Datasheet

ADS805
9
SBAS073B
www.ti.com
FIGURE 2. DC-Coupled, Single-Ended Input Configuration with DC-level Shift.
INPUT
FULL-SCALE REQUIRED
MODE RANGE V
REF
CONNECT TO
Internal 2Vp-p +1V SEL V
REF
Internal 5Vp-p +2.5V SEL GND
Internal 2V ≤ FSR < 5V 1V < V
REF
< 2.5V R
1
V
REF
and SEL
FSR = 2 • V
REF
V
REF
= 1 + (R
1
/R
2
)R
2
SEL and Gnd
External 1V < FSR < 5V 0.5V < V
REF
< 2.5V SEL +V
S
V
REF
Ext. V
REF
TABLE I. Selected Reference Configuration Examples.
V
IN
IN
IN
CM
22Ω
22Ω
100pF
R
T
100pF
+
4.7µF
0.1µF
ADS805
1:n
0.1µF
R
G
FIGURE 3. Transformer-Coupled Input.
REFERENCE OPERATION
Integrated into the ADS805 is a bandgap reference circuit
including logic that provides either a +1V or +2.5V reference
output, by simply selecting the corresponding pin-strap con-
figuration. Different reference voltages can be generated by
the use of two external resistors, which will set a different
gain for the internal reference buffer. For more design flexibil-
ity, the internal reference can be shut off and an external
reference voltage used. Table I provides an overview of the
possible reference options and pin configurations.
offset. The selection criteria for an appropriate op amp should
include the input bias current, output voltage swing, distortion,
and noise specification. Note that in this example the overall
signal phase is inverted. To reestablish the original signal
polarity, it is always possible to interchange the IN and
IN
connections.
SINGLE-ENDED-TO-DIFFERENTIAL
CONFIGURATION (TRANSFORMER-COUPLED)
In order to select the best suited interface circuit for the
ADS805, the performance requirements must be known. If
an ac-coupled input is needed for a particular application, the
next step is to determine the method of applying the signal;
either single-ended or differentially. The differential input
configuration may provide a noticeable advantage of achiev-
ing good SFDR performance based on the fact that, in the
differential mode, the signal swing can be reduced to half of
the swing required for single-ended drive. Secondly, by
driving the ADS805 differentially, the even-order harmonics
will be reduced. Figure 3 shows the schematic for the
suggested transformer-coupled interface circuit. The resistor
across the secondary side (R
T
) should be set to get an input
impedance match (e.g., R
T
= n
2
• R
G
).
One application example that will benefit from the differential
input configuration is the digitization of IF signals. The wide
track-and-hold input bandwidth makes the ADS805 well
suited for IF down conversion in both narrow and wideband
applications. The ADS805 maintains excellent dynamic per-
formance in multiple Nyquist regions covering a variety of IF
frequencies (see the Typical Characteristics). Using the
ADS805 for direct IF conversion eliminates the need of an
analog mixer along with subsequent functions like amplifiers
and filters, thus reducing system cost and complexity.
V
IN
2Vp-p
0
+1V
–1V
R
F
R
IN
NOTE: R
F
= R
IN
, G = –1
+V
S
+V
S
OPA691
ADS805
R
S
50Ω
R
3
2kΩ
22pF
10µF
+
+2.5V
R
2
R
1
R
4
2kΩ
IN
IN
REFB
(+1V)
V
REF
SEL
REFT
0.1µF
0.1µF