ADS823_14 Datasheet, PDF (10/20 Pages) Texas Instruments – 10-Bit, 60MHz 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
200Ω
R₁
1kΩ
+5V
R_S
50Ω
+5V
1/2
OPA2681
RSEL
+V_S
IN
10pF
ADS823
ADS826
CM (+2.5V)
IN
0.1µF
REFB
(+1.5V)
50Ω
REFT
(+3.5V)
INT/EXT
0.1µF
0.1µF
R_F
1kΩ
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 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
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 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 = n² • 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 reference
circuit. The internal blocks are the bandgap voltage refer-
ence, the drivers for the top and bottom reference, and the
RSEL
+V_S INT/EXT
ADS823
ADS826
50kΩ
50kΩ
Bandgap Reference and Logic
V_REF
R_G
0.1µF _1:n
V_IN
22Ω
R_T
22Ω
IN
47pF
ADS823
ADS826
IN _{CM RSEL INT/EXT}
47pF
+
10µF
+5V
0.1µF
+1
+1
400Ω
400Ω
400Ω
400Ω
REFT ByT
CM
ByB
REFB
Bypass Capacitors: 0.1µF each (optionally, 2.2µF
tantalum capacitors maybe added to ByT and ByB
pins for the best results).
FIGURE 5. Transformer-Coupled Input.
10
FIGURE 6. Equivalent Reference Circuit with Recommended
Reference Bypassing.
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ADS823, ADS826
SBAS070B