Datasheet

ADS828
10
SBAS126A
FIGURE 4. DC-Coupled Interface Circuit with Level Shifting Using Dual Current-Feedback Amplifier OPA2681.
FIGURE 5. Transformer Coupled Input.
FIGURE 6. Equivalent Reference Circuit with Recommended
Reference Bypassing.
SINGLE-ENDED-TO-DIFFERENTIAL CONFIGURATION
(Transformer Coupled)
If the application requires a signal conversion from a single-
ended source to feed the ADS828 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 ADS828 see matched impedances, and the differential
signal swing can be reduced to half of the swing required for
single-ended drive. Figure 5 shows the schematic for the
suggested transformer-coupled interface circuit. The compo-
nent values of the R-C low-pass may be optimized depend-
ing 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 Voltage Standing Wave Ratio (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
the resistive reference ladder. The bandgap reference circuit
2Vp-p
NOTE: R
F
= R
IN
, G = –1
V
IN
R
2
200Ω
R
1
1kΩ
ADS828
R
S
50Ω
10pF
0.1µF
IN
IN
CM (+2.5V)
INT/EXT
R
F
499Ω
R
IN
499Ω
V
CM
= +1.25V
REFB
(+1.5V)
REFT
(+3.5V)
1/2
OPA2681
1/2
OPA2681
R
F
1kΩ
50Ω
0.1µF
0.1µF
RSEL
+V
S
+5V
+5V
V
IN
IN
IN
CM
22Ω
22Ω
47pF
R
T
47pF
+
10µF
0.1µF
INT/EXTRSEL
+5V
ADS828
1:n
0.1µF
R
G
ADS828
REFT
ByT CM ByB REFB
Bypass Capacitors: 0.1µF || 2.2µF each
Bandgap Reference and Logic
V
REF
400Ω 400Ω 400Ω
400Ω
+1+1
+V
S
50kΩ 50kΩ
INT/EXTRSEL