Typical measurement applications of Mixed Signal Oscilloscopes

Aside from capturing digital and analog signals on mixed signal devices, the primary measurement application of mixed-signal oscilloscopes (MSO) involve verifying and debugging microcontrollers (MCU)/digital signal processors (DSP)-based mixed-signal with data buses and embedded address.

Generally, today’s MSOs offer 16 channels of digital acquisition, leading to the mistaken assumption by engineers that MSOs are limited to eight-bit processing applications. It has to be noted that MSOs are used primarily to monitor digital and analog I/O, which consist of all the signals available in DSP- and MCU-based designs. Moreover, users should not get into the habit of relating the MSO’s number of digital channels of acquisition with the number of bits of processing within an internal bus-based DSP or MCU, as it is usually irrelevant. Having sixteen digital channels and two to four analog channels is more than adequate for users to verify and monitor dedicated/specific functions of eight-bit, 16-bit or even 32-bit DSP/MCU-based designs.

Monitoring data lines and parallel address in an external bus-based design, like a 32-bit microprocessor computer, is not the MSO’s primary measurement application. This could be best addressed with a logic analyzer. In addition, if users need to simultaneously time-correlate analog signals and/or analog characteristics of digital signals, multiple vendors offer a two-box solution of oscilloscope and logic analyzer. With this type of solution, users must accept the logic analyzer’s more complex use-model including its single-shot or slow waveform update rates.

An MSO offering 16-logic channels with two to four analog channels is enough to measure critical timing parameters even in 32-bit systems that feature external memory devices.

The MSO’s digital and analog acquisition performance is more important than the number of channels. The most basic specifications of the analog acquisition performance of the oscilloscope are sample rate and bandwidth.

The bandwidth of the oscilloscope should be approximately five times higher than its system’s highest clock rate. For singleshot/real-time measurements, the sample rate of the oscilloscope should be at least four times more than its bandwidth, or faster.

However, some logic analyzer and oscilloscope users do not fully understand the required acquisition performance of logic analyzers and MSOs. The digital acquisition performance of an MSO should be commensurate with its analog acquisition performance, and combining a low-performance logic analyzer with a high-performance oscilloscope does not make sense.

When using digital/logic acquisition channels, the measurement resolution of the device is restricted to plus or minus one sample period. Thus, if a user attempts to capture digital signals with up to 200 MHz toggle/clock rate, each of its high or low pulse could be as narrow as 2.5 ns. This implies that if the digital acquisition system of the MSO samples at up to two GSa/s, the timing measurements per edge of the pulse could be in error by ±500 ps, which translates to 40 percent error on a 2.5 ns pulse. This explains why digital channel acquisition sample rates must be at least twice the oscilloscope’s bandwidth or higher.

Another important specification of an MSO is probing bandwidth, which includes both the digital and analog system probing.