XRD64L43 Datasheet, PDF (10/20 Pages) Exar Corporation – Dual 10-Bit 40MSPS CMOS ADC

XRD64L43 Preliminary
APPLICATION SECTION
Voltage References
The top ladder voltage for the XRD64L43 can be provided
from an internal bandgap reference. The bandgap reference
and its feedback path, Pins 1 and 2 respectively, can be used
to set the voltage for VRHF. Select Rf and Ri (if gain is
necessary) so that VRHF=VBG(1+Rf/Ri). The internal
bandgap voltage is 1.24 volts. The XRD64L43 has a low
impedence ladder, therefore, the typical value for Rf and Ri
is 10K (Rf and Ri are recommended to be greater than
5K).See Figure 2. for a simplified diagram. Decoupling caps
on the sense inputs to AGND should be used to reduce
injectioin of high-frequency noise.
Ri
Rf
VBG
VFBK
VRHF
VRHS
VRLS
VRLF
Bandgap
XRD64L43
Resistive Ladder
AVdd
VBG
VFBK
VRHF
VRHS
VRLS
VRLF
Bandgap
XRD64L43
Resistive Ladder
Figure 3. Voltage Reference Provided by an Ex-
ternal Source as Direct Inputs
Single-Ended Inputs
The XRD64L43 can be used in either single-ended or
differential input mode. For differential inputs, see the
Differential Inputs Section. Single-ended inputs mini-
mize the amount of external components necessary to
interface with the XRD64L43. The common inputs,
VINA(-) and VINB(-) should be tied to ground. VINA(+)
and VINB(+) can be used to apply direct inputs to the
XRD64L43. Figure 4. is a simplied diagram for single-
ended inputs. Pin 16, DIFF should be held low to select
single-ended inputs.
Figure 2. Voltage Reference Generated from the
Internal Bandgap Voltage w/gain
External voltage references can be forced at VRHF and
VRLF. If VRHF and VRLF are driven externally, VFBK
should be connected to AVdd, which tri-states the
bandgap reference. Direct inputs or inputs driven by
external amplifiers can be used to drive the ladder
reference voltages of the XRD64L43. See Figure 3. for
a simplified diagram. The sense inputs are intended for
sensing purposes only and care must be taken to insure that
no current flow be present in the sense lines.
Input A
VINA(+)
VINA(-)
50
Input B
VINB(+)
VINB(-)
50
Figure 4. Single-Ended Inputs for the XRD64L43
Rev. P1.00
10