Datasheet

ADS850
SBAS154C
12
www.ti.com
IN
IN
REFT
R
1
CMV
R
2
0.1µF
0.1µF
ADS850
REFB
FIGURE 6. Alternative Circuit to Generate Common-Mode
Voltage.
3.5V
1.5V
IN
IN
+2V
DC
SEL
V
REF
1.24kΩ
4.99kΩ
0.1µF
10µF
+2.5V
+
ADS850
+5V
V
IN
FIGURE 7. External Reference, Input Range 1.5V to 3.5V
(2Vp-p), Single-Ended, with +2.5V Common-
Mode Voltage.
OVR
MSB
Under = H
Over = H
FIGURE 8. External Logic for Decoding Under- and Over-
Range Condition.
ADS850
CMREFB
0.1µF
10µF 10µF
V
REF
0.1µF0.1µF
REFT
0.1µF
++
FIGURE 5. Recommended Reference Bypassing Scheme.
The top reference (REFT) and the bottom reference (REFB)
are brought out mainly for external bypassing. For proper
operation with all reference configurations, it is necessary to
provide solid bypassing to the reference pins in order to keep
the clock feedthrough to a minimum. Figure 5 shows the
recommended reference decoupling configuration.
In addition, the Common-Mode Voltage (CMV) may be used
as a reference level to provide the appropriate offset for the
driving circuitry. However, care must be taken not to appre-
ciably load this node, which is not buffered and has a high
impedance. An alternate method of generating a common-
mode voltage is given in Figure 6. Here, two external preci-
sion resistors (tolerance 1% or better) are located between
the top and bottom reference pins. The common-mode level
will appear at the midpoint. The output buffers of the top and
bottom reference are designed to supply approximately 2mA
of output current.
EXTERNAL REFERENCE OPERATION
Depending on the application requirements, it might be
advantageous to operate the ADS850 with an external
reference. This may improve the DC accuracy if the external
reference circuitry is superior in its drift and accuracy. To use
the ADS850 with an external reference, the user must
disable the internal reference, as shown in Figure 7. 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.
DIGITAL INPUTS AND OUTPUTS
Over Range (OVR)
One feature of the ADS850 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 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 clue logic can be built that
detects the overrange and underrange conditions, as shown
in Figure 8. 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.