Transformers also tend to suffer more from gain and phase mismatching than high performance differential amplifiers. However it is more difficult to control signal path gain using a transformer while still maintaining impedance matching. Because transformers are passive, they generally have lower distortion than differential amplifiers. A transformer is a passive component and therefore consumes no power the input power is equal to the output power. The latter two factors present the biggest challenge and will be dealt with here in more detail.įor the analog input circuit there are just two component choices to consider a wideband differential amplifier or a balun transformer (the analog inputs must be driven differentially to optimise dynamic performance).
Generally the greatest system design challenges are associated with the differential analog input circuit, the low jitter differential clock circuit and capturing and processing the high speed digital data. The higher the conversion speed, the greater the instantaneous analog bandwidth that can be sampled. ADC sampling frequency and bandwidth directly dictate the performance of wideband RADAR systems, satellite receivers and radio telescopes. High sampling frequencies allow fast signals to be captured, displayed and analysed. Digital oscilloscope performance is directly related to the ADC sampling rate in the analog front end. System performance is mostly dominated by ADC sampling speed and analog input bandwidth, and a high performance FPGA or ASIC is required for data capture. The architecture of these systems share common features. Gigasample converters are used in a wide variety of applications precise measurement equipment (digital oscilloscopes, mass spectrometers, LIDAR modules), communications (point to point wireless links, wideband RADAR, satellite receivers) and scientific systems (particle detection, radio telescopes), to name a few. The good news is that reference designs are available that make the design of such sampling systems manageable, even without a strong RF or FPGA design background. This article will explore various aspects of such designs, touching on input signal conditioning, clock generation, signal routing and data capture, and in doing so will answer the above questions. Typical questions system designers might ask themselves are Do I have adequate RF knowledge? Do I need to become familiar with smith chart theory again and get the charts out of the bottom drawer? What kinds of amplifiers exist with GHz bandwidth and low distortion over that entire bandwidth? How will I generate a clock signal for a GSPS converter? How can I possibility capture and process data at gigabits per second? Is my lab adequately equipped to use and evaluate such technology? This is truly a mixed signal environment in which all the sub circuits have to be considered carefully to allow the analog to digital converter (ADC) to deliver the optimum dynamic performance.
Working with GHz analog signals is often referred to as a 'black art', so even experienced circuit designers are often hesitant when presented with such a challenge. Ultra high-speed data conversion offers many challenges to the system designer.