Datasheet

ADS822, ADS825
9
SBAS069B
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 References (REFB).
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 OPA686, or
current-feedback op amps like the OPA691.
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 ADC. In order to accomplish this, the
interface circuit has to provide a DC level shift to the analog
input signal. See Figure 4 for a circuit that 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 dividers R
1
, R
2
. 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 cor-
rected for by adjusting the resistor values of the divider, R
1
and R
2
. 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.
+V
IN
0V
–V
IN
OPA690
V
IN
R
F
402Ω
1.62kΩ
R
G
402Ω
ADS822
ADS825
R
S
50Ω
10pF
0.1µF
IN
IN
CM
INT/EXT
GND
REFT
+3.5V
1.62kΩ
50Ω
V
CM
+2.5V
REFB
+1.5V
0.1µF
0.1µF
RSEL
+V
S
+5V
+5V
OPA642
V
IN
R
F
402Ω
1.62kΩ
R
G
402Ω
ADS822
ADS825
R
S
24.9Ω
1.62kΩ
100pF
0.1µF
0.1µF
IN
IN
CM
REFB
+1.5V INT/EXT
GND
REFT
+3.5V
RSEL
+V
S
+5V
+5V
–5V