Build a digital-storage oscilloscope for your high-performance logic analyzer.

Combine this add-on device, March's Logic Analyzer, and almost any PC, and get a full-featured digital-storage oscilloscope.

Everyone who works with electronics would like to have a decent oscilloscope as a part of their bench equipment. Unfortunately, top-end oscilloscopes command top-end prices, putting them out of reach of some hobbyists. No doubt you've studied the article "Build a High-Performance Logic Analyzer" that was featured in the March 1998 issue of Electronics Now. That device sported an expansion port to which you could attach additional add-on modules.

As promised, this month we are presenting such an add-on module. With this module, you can turn the High-Performance Logic Analyzer into a 40-million-samples-per-second dual-trace digital-storage oscilloscope. Like the Logic Analyzer, the digital-storage oscilloscope, or DSO, connects to any PC and only requires DOS to run the host software with a CGA or greater display. Those computer requirements mean that you can easily recycle that old PC that can't run any modern software, but is still too good to just get rid of.

The unit itself can sample two analog channels and four digital inputs simultaneously. The sampling rate can be chosen from any of 10 built-in rates ranging from 40 MHz down to 312.5 kHz. If those rates don't fit your needs, you can use either of the logic analyzer's two external clock inputs. The oscilloscope features include the ability to use standard 10X scope probes, three trigger modes including a sweep-triggered mode with settable level and slope, plus triggering of the storage cycle with the digital-logic inputs.

Like the logic analyzer that it works with, the DSO can store up to 2048 samples. Additional advantages of a computer-based DSO include the ability to capture and view a transient signal, both before and after the trigger event, and the ability to save the captured data to disk for printing and later study.

Designing a DSO. The functions of a logic analyzer and digital-storage oscilloscope are in many ways very similar. In a DSO, digitized analog signals are stored for later display, much like a logic analyzer, which stores digital signals for later display.

To take advantage of that fact, the companion logic analyzer was designed to hold all of the functions that are common to both devices. That way, the DSO module is simply an analog front-end for the logic analyzer. The main advantage to that approach is that it helps keep the cost of the project down. The result is having two very powerful test instruments, a logic analyzer and a digital-storage oscilloscope, for about the same cost as a digital-storage oscilloscope alone.

Circuit Description. The schematic diagram for the DSO is shown in Fig. 1. The two analog channels are identical, so only one channel will be described; the other channel works the same way. The input signal is buffered by IC1-a, a high-speed FET-input op-amp. There is no gain in that first stage as its only function is to prevent loading the source of the input signal - which is why the op-amp has a FET-based input. The actual input impedance is set by R1 and the scope-probe resistance (usually 9 megohms for a 10X probe).

The buffered signal is then fed into high-speed op-amp IC2-a, which is configured as an inverting amplifier. The amplifier's gain can be varied with R7. An offset voltage is also added so that the amplifier signal will be within the proper voltage range needed for digital conversion. The offset voltage source comes from IC4, the analog-to-digital converter. The reference voltage is buffered by IC3-b, and then inverted by IC3-a. The amount of offset can be varied with R13 in order to adjust the …