+5V
R
F
499Ω
R
IN
499Ω
V
IN
2Vp-p
1/2
OPA2681
R
S
39Ω
IN
47pF
RSEL
+V
S
ADS831
NOTE: R
F
= R
IN
, G = –1
CM (+2.5V)
IN
+5V
R
2
301Ω
V
CM
= +1.25V
R
1
499Ω
R
F
1kΩ
0.1µF
REFB
(+2.0V)
REFT
(+3.0V)
0.1µF
1/2
OPA2681
INT/EXT
50Ω
0.1µF
FIGURE 4. DC-Coupled Interface Circuit with Dual Current-Feedback Amplifier OPA2681. The OPA2680 can be used in place
of the OPA2681 if a voltage feedback amplifier is preferred.
SINGLE ENDED-TO-DIFFERENTIAL CONFIGURATION
(Transformer Coupled)
If the application requires a signal conversion from a single-
ended source to feed the ADS831 differentially, a RF trans-
former might be a good solution. The selected transformer
must have a center tap in order to apply the common-mode
DC voltage necessary to bias the converter inputs.
AC grounding the center tap will generate the differential
signal swing across the secondary winding. Consider a step-
up transformer to take advantage of a signal amplification
without the introduction of another noise source. Further-
more, the reduced signal swing from the source may lead to
an improved distortion performance.
The differential input configuration may provide a notice-
able advantage of achieving good SFDR performance over
a wide range of input frequencies. In this mode both inputs
of the ADS831 see closely matched impedances, and the
differential signal swing is reduced to half of the swing
required for single-ended drive. Figure 5 shows the sche-
matic for the suggested transformer coupled interface cir-
R
G
0.1µF 1:n
V
IN
47pF
R
T
22Ω
IN
47pF
+5V
+
10µF
0.1µF
CM
RSEL INT/EXT
ADS831
22Ω
IN
cuit. The component values of the R-C lowpass may be
optimized depending on the desired roll-off frequency. The
resistor across the secondary side (R
T
) should be calculated
using the equation R
T
= n
2
x R
G
to match the source
impedance (R
G
) for good power transfer and VSWR.
REFERENCE OPERATION
Figure 6 depicts the simplified model of the internal refer-
ence circuit. The internal blocks are the bandgap voltage
reference, the drivers for the top and bottom reference, and
RSEL
ADS831
50kΩ
+V
S
INT/EXT
50kΩ
Bandgap Reference and Logic
V
REF
+1
+1
400Ω
400Ω
REFT
CM
REFB
Bypass Capacitors: 0.1µF || 2.2µF each
FIGURE 5. Transformer Coupled Input.
FIGURE 6. Equivalent Reference Circuit with Recommended
Reference Bypassing.
9
ADS831
SBAS087A
TI [ TEXAS INSTRUMENTS ]
