signal (U10 pin PD 12), so a small buffer op amp will be needed.
To use this part, close attention will be needed to properly de-couple the power supplies, filter the PWM out from U10 pin PB11 for a clean DC offset voltage (may also require a Buffer Op Amp), and layout and shielding to minimize noise. The inputs should have a 10,000 Ohm or higher resistor in series to limit overload. A 100 kOhm HV resistor would be better (protects to 1000V input) but it may affect the max bandwidth a bit.
The part is less than $2.00 (about $1.00 in a full reel quantity).
Another possibility is to use the TI PGA117, which has 10 multiplexed inputs. This would add the ability to use it as a 2 channel analog input with an additional 8 channels of logic. Unfortunately this would require something like a 10 pin 1 mm header and a cut out in the case to access the connector for the 8 Logic Inputs. I suspect that the uC would require additional memory (external) as well. Just a thought.
Auto Power Off and DC Input. I think most people would not want a simple Auto Power Off, unless it could be adjusted for time and disabled. This may cost more than it is worth. With a good charging circuit, battery life should be good, and when connected to external USB power, it is not an issue. Small USB power supplies are readily available for a few dollars on e-bay and elsewhere. DC input would be difficult to implement, especially with some Overload Protection for the DSO Nano.
Shazam