Datasheet

ADS900
10
SBAS058A
OPA680
V
IN
R
F
V
CM
= 1.5V
I = 250µA
0.1µF
IN
CM
+1.5V
REFB
+1.25V
R
S
R
IN
+3V
+5V
R
1
R
2
V
OS
0.1µF
22pF
0.1µF
ADS900
IN
R
1
1kΩ
OPA68x
V
IN
R
F
402Ω
V
CM
= 1.5V
C
1
0.1µF
0.1µF
IN
IN
CM
+5V
R
S
50Ω
+3V
R
G
402Ω
C
G
0.1µF
22pF
ADS900
As a passive component, a transformer can be used to step-
up the signal amplitude without adding noise or distortion.
At the same time it electrically isolates the front-end from
the converter. In order to achieve optimum performance and
to bias the converter inputs up to the correct common-mode
voltage the mid-reference pin “CM” can be tied directly to
the center tap of the transformer.
Figure 6 shows an example for a single-ended DC-coupled
interface circuit using one high-speed op amp to level-shift
the ground-referenced input signal to condition it for the
input requirements of the ADS900. With a +3V supply the
input signal swings 1Vp-p centered around a typical com-
mon-mode voltage of +1.5V. This voltage can be derived
from the internal bottom reference (REFB = +1.25V) and
then fed back through a resistor divider (R
1
, R
2
) to level shift
the driving op amp (OPA680). A capacitor across R
2
will
shunt most of the wideband noise to ground. Depending on
the configured gain the values of resistors R
1
and R
2
must be
adjusted since the offsetting voltage (V
OS
) is amplified by
the non-inverting gain, 1+(R
F
/R
IN
). This example assumes
the sum of R
1
and R
2
to be 5kΩ, drawing only 250µA from
the bottom reference. Considerations for the selection of a
FIGURE 6. Single-Ended DC-Coupled Input Circuit.
FIGURE 4. Driver Circuit Using Single Supply.
FIGURE 5. Single-Ended to Differential Drive Circuit Using a Transformer.
OPA65x
V
IN
R
1
RF Transformer: Minicircuits TT1-6
0.1µF
IN
CM
R
S
T
1
R
2
22pF
R
T
22pF
ADS900
+3V
IN