ADS824 Burr-Brown Corporation, ADS824 Datasheet - Page 10

• Manufacturer Part Number: ADS824
• Description: 10-Bit/ 70MHz Sampling ANALOG-TO-DIGITAL CONVERTER
• Manufacturer: Burr-Brown Corporation

FIGURE 4. DC-Coupled Interface Circuit with Level Shifting Using 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 ADS824 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 ADS824 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 com-
FIGURE 5. Transformer Coupled Input.
V
IN
R
G
0.1µF
2Vp-p
NOTE: R
1:n
®
F
ADS824
V
= R
IN
IN
R
, G = –1
T
V
22
22
CM
499
= +1.25V
R
IN
+
47pF
10µF
47pF
1k
R
OPA2681
IN
IN
1
499
200
1/2
R
R
CM
F
2
0.1µF
ADS824
RSEL
+5V
INT/EXT
50
R
S
OPA2681
1k
+5V
R
1/2
F
10
ponent values of the R-C low-pass may be optimized de-
pending on the desired roll-off frequency. The resistor across
the secondary side (R
equation R
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
FIGURE 6. Equivalent Reference Circuit with Recommended
10pF
ADS824
0.1µF
50
T
Reference Bypassing.
= n
REFT
400
CM (+2.5V)
Bypass Capacitors: 0.1µF || 2.2µF each
IN
2
IN
+1
x R
Bandgap Reference and Logic
RSEL
G
ByT
400
(+1.5V)
50k
to match the source impedance (R
REFB
T
) should be calculated using the
V
0.1µF
+V
REF
S
CM
50k
400
ADS824
(+3.5V)
REFT
RSEL
INT/EXT
0.1µF
ByB
+1
400
INT/EXT
+5V
+V
REFB
S
G
)