Datasheet

ADS822, ADS825
10
SBAS069B
FIGURE 4. DC-Coupled Interface Circuit with 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 ADS822 and ADS825 differentially,
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 converter
inputs. AC-grounding the center tap will generate the differ-
ential signal swing across the secondary winding. Consider
a step-up transformer to take advantage of a signal amplifi-
cation 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 noticeable
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 sche-
matic for the suggested transformer-coupled interface circuit.
The component values of the R-C low-pass may be opti-
mized 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 Voltage Standing Wave Ratio
(VSWR).
REFERENCE OPERATION
Figure 6 depicts the simplified model of the internal reference
circuit. The internal blocks are the bandgap voltage refer-
ence, the drivers for the top and bottom references, and the
2Vp-p
NOTE: R
F
= R
IN
, G = –1
V
IN
R
2
200Ω
R
1
1kΩ
ADS822
ADS825
R
S
50Ω
10pF
0.1µF
IN
IN
CM (+2.5)
INT/EXT
R
F
499Ω
R
IN
499Ω
V
CM
= +1.25V
REFB
(+1.5V)
REFT
(+3.5V)
1/2
OPA2691
1/2
OPA2691
R
F
1kΩ
50Ω
0.1µF
0.1µF
RSEL
+V
S
+5V
+5V
A1
A2
V
IN
IN
IN
CM
22Ω
22Ω
47pF
R
T
47pF
+
10µF
0.1µF
INT/EXTRSEL
+5V
ADS822
ADS825
1:n
0.1µF
R
G
ADS822
REFT
ByT CM ByB REFB
Bandgap Reference and Logic
V
REF
400Ω 400Ω 400Ω
400Ω
+1+1
+V
S
50kΩ 50kΩ
INT/EXTRSEL
Bypass Capacitors: 0.1µF || 2.2µF each (optionally, 2.2µF tantalum
capacitors maybe added to ByT and ByB pins for the best results).