Datasheet

ADS805
11
SBAS073B
www.ti.com
4V
1V
IN
IN
SELV
REF
+2.5V ext.
V
REF
= 1V 1 +
R
1
R
2
FSR = 2 • V
REF
ADS805
R
1
5kΩ
+1.5V
R
2
10kΩ
V
IN
3.5V
1.5V
INV
IN
IN
+1V
SELV
REF
+2.5V ext.
ADS805
V
IN
5V
0V
IN
IN
+2.5V
SELV
REF
ADS805
SELECTING THE INPUT RANGE AND REFERENCE
Figures 7 through 9 show a selection of circuits for the most
common input ranges when using the internal reference of
the ADS805. All examples are for single-ended input and
operate with a nominal common-mode voltage of +2.5V.
EXTERNAL REFERENCE OPERATION
Depending on the application requirements, it might be
advantageous to operate the ADS805 with an external refer-
ence. This may improve the DC accuracy if the external
reference circuitry is superior in its drift and accuracy. To use
the ADS805 with an external reference, the user must
disable the internal reference, as shown in Figure 10. By
connecting the SEL pin to +V
S
, the internal logic will shut
down the internal reference. At the same time, the output of
the internal reference buffer is disconnected from the V
REF
pin, which now must be driven with the external reference.
Note that a similar bypassing scheme should be maintained
as described for the internal reference operation.
FIGURE 10. External Reference, Input Range 0.5V to 4.5V
(4Vp-p), with +2.5V Common-Mode Voltage.
FIGURE 8. Internal Reference with 1.5V to 3.5V Input Range.
FIGURE 9. Internal Reference with 1V to 4V Input Range.
FIGURE 7. Internal Reference with 0V to 5V Input Range.
DIGITAL INPUTS AND OUTPUTS
Over-Range (OVR)
One feature of the ADS805 is its ‘Over-Range’ (OVR) digital
output. This pin can be used to monitor any out-of-range
condition, which occurs every time the applied analog input
voltage exceeds the input range (set by V
REF
). The OVR
output is LOW when the input voltage is within the defined
input range. It becomes HIGH when the input voltage is
beyond the input range. This is the case when the input
voltage is either below the bottom reference voltage or above
the top reference voltage. OVR will remain active until the
analog input returns to its normal signal range and another
conversion is completed. Using the MSB and its complement
in conjunction with OVR, a simple decode logic can be built
that detects the over-range and under-range conditions, (see
Figure 11). It should be noted that OVR is a digital output
which is updated along with the bit information corresponding
to the particular sampling incidence of the analog signal.
Therefore, the OVR data is subject to the same pipeline
delay (latency) as the digital data.
4.5V
V
IN
0.5V
IN
IN
+2.5V ext.
SEL
V
REF
1.24kΩ
+2V
DC
4.99kΩ
0.1µF
10µF
REF1004
+2.5V
+
ADS805
+5V