9042 Datasheet, PDF (22/26 Pages) Maxwell Technologies – 12-Bit, 41 MSPS A/D Converter

12-Bit, 41 MSPS A/D Converter
9042
Another option for ac-coupling is a transformer. The impedance ratio and frequency characteristics of the transformer
are determined by examining the characteristics of the input signal source (transformer primary connection), and the
9042 input characteristics (transformer secondary connection). “RT” should be chosen to satisfy termination require-
ments of thesource, given the transformer turns ratio. A blocking capacitor is required to prevent 9042 dc bias currents
from flowing through the transformer.
Figure 28. Transformer-Coupled Analog Input Signal
When calculating the proper termination resistor, note that the external load resistor is in parallel with the 9042 analog
input resistance, 250 ohms. The external resistor value can be calculated from the following equation:
where Z is desired impedance.
9042
A dc-coupled input configuration (shown below) is limited by the drive amplifier performance. The 9042’s on-chip refer-
ence is buffered using the OP279 dual, rail-to-rail operational amplifier. The resulting voltage is combined with the
analog source using an AD9631. Pending improvements in drive amplifiers, this dc-coupled approach is limited to ~75
dB–80 dB of dynamic performance depending on which drive amplifier is used. The AD9631 and OP279 run off ±5 V.
Figure 29. DC-Coupled Analog Input Circuit
01.07.05 REV 7
All data sheets are subject to change without notice 22
©2005 Maxwell Technologies
All rights reserved.