ADS807 Datasheet, PDF (12/17 Pages) Texas Instruments – 12-Bit, 53MHz Sampling ANALOG-TO-DIGITAL CONVERTER

Single-Ended, AC-Coupled, Dual-Supply Interface
The circuit provided in Figure 4 shows typical connections for
using the ADS807 in a single-ended input configuration. The
bias requirements for AC-coupling are provided by a single
resistor to the CM output lead. The single-ended mode of
operation should be considered for ease of interface com-
plexity and applications where the dynamic performance can
be compromised. The series resistor R_S, along with the shunt
capacitance, provide the means to adjust the bandwidth and
optimize the performance towards good signal-to-noise ratio.
In addition, the amplifier configuration can be easily modified
for an anti-aliasing filter based on a 2nd-order Sallen-Key or
Multiple-Feedback topology.
The interface example shown in Figure 4 operates with the
full-scale range of the ADS807 set to 2Vp-p, leaving suffi-
cient headroom for the output of the OPA642 to drive the
converter and maintain low signal distortion.
DC-Coupled, Differential Driver with Level Shift
Several applications will require that the bandwidth of the
signal path include DC, in which case, the signal has to be
DC-coupled to the A/D converter. An op amp based interface
circuit can be configured to scale and level shift the input
signal to be compatible with the selected input range of the
A/D converter. The circuit shown in Figure 5 employs a dual
op amp, OPA2681, to drive the input of the ADS807 differ-
entially. The single-supply, general-purpose op amp OPA234
is added to buffer the common-mode voltage of +2.5V,
available at the CM pin, and apply it to the input of the driver
amplifier. This sets the correct DC voltage to bias the inputs
of the ADS807. It should be noted that any DC voltage
differences between the IN and IN inputs of the ADS807 will
result in an offset error.
Using the OPA2681, this circuit can be operated either with
a single or a dual ±5V supply.
+5V
V_IN
OPA642
–5V
R_F
402Ω
R_G
402Ω
R_S
16.5Ω
0.1µF
IN
68pF
1.82kΩ
CM
IN
0.1µF
ADS807E
FIGURE 4. AC-Coupling the Dual-Supply Amplifier OPA642 to the ADS807 for a 2Vp-p Full-Scale Input Range.
499Ω
249Ω
V_IN
24.9Ω
OPA2681
249Ω
499Ω
22pF
IN
ADS807E
249Ω
499Ω
499Ω
249Ω
24.9Ω
0.1µF
IN
22pF
24.9Ω
OPA234
0.1µF
1kΩ
CM
0.1µF
FIGURE 5. DC-Coupled Input Driver with Level Shifting.
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ADS807
SBAS072A