ADS824 Datasheet, PDF (9/12 Pages) Burr-Brown (TI) – 10-Bit, 70MHz Sampling ANALOG-TO-DIGITAL CONVERTER

V_IN
+V_IN
–V_IN
0.1µF
V_CM +2.5V
+5V
50Ω
OPA680
0V
R_F
402Ω
R_G
402Ω
0.1µF
1.0kΩ
+5V
1.0kΩ
R_S
REFB REFT RSEL +V_S
+1.5V +3.5V
50Ω
IN
10pF
ADS824
CM
IN
0.1µF
INT/EXT GND
FIGURE 2. AC-Coupled Input Configuration for a 2Vp-p Full-Scale Range and a Common-Mode Voltage, V_CM, at +2.5V Derived
From The Internal Top (REFT) and Bottom Reference (REFB).
+5V
V_IN
OPA642
–5V
R_F
402Ω
R_G
402Ω
R_S
24.9Ω
0.1µF
+5V
1.0kΩ
100pF
REFT RSEL +V_S
+3.5V
IN
1.0kΩ
CM
IN
0.1µF
ADS824
REFB
+1.5V INT/EXT GND
FIGURE 3. AC-Coupling the Dual Supply Amplifier OPA642 to the ADS823 for a 2Vp-p Full-Scale Input Range.
For applications requiring the driving amplifier to provide a
signal amplification, with a gain ≥ 5, consider using decom-
pensated voltage-feedback op amps, like the OPA643, or
current-feedback op amps like the OPA681 and OPA658.
DC-coupled with Level Shift
Several applications may 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. In order to accomplish
this, the interface circuit has to provide a DC level shift to
the analog input signal. The circuit shown in Figure 4
employs a dual op amp, A1, to drive the input of the
ADS824 and level shift the signal to be compatible with the
selected input range. With the RSEL pin tied to the supply
and the INT/EXT pin to ground, the ADS824 is configured
for a 2Vp-p input range and uses the internal references. The
complementary input (IN) may be appropriately biased
using the +2.5V common-mode voltage available at the
CM pin. One-half of amplifier A1 buffers the REFB pin and
drives the voltage divider R₁, R₂. Because of the op amp’s
noise gain of +2V/V, assuming R_F = R_IN, the common-mode
voltage (V_CM) has to be re-scaled to +1.25. This results in
the correct DC level of +2.5V for the signal input (IN). Any
DC voltage differences between the IN and IN inputs of the
ADS824 effectively produces an offset, which can be cor-
rected for by adjusting the resistor values of the divider, R₁
and R₂. The selection criteria for a suitable op amp should
include the supply voltage, input bias current, output voltage
swing, distortion, and noise specification. Note that in this
example, the overall signal phase is inverted. To re-establish
the original signal polarity, it is always possible to inter-
change the IN and IN connections.
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9
ADS824