Hi DwayneR and Slimfish,
DwayneR is correct. That is exactly why I suggested AC only, capacitive coupled inputs. To do this input circuit as a DC coupled, variable gain amplifier would cost a lot in terms of complexity, space, and money.
Regarding the input sensitivity mentioned earlier by Slimfish. The concept is to attenuate the input to accommodate the maximum P-P voltage you want to display on the scope. If you use a 10:1 probe, 30 V P-P becomes 3 V P-P, which is within the range of the ADC. With the probe switched to 1:1 the max input is 3 V P-P. By putting a 10:1 resistive divider on the input of the PGA113, these become 300V with a 10:1 probe and 30V with a 1:1 probe. If the maximum gain of the PGA113 is limited to 40 dB (gain of 100), then with a 1:1 probe the minimum input range is 300 mV P-P, which would be 37.5 mV per division (8 vertical divisions). This could be displayed as 30 mV / per division (with the extra 2 divisions not displayed, but available to see with Vertical offset).
By using a 3:1 attenuator in the PGA113 input, the input range would be:
10 V P-P/ division (80 V P-P for full display height) with a 10:1 Probe and
10 mV P-P/ division with a 1:1 Probe and max PGA113 gain of 100.
At the gain of 100, the PGA113 frequency response would be limited to about 300 kHz. At all other gains the PGA113 would exceed the Nyquist limit (500 kHz) of the ADC in the microcontroller.
If higher input voltage is desired, a 100:1 probe would allow a maximum 1kV input. Another possibility is to reduce the max Gain of the PGA113 to 10, reduce the attenuator and get 1 mV/ division input. I suspect that this would be useless because it would be very noisy.
Note that this design concept is based on using the protection diodes inside the PGA113 and requires enough series input resistance to limit the current to what the diodes can handle continuously at the maximum anticipated input voltage.
I think for this type of product, the 10V – 10 mV per division (100V Max input) input range using a switchable 1:1 – 10:1 Probe should meet most user’s needs.
Shazam