Datasheet

ADS802
12
SBAS039B
www.ti.com
FIGURE 6. A Low-Distortion, DC-Coupled, Single-Ended to Differential Input Driver Circuit.
FIGURE 7. Single-Ended Input Connection.
604Ω
301Ω
301Ω
301Ω
604Ω
49.9Ω
301Ω
604Ω
2.49kΩ
2.49kΩ
+2.25V
OPA842
OPA130
301Ω
0.1µF
OPA842
OPA842
+5V
–5V
+5V
(2)
+5V
–5V
+5V
+5V
+5V
–5V
BAS16
(1)
BAS16
(1)
301Ω
24.9Ω
Input Level
Shift Buffer
Optional
High Impedance
Input Amplifier
DC-Coupled
Input Signal
26 IN
22 CM
27 IN
ADS802
NOTES: (1) A Philips BAS16 diode or equivalent
may be used. (2) Supply bypassing not shown.
22pF
22pF
604Ω
0.1µF
0.1µF
22
26
27
CM
IN
IN
ADS802
0.1µF
Single-Ended
Input Signal
Full-Scale = +0.25V to +4.25V with internal references.
22pF
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 ADS802. 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 1.5V.
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
characteristics for expected performance versus full-scale
input range.
The circuit in Figure 8 works completely on a single +5V
supply. As a reference element, it uses micro-power refer-
ence REF1004-2.5 that is set to a quiescent current of
0.1mA. Amplifier A
2
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 A
1
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, pull-down resistors is not critical and can
be varied to optimize power consumption. The need for pull-
up, pull-down resistors depends only on the drive capability
of the selected drive amplifiers, and thus can be omitted.
PC-BOARD LAYOUT AND BYPASSING
A well-designed, clean pc-board layout will assure proper
operation and clean spectral response. Proper grounding and
bypassing, short lead lengths, and the use of ground planes
are particularly important for high-frequency circuits. Multilayer
pc-boards are recommended for best performance, but if
carefully designed, a two-sided pc-board with large, heavy