ADS7835ESLASH250 datasheet(13/21 Pages) TI2

13 / 21 page

ADS7835

(tCKP).

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.

With this in mind, power to the ADS7835 should be clean

and well bypassed. A 0.1

µF ceramic bypass capacitor should

be placed as close to the device as possible. In addition, a

µF to 10µF capacitor is recommended. If needed, an even

larger capacitor and a 5

Ω 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.

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.

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