Datasheet

ADS804
11
SBAS068B
www.ti.com
4V
1V
IN
IN
SELV
REF
+2.5V Ext.
V
REF
= 1V 1 +
R
1
R
2
FSR = 2 • V
REF
ADS804
R
1
5kΩ
+1.5V
R
2
10kΩ
V
IN
3.5V
1.5V
IN
IN
+1V
SELV
REF
+2.5V Ext.
V
IN
ADS804
V
IN
5V
0V
IN
IN
+2.5V
SELV
REF
ADS804
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 ADS804. 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 ADS804 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 ADS804 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.
DIGITAL INPUTS AND OUTPUTS
Over-Range (OVR)
One feature of the ADS804 is its ‘Over-Range’ digital output
(OVR). 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 LO when the input voltage is within the defined input
range. It becomes HI 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 refer-
ence 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 conjunc-
tion with OVR a simple clue 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.
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.
4.5V
0.5V
IN
IN
+2.5V ext.
+2V
DC
SEL
V
REF
1.24kΩ
4.99kΩ
0.1µF
10µF
REF1004
+2.5V
+
ADS804
+5V
V
IN