The very first design decision we had to consider was the type of input probe to use as this greatly determined the design parameters of our analog front-end. A typical BNC oscilloscope will have options for either a 1:1 or 10:1 passive probe configuration. This ratio describes the attenuation the signal goes through as it enters the oscilloscope and can be used to view larger amplitude signals that the analog front end might otherwise be able to handle. The circuit configuration for a 10:1 passive scope probe is shown below.
Initially, we decided to use a 10:1 probe circuit in order to accommodate larger input signals. This was largely motivated by two factors: uncertainty of board noise and desire for a wide dynamic range of inputs. We later revised this decision to a 20:1 circuit, based on the same principles, due the the maximum input voltage to the Variable Gain Amplifier. Additionally, as we began picking ICs we realized that it had become necessary to DC bias the attenuated input signal before it went into the filter IC. Therefore, we implemented a typical DC bias circuit by modifying the attenuator and adding and extra resistor to 3.3V.