(tCKP).
larger capacitor and a 5Ω or 10Ω series resistor may be used
to lowpass filter a noisy supply.
The ADS7835 draws very little current from an external
reference on average as the reference voltage is internally
buffered. However, glitches from the conversion process
appear at the VREF input and the reference source must be
able to handle this. Whether the reference is internal or
external, the VREF pin should be bypassed with a 0.1µF
capacitor. An additional larger capacitor may also be used,
if desired. 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.
LAYOUT
For optimum performance, care should be taken with the
physical layout of the ADS7835 circuitry. This is particu-
larly true if the CLK input is approaching the maximum
input rate.
The basic SAR architecture is sensitive to glitches or sudden
changes on the power supply, reference, ground connec-
tions, and digital inputs that occur just prior to latching the
output of the analog comparator. Thus, during any single
conversion for an n-bit SAR converter, there are n “win-
dows” in which large external transient voltages can easily
affect the conversion result. Such glitches might originate
from switching power supplies, nearby digital logic, and
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. The error can change if the
external event changes in time with respect to the CLK
input.
The GND pin should be connected to a clean ground point.
In many cases, this will be the “analog” ground. Avoid
connections which are too near the grounding point of a
microcontroller or digital signal processor. If needed, 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.
With this in mind, power to the ADS7835 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 10µF capacitor is recommended. If needed, an even
®
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ADS7835
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