CH0

CH1

CH2

CH3

CH4

CH5

CH6

CH7

+VDD

SDA

SCL

A1

A0

COM

REF

GND

= Via to ground plane

Place power supply

capacitor as close as

possible to VDD pin

ADS7828-Q1

ADS7828-Q1

www.ti.com

SBAS456B – DECEMBER 2008 – REVISED JANUARY 2016

9 Power Supply Recommendations

The device requires only one power supply, VDD, which can have an input voltage between 2.7 V and 5.0 V.

The value of VDD affects the input voltage range and the digital logic high and low levels. A decoupling capacitor

should be placed close to the VDD pin. TI recommends a value of at least 0.1 µF.

10 Layout

10.1 Layout Guidelines

For optimum performance, care should be taken with the physical layout of the ADS7828 circuitry. The basic

SAR architecture 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. Therefore, during any

single conversion for an "n-bit" SAR converter, there are n "windows" 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.

With this in mind, power to the ADS7828 should be clean and well-bypassed. A 0.1-

μF ceramic bypass capacitor

should be placed as close to the device as possible. A 1-

μF to 10-μF capacitor may also be needed if the

impedance of the connection between +VDD and the power supply is high.

The ADS7828 architecture offers no inherent rejection of noise or voltage variation in regards to using an

external reference input. This is of particular concern when the reference input is tied to the power supply. Any

noise and ripple from the supply will appear directly in the digital results. While high-frequency noise can be

filtered out, voltage variation due to line frequency (50 Hz or 60 Hz) can be difficult to remove.

The GND pin should be connected to a clean ground point. In many cases, this will be the "analog" ground.

Avoid connections that are too near the grounding point of a microcontroller or digital signal processor. The ideal

layout will include an analog ground plane dedicated to the converter and associated analog circuitry.

10.2 Layout Example

Figure 27. Layout Example

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