ADS7864
www.ti.com
SBAS141A–SEPTEMBER 2000–REVISED MARCH 2005
LAYOUT
10µF capacitor is recommended. If needed, an even
larger capacitor and a 5Ω or 10Ω series resistor may
be used to low-pass filter a noisy supply. On average,
the ADS7864 draws very little current from an exter-
nal reference as the reference voltage is internally
buffered. If the reference voltage is external and
originates from an op amp, make sure that it can
drive the bypass capacitor or capacitors without
oscillation. A bypass capacitor must not be used
when using the internal reference (tie pin 33 directly
to pin 34). The AGND and DGND pins should be
connected to a clean ground point. In all cases, this
should be the ‘analog’ ground. Avoid connections
which are too close to the grounding point of a
microcontroller or digital signal processor. If required,
run a ground trace directly from the converter to the
power supply entry point. The ideal layout will include
an analog ground plane dedicated to the converter
and associated analog circuitry.
For optimum performance, care should be taken with
the physical layout of the ADS7864 circuitry. This is
particularly true if the CLOCK input is approaching
the maximum throughput rate. The basic SAR archi-
tecture is sensitive to glitches or sudden changes on
the power supply, reference, ground connections and
digital inputs that occur just prior to latching the
output of the analog comparator. Thus, driving any
single conversion for an n-bit SAR converter, there
are n 'windows' in which large external transient
voltages can affect the conversion result. Such
glitches might originate from switching power
supplies, nearby digital logic or high power devices.
The degree of error in the digital output depends on
the reference voltage, layout, and the exact timing of
the external event. These errors can change if the
external event changes in time with respect to the
CLOCK input. With this in mind, power to the
ADS7864 should be clean and well-bypassed. A
0.1µF ceramic bypass capacitor should be placed as
close to the device as possible. In addition, a 1µF to
20
TI [ TEXAS INSTRUMENTS ]
