Cloning an A/D converter that hit end-of-life presented engineering challenges
March 21, 2012 | Tony Khazen | 222903188
Tony Khazen, Director, DATEL, a Business Unit of Murata Power Solutions outlines some of the challenges faced when selecting a 'cloned' analog-to-digital converter to replace an obsolete model.Data Converter products are widely available from multiple sources. There are very few pin-to-pin replacement and directly compatible products within the industry. An analog-to-digital converter of a particular resolution and speed from several suppliers may be quite dissimilar in package size or type, pin-out, functionality and performance.
Some converters may require multiple supply voltages while others may require only one. Some may need multiple ground pins while others may use separate analog and digital ground returns. The digital timing and analog front end are typically different between manufacturers. This dissimilarity came to light when a prominent military equipment supplier was hit with an end-of-life situation on one of the A/D converters that was used in their equipment.
This supplier of radar systems was in urgent need of a drop-in replacement A/D converter. They approached Datel seeking a “cloned” A/D for the obsolete model in the original design. Our engineering team accepted the opportunity but the challenges were more than anticipated. Certainly, the first task was to fit the replica in the same size package and map the circuit to the same pin-outs.
Timing was a tough undertaking, especially the “output data valid” time that had to mesh with the existing system timing. Aperture Delay is an important parameter for radar applications, especially for I and Q demodulation. Much design time was required to perfectly match the performance of the obsolete part. Because two units are required per radar system, (one for the I and one for the Q channel) the aperture delay could not vary from one unit to another.
Figure 1 shows the measurement of the Aperture Delay while Figure 2 shows the variance of Aperture Delay over temperature and between different units. Similarly, Aperture Jitter is another key parameter when sampling a high input frequency waveform. The design required sub-4 picoseconds in order to preserve the accuracy of the conversion and maintain the desired Signal-to-Noise ratio of 83 dB.
Figure 1. Aperture delay measurement
Figure 2. Variance of aperture delay over temperature and different units
The analog input characteristics of the A/D converter demanded special attention because it was being driven by a transmission circuit. The analog input voltage range, along with its bandwidth and settling time accuracy, had to closely match the counterpart A/D.
Most importantly, the input impedance characteristics of the A/D had to be tweaked such that the Voltage Standing Wave Ratio (VSWR), a critical parameter in a communication system, matched the performance of the original design over a defined frequency range. VSWR gives an indication of the amount of signal reflection back to the input signal from the A/D converter or, in another words, it is the amount of return loss. As shown in Figure 3, the VSWR ratio has to be actively trimmed and tweaked on every unit in order to meet the requirement.
Figure 3. VSWR unit trim to achieve specification
The layout of the A/D required delicate routing of signals, ground traces, and distribution of the power supplies in order to mimic the performance of the old A/D. Extra substrate layers and conductor runs were used in the design to accommodate the pin-out assignment and to avoid any signal coupling that could deteriorate the accuracy of the converter. The power supply and ground planes along with bypass capacitor layout were critical in order to avoid any kickback and contamination of sensitive signal paths as well as analog sections of the converter.
In its completion, the design was a form-and-fit solution that allowed our customer to drop the new component into the existing socket without change to the PCB or the system. This end user was able to keep the same documentation, test systems, and procedures – and simply add Datel as the qualified supplier of the A/D converter.
The new design showed an improvement in the overall accuracy of the radar system – a by-product of the cooperation between both of the companies’ engineering teams. While an equivalent part was the only requirement, the Datel A/D product was a functional upgrade to the system. The two-tone modulation FFT test for a typical unit is shown in Figure 4, where all the spurs created are less than -90dBFS. This experiment gave our customer a strong confidence in our engineering and process capabilities. Two years later, the same customer approached Datel to provide a pin-to-pin upgrade of the product at twice the sampling rate and with an improved Signal-to-Noise Ratio.
Figure 4. Two-tone intermodulation test
Military applications tend to run for long periods of time, and therefore, a long-term supplier is required for this market. At Datel we have been a supplier for over 35 years to many military and medical programs, and understand the importance of keeping products alive and viable for a long period.
About the author
Tony Khazen is currently the Director of the DATEL, a Business Unit of Murata Power Solutions. Khazen has been employed by the company for 23 years. Khazen started as a Design Engineer at DATEL in 1988 and in 1992 he was appointed as the Data Acquisition Engineering Manager.
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