ADS822 Datasheet, PDF (9/12 Pages) Burr-Brown (TI) – 10-Bit, 40MHz Sampling ANALOG-TO-DIGITAL CONVERTERS

V_IN
+V_IN
–V_IN
0.1µF
V_CM +2.5V
+5V
50Ω
OPA680
0V
R_F
402Ω
R_G
402Ω
0.1µF
1.62kΩ
+5V
1.62kΩ
R_S
REFB REFT RSEL +V_S
+1.5V +3.5V
50Ω
IN
10pF
ADS822
ADS825
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
1.62kΩ
IN
100pF
1.62kΩ
CM
IN
0.1µF
REFT RSEL +V_S
+3.5V
ADS822
ADS825
REFB
+1.5V INT/EXT GND
FIGURE 3. AC-Coupling the Dual Supply Amplifier OPA642 to the ADS822 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
ADS822 and ADS825, and level shifts 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
ADS822 and ADS825 are configured for a 2Vp-p input
range and use 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. Due to 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 +1.25V. 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
ADS822 and ADS825 effectively produces an offset, which
can be corrected for by adjusting the resistor values of the
divider, R₁and 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 in-
verted. To re-establish the original signal polarity, it is
always possible to interchange the IN and IN connections.
®
9
ADS822, ADS825