+5V
+VS
RF
499Ω
RIN
499Ω
RSEL
RS
VIN
50Ω
1/2
IN
OPA2681
2Vp-p
10pF
ADS822
ADS825
NOTE: RF = RIN, G = –1
CM (+2.5)
IN
0.1µF
REFB
REFT
+5V
(+1.5V)
(+3.5V)
INT/EXT
50Ω
R2
200Ω
0.1µF
1/2
VCM = +1.25V
OPA2681
0.1µF
R1
1kΩ
RF
1kΩ
FIGURE 4. DC-Coupled Interface Circuit with Dual Current-Feedback Amplifier OPA2681.
SINGLE-ENDED-TO-DIFFERENTIAL CONFIGURATION
(Transformer Coupled)
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 (RT) should be calculated
using the equation RT = n2 • RG to match the source impedance
(RG) for good power transfer and Voltage Standing Wave Ratio
(VSWR).
If the application requires a signal conversion from a single-
ended source to feed the ADS822 and ADS825 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.
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
+VS
INT/EXT
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 ADS822 and ADS825 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
ADS822
50kΩ
50kΩ
Bandgap Reference and Logic
VREF
RG
+1
+1
0.1µF
22Ω
1:n
VIN
IN
IN
47pF
ADS822
ADS825
400Ω
400Ω
400Ω
400Ω
RT
22Ω
REFT
ByT
CM
ByB
REFB
CM RSEL INT/EXT
47pF
+5V
+
10µF
0.1µF
Bypass Capacitors: 0.1µF || 2.2µF each
FIGURE6.EquivalentReferenceCircuitwithRecommended
Reference Bypassing.
FIGURE 5. Transformer Coupled Input.
®
ADS822, ADS825
10
BB [ BURR-BROWN CORPORATION ]
