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Design IdeasJanuary 16, 1997 |
When you work with mixed-signal designs, it's often useful to view the timing of several analog and digital channels on the scope at once. Figure 1 accomplishes this task using two buffered multiplexer ICs and two TTL logic chips. IC1 and IC2 each combine a four-channel multiplexer with an op-amp on the output. The output of the multiplexer connects to the noninverting input of the op amp, and the inverting input is available as an external pin on the IC. This feature allows the multiplexer to have a buffered output and allows each channel to have different gain.
The key to Figure 1's operation is to have an oscilloscope with a gated sweep output. This feature is available on the rear of most scopes, usually with TTL output levels. On the Tektronix 2465A, the gated sweep output is a TTL Level 1 when the trace is sweeping and a TTL Level 0 when no trace is present. By applying this sweep signal to a negative edge-triggered counter, which in turn controls an analog multiplexer, and by giving each multiplexer channel a different dc offset, you can view eight analog signals in real time on the oscilloscope.
Two oscilloscope channels are necessary: one for the signal input and one for a dedicated trigger input. In operating the eight channels, you must select one as the trigger signal and feed this signal into the second channel on the scope. You must set the scope to trigger on this second channel.
The circuit feeds the gate signal from the scope into a 7404 inverter and then into the 74193 counter. Two of the four binary outputs of the 74193 select a channel from either IC1 or IC2. The third binary output, along with a second inverter, selects one of the two multiplexer chips and thus selects one of eight channels.
A resistor network at the input of each multiplexer divides the input signal by 10 so that the circuit can view ±15V signals using IC1 and IC2, which operate from ±5V. Dividing the input is easy and allows you to view eight TTL channels side by side on the screen or a mix of TTL and larger amplitude analog signals.
The triggering of this circuit is very stable, and an adjustable offset allows you to line up input signals in any convenient order without reconnecting input leads. The circuit works easily with 1-MHz signals and should be able to work with higher frequencies as well. (DI #1977)
| Figure 1 |
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| Using two multiplexer ICs and some TTL logic (a), you can view eight analog or digital (or some of both) signals on the oscilloscope (b). |
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