+5V
R
F
499Ω
R
IN
499Ω
V
IN
2Vp-p
1/2
OPA2681
R
S
50Ω
IN
10pF
RSEL
+V
S
ADS823
ADS826
NOTE: R
F
= R
IN
, G = –1
CM (+2.5V)
IN
+5V
R
2
200Ω
V
CM
= +1.25V
R
1
1kΩ
R
F
1kΩ
0.1µF
REFB
(+1.5V)
REFT
(+3.5V)
0.1µF
1/2
OPA2681
INT/EXT
50Ω
0.1µF
FIGURE 4. DC-Coupled Interface Circuit with Level-Shifting, Dual Current-Feedback Amplifier OPA2681.
SINGLE-ENDED TO DIFFERENTIAL CONFIGURATION
(Transformer Coupled)
If the application requires a signal conversion from a single-
ended source to feed the ADS823 and ADS826 differen-
tially, a RF transformer 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 con-
verter inputs. AC grounding the center tap will generate the
differential signal swing across the secondary winding. Con-
sider a step-up transformer to take advantage of a signal
amplification without the introduction of another noise source.
Furthermore, 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 ADS823 and ADS826 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
R
G
0.1µF 1:n
V
IN
47pF
R
T
22Ω
IN
47pF
+5V
+
10µF
0.1µF
CM
RSEL INT/EXT
ADS823
ADS826
22Ω
IN
circuit. The component values of the R-C low-pass 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
• 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
ADS823
ADS826
50kΩ
+V
S
INT/EXT
50kΩ
Bandgap Reference and Logic
V
REF
+1
+1
400Ω
400Ω
400Ω
400Ω
REFT
ByT
CM
ByB
REFB
Bypass Capacitors: 0.1µF || 2.2µF each
FIGURE 5. Transformer Coupled Input.
®
FIGURE 6. Equivalent Reference Circuit with Recommended
Reference Bypassing.
10
ADS823, ADS826
BURR-BROWN [ BURR-BROWN CORPORATION ]
