ADS801 Datasheet, PDF (12/13 Pages) Burr-Brown (TI) – 12-Bit, 25MHz Sampling ANALOG-TO-DIGITAL CONVERTER

Optional
High Impedance
301Ω
Input Amplifier _+5V(2)
DC-Coupled
Input Signal
OPA642
–5V
24.9Ω
301Ω
301Ω
+5V
604Ω
+5V
301Ω
BAS16⁽¹⁾
OPA642
0.1µF
–5V
604Ω ^604Ω
+5V
+5V
2.49kΩ
0.1µF
49.9Ω
+5V
OPA130
27 IN
22pF
ADS801
2.49kΩ ^+2.25V
22 CM
OPA642
BAS16⁽¹⁾
301Ω
0.1µF
–5V
Input Level
Shift Buffer
26 IN
22pF
301Ω
604Ω
NOTES: (1) A Philips BAS16 diode or equivalent
may be used. (2) Supply bypassing not shown.
FIGURE 6. A Low Distortion DC-Coupled, Single-Ended to Differential Input Driver Circuit.
Single-Ended
Input Signal
22 CM
0.1µF
26 IN
ADS801
27 IN
22pF
Full Scale = +0.25V to +4.25V with internal references.
REFB_EXT), with the common-mode being centered at
(REFT_EXT + REFB_EXT)/2. Refer to the typical performance
curves for expected performance vs full scale input range.
The circuit in Figure 8 works completely on a single +5V
supply. As a reference element, it uses the micro-power
reference 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.
FIGURE 7. 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 refer-
ences 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 ADS801.
Changing the full scale range to a lower value has the benefit
of easing the swing requirements of external input amplifi-
ers. 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 and the value of the external bottom reference
(REFB_EXT) is greater than or equal to +1.1V and the differ-
ence 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 –
®
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/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 capa-
bility of the selected drive amplifier 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. Mul-
tilayer PC boards are recommended for best performance
but if carefully designed, a two-sided PC board with large,
heavy ground planes can give excellent results. It is recom-
mended that the analog and digital ground pins of the
ADS801 be connected directly to the analog ground plane.
In our experience, this gives the most consistent results. The
A/D power supply commons should be tied together at the
analog ground plane. Power supplies should be bypassed
with 0.1µF ceramic capacitors as close to the pin as possible.
ADS801
12