Most oscilloscopes are required to be DC-coupled in order to monitor DC or low-frequency signals. This requirement forces the design to use DC-coupled, fully differential amplifiers to convert from single-ended signaling at the front panel to differential signaling at the ADC. This design uses two differential amplifiers. The first amplifier shown in Figure 8-1 is the LMH5401 that converts from single-ended to differential signaling. The LMH5401 interfaces with the front panel through a programmable termination network and has an offset adjustment input. The amplifier has an 8-GHz, gain-bandwidth product that is sufficient to support a 1-GHz bandwidth oscilloscope. A second amplifier, the LMH6401, comes after the LMH5401 to provide a digitally programmable gain control for the oscilloscope. The LMH6401 supports a gain range from –6 dB to 26 dB in 1-dB steps. If gain control is not necessary or is performed in a different location in the signal chain, then this amplifier can be replaced with a second LMH5401 for additional fixed gain or omitted altogether.

The input of the oscilloscope contains a programmable termination block that is not covered in detail here. This block enables the front-panel input termination to be programmed. For instance, many oscilloscopes allow the termination to be programmed as either 50-Ω or 1-MΩ to meet the needs of various applications. A 75-Ω termination can also be desired to support cable infrastructure use cases. This block can also contain an option for DC blocking to remove the DC component of the external signal and therefore pass only AC signals.

A precision DAC is used to configure the offset of the oscilloscope front-end to prevent saturation of the analog signal chain for input signals containing large DC offsets. The DAC8560 is shown in Figure 8-1 along with signal-conditioning amplifiers OPA703 and LMH6559. The first differential amplifier, LMH5401, is driven by the front panel input circuitry on one input, and the DC offset bias on the second input. The impedance of these driving signals must be matched at DC and over frequency to ensure good even-order harmonic performance in the single-ended to differential conversion operation. The high bandwidth of the LMH6559 allows the device to maintain low impedance over a wide frequency range.

An antialiasing, low-pass filter is positioned at the input of the ADC to limit the bandwidth of the input signal into the ADC. This amplifier also band-limits the front-end noise to prevent aliased noise from degrading the signal-to-noise ratio of the overall system. Design this filter for the maximum input signal bandwidth specified by the oscilloscope. The input bandwidth can then be reconfigured through the use of digital filters in the FPGA or ASIC to limit the oscilloscope input bandwidth to a bandwidth less than the maximum.