Electronic Design

Handy Oscilloscope Accessory Produces Multichannel Display

In circuit development and testing, it's often necessary to display more channels than today's oscilloscopes provide. One simple solution is to trigger several oscilloscopes in parallel. However, a single scope is more compact and allows more precise timing measurements to be made between channels. Also, by subtracting two input pulses, their time difference may be observed directly. Previously reported eight-channel display circuits lack inverting and adding features. Plus, their inputs are incompatible with common low-capacitance probes.

The accessory shown in the figure can convert one channel of a single-trace (or dual-trace) scope to operate as a dual-trace unit, complete with the usual Add, Invert, Chop, and Alternate modes. The circuit may easily be augmented using the MAX4310 multiplexer family to supply four or eight channels.

The AD8055 (or AD8056 dual version) input buffer amplifiers have an input impedance of 10 MΩ shunted with 2 pF, allowing the circuit's input impedance to be 1 MΩ shunted with 30 pF. This capacitance may be adjusted to match the oscilloscope's own input capacitance so that its low-capacitance probes may be used, unchanged, at inputs A and B. All amplifiers are provided with decoupling capacitors (not shown), as recommended by the manufacturers.

The scope's external-trigger input should be employed for triggering on an appropriate trigger signal, which may be signal A. Channel A has a dedicated, back-terminated buffer amplifier for triggering. Thus, any reflections returning from the scope's external trigger can't degrade the Channel A signal, which will be displayed intact. Furthermore, the AD8056 provides excellent trigger-line isolation from any interference due to the multiplexer. This is crucial for precision timing measurements between channels. Without such isolation buffers, toggling the MAX4310 would cause small changes in signal A that, in turn, would cause channel-dependent triggering and timing fluctuations—and, ultimately, error.

For the Chop mode shown, the MAX4310 multiplexer is switched by the 4060 oscillator/counter. Pin 1 of the multiplexer is switched in the 120- to 250-Hz range, which is adequate for all of the faster scope sweep settings. For most purposes, this generally provides a flicker-free, dual-trace display. The 4060B has additional counter outputs, which may be used to switch a MAX4311 (four channels) or a MAX4312 (eight channels), as required.

At slower sweep speeds, it may be necessary to use the Alternate mode, rather than the Chop mode. For the Alternate mode, the oscillator should be disabled, and the oscilloscope's Gate-Out signal must be connected to trigger the 4060B's clock input (pin 11).

All channels, except channel A, need an AD8055 buffer, followed by an AD8138 differential amplifier for any channel that needs an Invert function. Switch S1 changes the sign of channel B. The versatile AD8138 includes an "output common-mode- voltage" (VOCM ) input, which is biased by a 2-kΩ potentiometer, to adjust the trace separation.

The second AD8055 is configured to provide a sum output of (A ± B) in a 50-Ω terminated output cable. This amplifier, as well as the multiplexer amplifier, are back-terminated with 50 Ω, so any reflections from the scope inputs are eliminated.

An ECL pulse generator was used at input A or B to measure the rise time of the complete circuit. The trigger, multiplexer, sum, and difference outputs each exhibited rise times less than 5 ns on a 1-GHz sampling oscilloscope. This indicates that the accessory suits oscilloscopes with bandwidths up to 50 MHz.

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