1kΩ
+5V
V
CM
= +2.5V
DC
+5V
0.1µF
V
IN
OPA681
47pF
+V
IN
0V
–V
IN
R
F
402Ω
CM
R
G
402Ω
0.1µF
IN
0.1µF
INT/EXT
GND
ADS831
R
S
39Ω
IN
REFB
+2.0V
REFT
+3.0V
RSEL
+V
S
1kΩ
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). The OPA680 can be used in place of the
OPA681 if a voltage feedback amplifier is preferred.
+5V
1kΩ
+5V
V
IN
OPA642
47pF
–5V
R
F
402Ω
1kΩ
CM
IN
R
G
402Ω
0.1µF
REFB
+2.0V
ADS831
R
S
24.9Ω
0.1µF
IN
REFT
+3.0V
RSEL
+V
S
INT/EXT
GND
FIGURE 3. AC-Coupling the Dual Supply Amplifier OPA642 to the ADS831 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, such as the OPA643, or
current feedback op amps 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
ADS831 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 ADS831 is
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 the amplifier (OPA2681)
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 +1.25V, resulting 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 ADS831 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 specification. 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.
ADS831
8
BURR-BROWN [ BURR-BROWN CORPORATION ]
