the resistive reference ladder. The bandgap reference circuit
includes logic functions that allow to set the analog input
swing of the ADS831 to either a 1Vp-p or 2Vp-p full-scale
range simply by tying the RSEL pin to a LOW or HIGH
potential, respectively. While operating the ADS831 in the
external reference mode, the buffer amplifiers for REFT and
REFB are disconnected from the reference ladder.
The common-mode voltage available at the CM pin may be
used as a bias voltage to provide the appropriate offset for
the driving circuitry. However, care must be taken not to
appreciably load this node, which is not buffered and has a
high impedance. An alternative way of generating a com-
mon-mode voltage is given in Figure 7. Here, two external
precision resistors (1% tolerance or better) are located
between the top and bottom reference pins. The common-
mode voltage, CMV, will appear at the midpoint.
As shown, the ADS831 has internal 50kΩ pull-up resistors
at the Range Select pin (RSEL) and Reference Select pin
(INT/EXT). Leaving those pins open configures the ADS831
for a 2Vp-p input range and external reference operation.
Setting the ADS831 up for internal reference mode requires
to bring the INT/EXT pin LOW.
EXTERNAL REFERENCE OPERATION
For even more design flexibility, the internal reference can
be disabled and an external reference voltage be used. The
utilization of an external reference may be considered for
applications requiring higher accuracy, improved tempera-
ture performance, or a wide adjustment range of the
converter’s full-scale range. Especially in multichannel
applications, the use of a common external reference has the
benefit of obtaining better matching of the full-scale range
between converters.
The reference buffers can be utilized to supply up to 1mA
(sink and source) to external circuitry. To ensure proper
operation with any reference configurations, it is necessary
to provide solid bypassing at the reference pins in order to
keep the clock feedthrough to a minimum (Figure 6). All
bypassing capacitors should be located as close to their
respective pins as possible.
The external references can vary as long as the value of the
external top reference REFTEXT stays within the range of
(VS – 1.25V) and (REFB + 0.8V), and the external bottom
reference REFBEXT stays within 1.25V and (REFT – 0.8V),
see Figure 8.
ADS831
REFT
+3.0V
REFB
+2.0V
The full-scale input signal range (FSR) of the ADS831 is
determined by the voltage difference across the reference
pins REFT and REFB (FSR = REFT – REFB), while the
common-mode voltage is defined by CMV = (REFT +
REFB)/2. In order to maintain good ac performance, it is
recommended that the typical common-mode voltage be
kept at +2.5V while setting the external reference voltages.
It is possible, however, to deviate from this common-mode
level without significantly impacting the performance. In
particular, DC-coupled applications may benefit from a
lower CMV as it increases the signal headroom of the
R1
1kΩ
R2
1kΩ
+
+
2.2µF
0.1µF
0.1µF
2.2µF
CMV
+2.5V
FIGURE 7. Alternative Circuit to Generate Common-Mode
Voltage.
+5V
B
A
A - Short for 1Vp-p Input Range
B - Short for 2Vp-p Input Range (Default)
+VS
INT/EXT
RSEL
GND
IN
VIN
ADS831
CMV
IN
REFT
GND
REFB
External Top Reference
REFT = REFB +0.8V to +3.75V
External Bottom Reference
REFB = REFT –0.8V to +1.25V
FIGURE 8. Configuration Example for External Reference Operation.
®
10
ADS831
BB [ BURR-BROWN CORPORATION ]
