1.62kΩ
+5V
V
CM
+2.5V
+5V
0.1µF
V
IN
OPA680
10pF
+V
IN
0V
–V
IN
R
F
402Ω
CM
R
G
402Ω
0.1µF
IN
0.1µF
INT/EXT
GND
ADS823
ADS826
50Ω
R
S
50Ω
IN
REFB
+1.5V
REFT
+3.5V
RSEL
+V
S
1.62kΩ
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
1.62kΩ
+5V
V
IN
OPA642
100pF
–5V
R
F
402Ω
1.62kΩ
CM
IN
R
G
402Ω
0.1µF
REFB
+1.5V
ADS823
ADS826
R
S
24.9Ω
0.1µF
IN
REFT
+3.5V
RSEL
+V
S
INT/EXT
GND
FIGURE 3. AC-Coupling the Dual Supply Amplifier OPA642 to the ADS823 and ADS826 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 includes 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 ADS823 and
ADS826, 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 ADS823 and ADS826
are configured for a 2Vp-p input range and uses the internal
references. The complementary input (IN) may be appropri-
ately biased using the +2.5V common-mode voltage avail-
able at the CM pin. One half of amplifier A1 buffers the
REFB pin and drives the voltage divider 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 ADS823 and ADS826 effectively produce
an offset, which can be corrected 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 specifi-
cation. Note that in this example the overall signal phase is
inverted. To re-establish the original signal polarity, it is
always possible to interchange the IN and IN connections.
®
9
ADS823, ADS826
BURR-BROWN [ BURR-BROWN CORPORATION ]
