ADS820_13 Datasheet, PDF (13/20 Pages) Texas Instruments – ANALOG-TO-DIGITAL CONVERTER

The ADS820 can also be configured with a single-ended input
full-scale range of +0.25V to +4.25V by tying the complemen-
tary input to the common-mode reference voltage, as shown
in Figure 8. This configuration will result in increased even-
order harmonics, especially at higher input frequencies. How-
ever, this trade-off may be quite acceptable for time-domain
applications. The driving amplifier must give adequate perfor-
mance with a +0.25V to +4.25V output swing in this case.
Single-Ended
Input Signal
22 CM
0.1µF
26 IN
ADS820
27 IN
22pF
Full Scale = +0.25V to +4.25V with internal references.
FIGURE 8. Single-Ended Input Connection.
EXTERNAL REFERENCES AND
ADJUSTMENT OF FULL-SCALE RANGE
The internal reference buffers are limited to approximately
1mA of output current. As a result, these internal +1.25V and
+3.25V references may be overridden by external references
that have at least 18mA (at room temperature) of output drive
capability. In this instance, the common-mode voltage will be
set halfway between the two references. This feature can be
used to adjust the gain error, improve gain drift, or to change
the full-scale input range of the ADS820. Changing the full-
scale range to a lower value has the benefit of easing the
swing requirements of external input amplifiers. The external
references can vary as long as the value of the external top
reference (REFT_EXT) is less than or equal to +3.4V, the value
of the external bottom reference (REFB_EXT) is greater than or
equal to +1.1V, and the difference between the external
references are greater than or equal to 800mV.
For the differential configuration, the full-scale input range
will be set to the external reference values that are
selected. For the single-ended mode, the input range is
2 • (REFT_EXT – REFB_EXT), with the common-mode being
centered at (REFT_EXT + REFB_EXT)/2. Refer to the typical
performance curves for expected performance versus full-
scale input range.
The circuit in Figure 9 works completely on a single +5V
supply. As a reference element, it uses micro-power refer-
ence REF1004-2.5, which is set to a quiescent current of
0.1mA. Amplifier A₂ is configured as a follower to buffer the
+1.25V generated from the resistor divider. To provide the
necessary current drive, a pull-down resistor, R_P, is added.
Amplifier A₁ is configured as an adjustable gain stage, with
a range of approximately 1 to 1.32. The pull-up resistor again
relieves the op amp from providing the full current drive. The
value of the pull-up and pull-down resistors is not critical and
can be varied to optimize power consumption. The need for
pull-up/down resistors depends only on the drive capability of
the selected drive amplifiers and thus can be omitted.
+5V
+5V
10kΩ
REF1004
+2.5V
0.1µF
A₁
1/2
OPA2234
2kΩ
10kΩ
10kΩ⁽¹⁾
10kΩ⁽¹⁾
6.2kΩ
10kΩ
A₂
1/2
OPA2234
NOTE: (1) Use parts alternatively for adjustment capability.
R_P
220Ω
Top
Reference
+2.5V to +3.25V
+1.25V
R_P
220Ω
Bottom
Reference
FIGURE 9. Optional External Reference to Set the Full-Scale Range Utilizing a Dual, Single-Supply Op Amp.
ADS820
13
SBAS037B
www.ti.com