ADS62P49_1 Datasheet, PDF (56/77 Pages) Texas Instruments – Dual Channel 14-/12-Bit, 250-/210-MSPS ADC With DDR LVDS and Parallel CMOS Outputs

ADS62P49 / ADS62P29
ADS62P48 / ADS62P28
SLAS635A – APRIL 2009 – REVISED JUNE 2009............................................................................................................................................................. www.ti.com
For applications where only a band of frequencies are used, the drive circuit can be tuned to present a low
impedance for the sampling glitches. Figure 99shows an example with 1:4 transformer, tuned for a band around
150 MHz.
Differential
input signal
1:4
5W
0.1mF ^{100 W}
100 W
72 nH
5W
INP
25 W
15 pF
25 W
INM
VCM
Figure 99. Drive Circuit with 1:4 Transformer
Input Common-Mode
To ensure a low-noise common-mode reference, the VCM pin is filtered with a 0.1µF low-inductance capacitor
connected to ground. The VCM pin is designed to directly drive the ADC inputs. The input stage of the ADC
sinks a common-mode current in the order of 3.6µA / MSPS (about 900µA at 250 MSPS).
REFERENCE
The ADS62Px9/x8 has built-in internal references REFP and REFM, requiring no external components. Design
schemes are used to linearize the converter load seen by the references; this and the on-chip integration of the
requisite reference capacitors eliminates the need for external decoupling. The full-scale input range of the
converter can be controlled in the external reference mode as explained below. The internal or external reference
modes can be selected by programming the serial interface register bit <REF>.
56
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