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ADS807 Datasheet(PDF) 11 Page - Texas Instruments

Part No. ADS807
Description  12-Bit, 53MHz Sampling ANALOG-TO-DIGITAL CONVERTER
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com
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ADS807 Datasheet(HTML) 11 Page - Texas Instruments

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ADS807
11
SBAS072A
www.ti.com
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 ADS807 set to the 2Vp-p input range, the top and
bottom references (REFT, REFB) provide an output voltage
of +3.0V and +2.0V, respectively. The CM output of the
ADS807 is used to bias the inputs of the driving amplifiers.
Using the OPA2681 on a single +5V supply, its ideal com-
mon-mode point is +2.5V, which coincides with the recom-
mended common-mode input level for the ADS807, thus
obviating the need for coupling capacitors between the
amplifiers and the converter.
The addition of a small series resistor (RS) between the
output of the op amps and the input of the ADS807 will be
beneficial in almost all interface configurations. It will decouple
the op amp’s output from the capacitive load and avoid gain
peaking, which can result in increased noise. For best
spurious and distortion performance, the resistor value should
be kept below 100
Ω. Furthermore, the series resistor in
combination with the shunt capacitor, establishes a passive
low-pass filter limiting the bandwidth for the wideband noise,
thus improving the SNR. The spurious-free dynamic range of
this single-supply front end is limited by the 2nd-harmonic
distortion. An improvement of several dB may be realized by
adding a pull-down resistor (RP) at the output of each
amplifier. This pulls a DC bias current out of the output stage
of the amplifier. It is set to approximately 5mA in Figure 3, but
will vary depending on the amplifier used.
FIGURE 2. AC-Coupled Differential Driver Interface with OPA642.
FIGURE 3. AC-Coupled, Differential Interface for Single-Supply Operation.
V
IN
ADS807E
68pF
IN
CM
IN
+5V
68pF
0.1
µF
0.1
µF
0.1
µF
A1
A2
OPA2681
R
G
499
R
F
499
499
R
S
24.9
V
CM = +2.5V
R
P
499
R
P
499
R
F
499
R
IN
249
R
S
24.9
V
IN
ADS807E
100pF
IN
IN
402
1.82k
1.82k
16.5
200
402
402
REFB
100pF
0.1
µF
0.1
µF
A1
OPA642
A2
OPA642
REFT
1.82k
1.82k
16.5


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