R
F
+1V
0
–1V
2Vp-p
V
IN
OPA681
100pF
R
IN
2kΩ
IN
REFT
+V
S
R
S
24.9Ω
R
1
+V
S
R
2
ADS804
+2.5V
0.1µF
+
IN
0.1µF
REFB
2kΩ
(+1V)
V
REF
SEL
10µF
NOTE: R
F
= R
IN
, G = –1
FIGURE 2. DC-Coupled, Single-Ended Input Configuration with DC-level Shift.
DC voltage differences between the IN and IN inputs of the
ADS804 effectively will produce an offset, which can be
corrected for by adjusting the values of resistors R1 and R2.
The bias current of the op amp may also result in an
undesired offset. The selection criteria of the 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 re-establish
the original signal polarity it is always possible to inter-
change the IN and IN connections.
SINGLE-ENDED-TO-DIFFERENTIAL
CONFIGURATION (TRANSFORMER COUPLED)
In order to select the best suited interface circuit for the
ADS804, 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
achieving 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 ADS804 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
).
REFERENCE OPERATION
Integrated into the ADS804 is a bandgap reference circuit
including logic that provides either a +1V or +2.5V refer-
ence output, by simply selecting the corresponding pin-strap
configuration. Different reference voltages can be generated
by the use of two external resistors, which will set a different
R
G
0.1µF
V
IN
100pF
R
T
ADS804
22Ω
IN
100pF
+
CM
1:n
22Ω
IN
4.7µF
0.1µF
FIGURE 3. Transformer-Coupled Input.
gain for the internal reference buffer. For more design
flexibility, the internal reference can be shut off and an
external reference voltage used. Table I provides an over-
view of the possible reference options and pin configura-
tions.
INPUT
FULL-SCALE
RANGE
2Vp-p
5Vp-p
2V≤ FSR < 5V
FSR = 2 x V
REF
External
1V < FSR < 5V
MODE
Internal
Internal
Internal
REQUIRED
V
REF
+1V
+2.5V
1V < V
REF
< 2.5V
V
REF
= 1 + (R
1
/R
2
)
0.5V < V
REF
< 2.5V
CONNECT
SEL
SEL
R
1
R
2
SEL
V
REF
TO
V
REF
GND
V
REF
and SEL
SEL and Gnd
+V
S
Ext. V
REF
TABLE I. Selected Reference Configuration Examples.
®
9
ADS804
BURR-BROWN [ BURR-BROWN CORPORATION ]
