Datasheet

ADS828
9
SBAS126A
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).
FIGURE 3. AC-Coupling the Dual Supply Amplifier OPA642 to the ADS828 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
ADS828 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 ADS828 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 R
1
, R
2
. 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
ADS828 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
OPA680
V
IN
R
F
402
1.0k
R
G
402
ADS828
R
S
50
10pF
0.1µF
IN
IN
CM
INT/EXT
GND
REFT
+3.5V
1.0k
50
V
CM
+2.5V
REFB
+1.5V
0.1µF
0.1µF
RSEL
+V
S
+5V
+5V
OPA642
V
IN
R
F
402
1.0k
R
G
402
ADS828
R
S
24.9
1.0k
100pF
0.1µF
0.1µF
IN
IN
CM
REFB
+1.5V INT/EXT
GND
REFT
+3.5V
RSEL
+V
S
+5V
+5V
5V