Simple oscilloscope accessory produces multi-channel displays

　　In circuit development and testing, it is often necessary to display more channels than the existing oscilloscopes provide. A simple solution is to trigger several oscilloscopes in parallel. However, a single oscilloscope is simpler and allows more accurate timing measurements between channels. In addition, by subtracting two input pulses, their time difference can be directly observed.

　　The accessory circuit shown in Figure 1 converts one channel of a single-trace (or dual-trace) oscilloscope to operate as a dual-trace unit with the usual Add, Invert, Chop, and Alternate modes. This circuit can be easily expanded to provide 4 or 8 channels using the MAX4310 multiplexer.

　　The AD8055 (or AD8056) input buffer amplifier has a 10MΩ input impedance (in parallel with 2pF), which allows the circuit to have a 1MΩ input impedance (in parallel with 30pF). This capacitance can be adjusted to match the input capacitance of the oscilloscope itself, so low capacitance probes can be used (at inputs A and B). All amplifiers should have decoupling capacitors.

　　The external trigger input of the oscilloscope should be triggered with an appropriate trigger signal, which can be signal A. Channel A has a dedicated reverse-end buffer amplifier, which is suitable for triggering. Therefore, any reflection return from the oscilloscope's external trigger cannot degrade the channel A signal, and the display will remain intact. In addition, the AD8056 provides trigger line isolation for interference caused by the multiplexer.

　　Isolation is critical for accurate timing measurements between channels. Without this isolation buffer, triggering the MAX4310 would result in small changes in signal A. This in turn would cause channel-dependent triggering and timing fluctuations, and at a minimum, result in errors.

　　For the Chop mode shown, the 4060 oscillator/counter switches the MAX4310 multiplexer. Pin 1 of the multiplexer switches between 120 and 250 Hz, a range that is suitable for all faster oscilloscope sweep settings. For most purposes, a flicker-free dual-trace display is generally provided. The 4060B has an additional counter output that can be used to switch the MAX4311 (4 channels) or MAX4312 (8 channels) if desired.

　　At lower scan speeds, the Alternate mode must be used instead of the Chop mode. For the Alternate mode, the oscillator should be disabled and the Gate-Out signal must be connected to trigger the 4060B's clock input (pin 11).

　　All channels except Channel A require an AD8055 buffer after the AD8138 differential amplifier, and any channel requires an inverting function. Switch S1 changes the sign of Channel B. The general AD8138 includes an "output common-mode voltage (VOCM)" input that is biased with a 2KΩ potentiometer to adjust the trace separation.

　　The second AD8055 is configured to provide (A±B) and output to a 50Ω terminated output cable. This amplifier, as well as the multiplexer amplifier, are back terminated with 50Ω, so any reflections from the oscilloscope input are eliminated.

　　The rise time of the entire circuit was measured using an ECL pulse generator at input A or B. The trigger, multiplexer, and differential output all showed rise times of less than 5ns on a 1GHz sampling oscilloscope. This indicates that this accessory is suitable for oscilloscopes with bandwidths up to 50MHz.