AD9281-EB Analog Devices Inc, AD9281-EB Datasheet - Page 10

• Manufacturer Part Number: AD9281-EB
• Description: BOARD EVAL FOR AD9281
• Manufacturer: Analog Devices Inc

Specifications of AD9281-EB

• Rohs Status: RoHS non-compliant

AD9281
REFERENCE AND REFERENCE BUFFER
The reference and buffer circuitry on the AD9281 is configured
for maximum convenience and flexibility. An illustration of the
equivalent reference circuit is show in Figure 26. The user can
select from five different reference modes through appropriate
pin-strapping (see Table I below). These pin strapping options
cause the internal circuitry to reconfigure itself for the appropri-
ate operating mode.
Mode
1 V
2 V
Programmable
External
1 V Mode (Figure 22)—provides a 1 V reference and 1 V input
full scale. Recommended for applications wishing to optimize
high frequency performance, or any circuit on a supply voltage
of less than 4 V. The part is placed in this mode by shorting the
REFSENSE pin to the VREF pin.
2 V Mode (Figure 23)—provides a 2 V reference and 2 V input
full scale. Recommended for noise sensitive applications on 5 V
supplies. The part is placed in 2 V reference mode by ground-
ing (shorting to AVSS) the REFSENSE pin.
1V
2V
0V
0V
5k
5k
10 F
10 F
10 F
10 F
Figure 22. 0 V to 1 V Input
Figure 23. 0 V to 2 V Input
Input Span
1 V
2 V
1 + (R1/R2)
= External Ref AVDD
Table I. Table of Modes
0.1 F
0.1 F
0.1 F
0.1 F
5k
5k
1V
VREF
IINA
IINB
IINA
IINB
VREF
REFSENSE
AD9281
AD9281
REFSENSE
I OR QREFB
I OR QREFT
I OR QREFB
I OR QREFT
REFSENSE Pin Figure
VREF
AGND
See Figure
QINA
QINB
QINA
QINB
0.1 F
0.1 F
1V
0V
2V
0V
10 F
10 F
0.1 F
0.1 F
0.1 F
0.1 F
22
23
24
25
–10–
Externally Set Voltage Mode (Figure 24)—this mode uses
the on-chip reference, but scales the exact reference level though
the use of an external resistor divider network. VREF is wired to
the top of the network, with the REFSENSE wired to the tap
point in the resistor divider. The reference level (and input full
scale) will be equal to 1 V × (R1 + R2)/R1. This method can be
used for voltage levels from 0.7 V to 2.5 V.
External Reference Mode (Figure 25)—in this mode, the on-
chip reference is disabled, and an external reference applied to
the VREF pin. This mode is achieved by tying the REFSENSE
pin to AVDD.
Reference Buffer—The reference buffer structure takes the
voltage on the VREF pin and level-shifts and buffers it for use
by various sub-blocks within the two A/D converters. The two
converters share the same reference buffer amplifier to maintain
the best possible gain match between the two converters. In the
interests of minimizing high frequency crosstalk, the buffered
references for the two converters are separately decoupled on
the IREFB, IREFT, QREFB and QREFT pins, as illustrated in
Figure 26.
REFERENCE
1V
0V
VREF = 1 + R2
EXT
1V
0.1 F
5k
1 F
R2
R1
Figure 24. Programmable Reference
10 F
R1
10 F
Figure 25. External Reference
VREF
REFSENSE
AVSS
0.1 F
0.1 F
AVDD
5k
AD9281
+
–
I OR QREFB
I OR QREFT
VREF
IINA
IINB
REFSENSE
+ –
1V
AD9281
I OR QREFB
I OR QREFT
QINA
QINB
0.1 F
0.1 F
10 F
1V
0V
0.1 F
0.1 F
10 F
0.1 F
0.1 F
REV. F