August 1998, Issue 97

BitScope
A Mixed-Signal Capture Engine

VOLTAGE RANGES

The BitScope DSO includes four internal attenuation ranges and four channel inputs. Channel A and B are BNC connectors that may have ´1 or ´10 probes connected. Channel C and D (pod inputs) have a fixed attenuator, and possibly, there’s some extra circuitry in the pod.

Table 1 details the range sensitivities. The ranges aren’t nearly as comprehensive as a bench CRO, but it covers those most useful to digital and analog circuits. As well, I intended for the pod connector to deal with unusual or high voltage signals by way of an active pod adapter.

It’s also possible to alter the gain of some ranges. Since the ADC output is an eight-bit number that ranges from 00 to FF, the final interface just needs to ratiometrically apply this hex value to the voltage range of each stage.

A little thought reveals that for a digital oscilloscope, volts per division and microseconds per division are quite arbitrary notions. Provided that the signal under consideration is within the ADC range and the sample-buffer size, a display can be of any size and grid spacing. Similarly, the notion of y offset becomes a display function, which has nothing to do with the sample engine.

IN YOUR HANDS

With this design, I hope to have presented a low-cost solution to the engineer’s needs for sophisticated test equipment. I have heeded the call for more open designs and liberation from the single-platform juggernaut.

In the coming months, I look forward to hearing from any of you who can think of applications for this device that I haven’t even dreamed of.

Norman Jackson is principal hardware design engineer for Discrete Time Systems P/L in Sydney, Australia. He designs DSP-based digital audio systems for use in film and TV postproduction. You may reach Norman at normj@ discrete.net.