will create a 1st-order, low-pass filter in conjunction with the
specified input capacitance of the ADS2806. Its cutoff fre-
quency can be adjusted even further by adding an external
shunt capacitor from each signal input to ground. The
optimum values of this R-C network depend on a variety of
factors that include the ADS2806 sampling rate, the se-
lected op amp, the interface configuration, and the particular
application (time domain versus frequency domain). Gener-
ally, increasing the size of the series resistor and/or capaci-
tor will improve the SNR performance, but depending on the
signal source, large resistor values may be detrimental to
achieving good harmonic distortion. In any case, optimizing
the R-C values for the specific application is encouraged.
see matched impedances. Figure 1 shows the schematic for
the suggested transformer coupled interface 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 VSWR.
The circuit example of Figure 1 shows the voltage feedback
amplifier OPA680 driving the RF transformer, which con-
verts the single-ended signal into a differential. The OPA680
can be employed for either single- or dual-supply operation.
For details on how to optimize its frequency response, refer
to the OPA680 data sheet (SBOS083) on our web site at
www.ti.com. With the 49.9Ω series output resistor, the
amplifier emulates a 50Ω source (RG). Any DC content of
the signal can be easily blocked by a capacitor (0.1µF) to
avoid DC loading of the op amp’s output stage.
Transformer Coupled, Single-Ended to Differential
Configuration
If the application requires a signal conversion from a single-
ended source to drive the ADS2806 differentially, an RF
transformer might be a good solution. The selected trans-
former must have a center tap in order to apply the com-
mon-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 ampli-
fication without the introduction of another noise source.
Furthermore, the reduced signal swing from the source may
lead to improved distortion performance.
AC-Coupled, Single-Ended to Differential Interface
with Dual-Supply Op Amps
Some applications demand a very high dynamic range and
low levels of intermodulation distortion, but usually allow the
input signal to be AC-coupled into the ADC. Appropriate
driver amplifiers need to be selected to maintain the excellent
distortion performance of the ADS2806. Often, these op
amps deliver the lowest distortion with a small, ground-
centered signal swing that requires dual power supplies.
Because of the AC-coupling, this requirement can be easily
accomplished, and the needed level shifting of the input
signal can be implemented without affecting the driver circuit.
The differential input configuration provides the noticeable
advantage of achieving high SFDR over a wide range of
input frequencies. In this mode, both inputs of the ADS2806
RG
0.1µF
VIN
49.9Ω
24.9Ω
1:n
IN
OPA680
47pF
1/2
R1
RT
24.9Ω
ADS2806Y
CM
+2.5V
IN
R2
47pF
+
0.1µF
10µF
One Channel of Two
FIGURE 1, Converting a Single-Ended Input Signal into a Differential Signal Using an RF-Transformer.
ADS2806
SBAS178B
10
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