+5V
604Ω
+5V
301Ω
BAS16(1)
Optional
High Impedance
Input Amplifier
27 IN
301Ω
OPA842
301Ω
2.49kΩ
0.1µF
22pF
+5V(2)
0.1µF
+5V
–5V
604Ω
DC-Coupled
Input Signal
ADS802
OPA842
604Ω
49.9Ω
+2.25V
OPA130
2.49kΩ
22 CM
+5V
–5V
+5V
24.9Ω
301Ω
Input Level
Shift Buffer
301Ω
BAS16(1)
0.1µF
26 IN
OPA842
22pF
–5V
604Ω
NOTES: (1) A Philips BAS16 diode or equivalent
may be used. (2) Supply bypassing not shown.
301Ω
FIGURE 6. A Low-Distortion, DC-Coupled, Single-Ended to Differential Input Driver Circuit.
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 • (REFTEXT – REFBEXT), with the common-mode being
centered at (REFTEXT + REFBEXT)/2. Refer to the typical
characteristics for expected performance versus full-scale
input range.
22 CM
0.1µF
ADS802
Single-Ended
Input Signal
26 IN
27 IN
22pF
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 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 (RP) is added.
Full-Scale = +0.25V to +4.25V with internal references.
FIGURE 7. Single-Ended Input Connection.
EXTERNAL REFERENCES AND ADJUSTMENT
OF FULL-SCALE RANGE
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, 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.
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 (REFTEXT) is less than or equal to +3.4V, the value
of the external bottom reference (REFBEXT) is greater than or
equal to +1.1V, and the difference between the external
references are greater than or equal to 1.5V.
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
ADS802
12
SBAS039B
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