the 74HC574s but should be kept at least 1/4" (6mm) distant
from the digital ground plane (if present). The analog and
digital grounds planes should not overlap at any point.
SIGNAL CONDITIONING
The ADS7811 input essentially consists of a switch and a
capacitor. In the acquisition or sample mode, the switch is
closed and the input signal drives the capacitor directly.
When a conversion is started, the switch is opened capturing
the input signal at that moment. This voltage is held on the
capacitor for the remainder of the conversion.
INTERMEDIATE LATCHES
The 74HC574s shown in Figure 1 isolate the ADS7811 from
digital signals on a microprocessor, digital signal processor
(DSP), or microcontroller bus. This is necessary because of
the precision needed within the ADS7811. The weight of a
single LSB in the ADS7811 is 76µV, and the comparator
must be able to resolve differences in voltage to this level.
External digital signals which transition during the conver-
sion can easily couple onto the substrate and produce volt-
ages larger than this.
While this provides for a wide bandwidth sample and hold
function and results in excellent AC performance, this archi-
tecture requires a high bandwidth, precision op amp to drive
the analog input. The op amp and configuration shown in
Figure 1 is highly recommended. The amplifier should be
placed within 1 to 2 inches (25 to 50mm) of the ADS7811,
and the layout guidelines in the OPA628 data sheet should
be strictly followed.
In place of the 74HC574s, it might be possible to use a FIFO
or similar type of memory device. For many systems, it may
be difficult to go directly from the ADS7811 into a
microcontroller or DSP even if the ADS7811 is not con-
nected to shared bus. The reason for this is that the outputs
are active only during the acquisition period.
®
9
ADS7811
BB [ BURR-BROWN CORPORATION ]
