Figure 2 shows an example of such an interface circuit
specifically designed to maximize the dynamic performance.
The voltage feedback amplifier, OPA642, maintains an
excellent distortion performance for input frequencies of up
to 15MHz. The two amplifiers (A1, A2) are configured as an
inverting and noninverting gain stage to convert the input
signal from single-ended to differential. The nominal gain for
this stage is set to +2V/V. The outputs of the OPA642s are
AC-coupled to the converter’s differential inputs. This will
keep the distortion performance at its best since the signal
range stays within the linear region of the op amp and
sufficient headroom to the supply rails can be maintained.
Four resistors located between the top (REFT) and bottom
(REFB) reference shift the input signal to a common-mode
voltage of approximately +2.5V.
tion front end to the ADS2806. With a minimum gain stability
of +3, the gain resistors have to be modified, as well as
optimizing the series resistor and shunt capacitance at each
of the converter inputs.
AC-Coupled, Single-Ended-to-Differential Interface
for Single-Supply Operation
The previously discussed interface circuit can be modified if
the system only allows for a single-supply operation, e.g.,
VS = +5V. Single-supply operation requires the driver ampli-
fier to be biased as well in order to process a bipolar input
signal. Typically, single-supply amplifiers do not achieve
distortion performance as well as dual-supply op amps. The
driver amplifier’s output swing must exceed the full-scale
input range of the converter. In addition, dual op amps, such
as the current-feedback OPA2681, should be considered
since they provide the closest open-loop gain and phase
matching between the two channels. Shown in Figure 3 is
a single-supply interface circuit for an AC-coupled input
signal. With the ADS2806 set to the 2Vp-p input range, the
The interface circuit of Figure 2 can be modified to extend
the bandwidth to approximately 25MHz, by replacing the
OPA642 with its decompensated version, the OPA643. The
OPA643 provides the necessary slew rate for a low distor-
402Ω
200Ω
1.82kΩ
1.82kΩ
REFT
0.1µF
VIN
16.5Ω
A1
OPA642
IN
100pF
402Ω
1/2
402Ω
ADS2806Y
0.1µF
16.5Ω
A2
IN
OPA642
100pF
REFB
1.82kΩ
1.82kΩ
One Channel of Two
FIGURE 2. AC-Coupled Differential Driver Interface with OPA642.
RF
499Ω
RIN
249Ω
0.1µF
RS
VIN
24.9Ω
1/2
OPA2681
IN
RP
68pF
499Ω
499Ω
VCM = +2.5V
1/2
CM
ADS2806Y
0.1µF
+5V
RS
24.9Ω
1/2
OPA2681
IN
68pF
RF
499Ω
RG
249Ω
RP
499Ω
One Channel of Two
0.1µF
FIGURE 3. AC-Coupled, Differential Interface for Single-Supply Operation.
ADS2806
SBAS178B
11
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