ADS1675 datasheet(28/40 Pages) TI2

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ADS1675

SBAS416D – DECEMBER 2008 – REVISED AUGUST 2010

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APPLICATION INFORMATION

6. Analog Inputs: The analog input pins must be

obtain

the

specified

performance

from

the

driven

differentially

achieve

specified

ADS1675,

the

following

layout

and

component

performance.

true

differential

driver

guidelines should be considered.

transformer (ac applications) can be used for this

1. Power Supplies: The device requires two power

purpose. Route the analog inputs tracks (AINP,

supplies for operation: DVDD and AVDD. A very

AINN) as a pair from the buffer to the converter

clean and stable AVDD supply is needed to

using short, direct tracks and away from digital

achieve optimal performance from the device. For

tracks. A 750pF capacitor should be used directly

both supplies, use a 10mF tantalum capacitor,

across the analog input pins, AINP and AINN. A

bypassed with a 0.1mF ceramic capacitor, placed

low-k dielectric (such as COG or film type) should

close to the device pins. Alternatively, a single

be used to maintain low THD. Capacitors from

10mF ceramic capacitor can be used. The

each analog input to ground should be used.

supplies should be relatively free of noise and

They should be no larger than 1/10 the size of

should not be shared with devices that produce

the difference capacitor (typically 100pF) to

voltage spikes (such as relays, LED display

preserve the ac common-mode performance.

drivers, etc.). If a switching power-supply source

7. Component Placement: Place the power supply,

is used, the voltage ripple should be low (less

analog

input,

and

reference

input

bypass

than

2mV).

The

power

supplies

may

capacitors as close as possible to the device

sequenced in any order.

pins. This placement is particularly important for

2. Ground Plane: A single ground plane connecting

the

small-value

ceramic

capacitors.

both AGND and DGND pins can be used. If

Surface-mount components are recommended to

separate digital and analog grounds are used,

avoid

the

higher

inductance

leaded

connect the grounds together at the converter.

components.

3. Digital Inputs: Source terminate the digital inputs

Figure 53 through Figure 55 illustrate the basic

to the device with 50

Ω series resistors. The

connections and interfaces that can be used with the

resistors should be placed close to the driving

ADS1675.

end of the digital source (oscillator, logic gates,

DSP, etc.). These resistors help reduce ringing

on the digital lines, which may lead to degraded

ADC performance.

4. Analog/Digital Circuits: Place analog circuitry

(input buffer, reference) and associated tracks

together, keeping them away from digital circuitry

(DSP,

microcontroller,

logic).

Avoid

crossing

digital tracks across analog tracks to reduce

noise coupling and crosstalk.

5. Reference Inputs: The ADS1675 reference input

has 400

Ω across VREFP and VREFN. The

driving amplifier must source current for this static

current, as well as dynamic switching current as a

result of the 32MHz clock. The reference driving

amplifier should be ready to source at least

10.5mA.

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