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Challenges in cloning ADC that hit end-of-life

Posted: 18 Jan 2013 ?? ?Print Version ?Bookmark and Share

Keywords:analogue-to-digital converter? Signal-to-Noise? Voltage Standing Wave Ratio?

Data converter products are broadly available from a number of sources. There are very few pin-to-pin replacement and directly compatible products within the industry. An analogue-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 analogue and digital ground returns. The digital timing and analogue 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-4ps 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 analogue input characteristics of the A/D converter demanded special attention because it was being driven by a transmission circuit. The analogue input voltage range, along with its bandwidth and settling time accuracy, had to closely match the counterpart A/D.

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