[Design View / Design Solution]
Designing For High Speed In Current-To-Voltage Conversion
As new communications systems reduce the number of RF up-conversions, design of the digital-to-analog stage becomes more challenging.
Output thermal noise density is different for each resistor. For R2A, it is:
For RINA, it is:
For R1B:
For R2B:
The total output noise density is:
The differential circuit’s output noise density is 4.8 nV/vHz. Total voltage noise for the 100-MHz band above 100 kHz is 47.7 µV, over half a 14-bit LSB. In some applications, with the DAC’s ENOB of 10, the op-amp noise may not be significant. But it needs to be RMS-averaged with other noise sources for the full picture.
This simple analysis considered a few of the factors in choosing circuits for the voltage output in a high-frequency communications system with digital synthesis and showed what was possible with a fast, low-noise op amp. Multitone communication systems will place higher demands on the DAC and op amp. But any system needs to be built and characterized in conditions similar to those in the final application to guarantee wideband performance.
JOHN ARDIZZONI, application engineer in Analog Devices’ High Speed Amplifier Group, has authored numerous papers and is a contributing writer for the company’s RAQ’s column. He received his BSEE from Merrimack College, North Andover, Mass. He can be reached at john.ardizzoni@analog.com.
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