ADS831 Datasheet, PDF (9/15 Pages) Burr-Brown (TI) – 8-Bit, 80MHz Sampling ANALOG-TO-DIGITAL CONVERTER

V_IN
2Vp-p
R_IN
499Ω
R_F
499Ω
1/2
OPA2681
NOTE: R_F = R_IN, G = –1
V_CM = +1.25V
R₂
301Ω
R₁
499Ω
+5V
R_S
39Ω
IN
47pF
RSEL
+V_S
ADS831
+5V
1/2
OPA2681
CM (+2.5V)
IN
0.1µF
REFB
(+2.0V)
50Ω
REFT
(+3.0V)
INT/EXT
0.1µF
0.1µF
R_F
1kΩ
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
22Ω
R_T
22Ω
IN
47pF
ADS831
IN _{CM RSEL INT/EXT}
47pF
+
10µF
+5V
0.1µF
FIGURE 5. Transformer Coupled Input.
ADS831
SBAS087A
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 = n² 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
+V_S INT/EXT
ADS831
50kΩ
50kΩ
Bandgap Reference and Logic
V_REF
+1
400Ω
REFT
+1
400Ω
CM
REFB
Bypass Capacitors: 0.1µF || 2.2µF each
FIGURE 6. Equivalent Reference Circuit with Recommended
Reference Bypassing.
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