ADC 2 Spécifications télécharger pdf (Page 25)

ADS7865

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SBAS441C –OCTOBER 2008–REVISED APRIL 2012

LAYOUT Depending on the circuit density of the board,

placement of the analog and digital components, and

For optimum performance, care should be taken with

the related current loops, a single solid ground plane

the physical layout of the ADS7865 circuitry. This

for the entire printed circuit board (PCB) or a

caution is particularly true if the CLOCK input

dedicated analog ground area may be used. In an

approaches the maximum throughput rate. In this

instance of a separated analog ground area, ensure a

case, it is recommended to have a fixed phase

low-impedance connection between the analog and

relationship between CLOCK and CONVST.

digital ground of the ADC by placing a bridge

underneath (or next to) the ADC. Otherwise, even

Additionally, the basic SAR architecture is quite

short undershoots on the digital interface with a value

sensitive to glitches or sudden changes on the power

lower than –300mV may lead to conduction of ESD

supply, reference, ground connections, and digital

diodes, causing current flow through the substrate

inputs that occur just before latching the output of the

and degrading the analog performance.

analog comparator. Therefore, when driving any

single conversion for an n-bit SAR converter, there

During the PCB layout process, care should also be

are n windows in which large external transient

taken to avoid any return currents crossing any

voltages can affect the conversion result. Such

sensitive analog areas or signals. No signal must

glitches might originate from switching power

exceed the limit of –300mV with regard to the

supplies, nearby digital logic, or high-power devices.

respective ground plane. Figure 37 illustrates the

The degree of error in the digital output depends on

recommended layout of the ground and power-supply

the reference voltage, layout, and the exact timing of

connections.

the external event. These errors can change if the

external event also changes in time with respect to

Supply

the CLOCK input.

The ADS7865 has two separate supplies: the BV

With this possibility in mind, power to the ADS7865

pin for the digital interface and the AV

pin for all

should be clean and well-bypassed. A 0.1μF ceramic

remaining circuits.

bypass capacitor should be placed as close to the

device as possible. In addition, a 1μF to 10μF BV

can range from 2.7V to 5.5V, allowing the

capacitor is recommended. If needed, an even larger ADS7865 to easily interface with processors and

capacitor and a 5Ω or 10Ω series resistor may be controllers. To limit the injection of noise energy from

used to low-pass filter a noisy supply. external digital circuitry, BV

should be filtered

properly. Bypass capacitors of 0.1μF and 10μF

If the reference voltage is external and originates

should be placed between the BV

pin and the

from an operational amplifier, be sure that it can drive

ground plane.

the reference capacitor without oscillation. The

connection between the output of the external AV

supplies the internal analog circuitry. For

reference driver and REF

should be of low optimum performance, a linear regulator (for

resistance (10Ω max) to minimize any code- example, the UA7805 family) is recommended to

dependent voltage drop on this path. generate the analog supply voltage in the range of

2.7V to 5.5V for the ADS7865 and the necessary

Grounding analog front-end circuitry.

All ground (AGND and BGND) pins should be Bypass capacitors should be connected to the ground

connected to a clean ground reference. These plane such that the current is allowed to flow through

connections should be kept as short as possible to the pad of the capacitor (that is, the vias should be

minimize the inductance of these paths. It is placed on the opposite side of the connection

recommended to use vias connecting the pads between the capacitor and the power-supply pin of

directly to the ground plane. In designs without the ADC).

ground planes, the ground trace should be kept as

wide as possible. Avoid connections that are too near Digital Interface

the grounding point of a microcontroller or digital

To further optimize device performance, a series

signal processor.

resistor of 10Ω to 100Ω can be used on each digital

pin of the ADS7865. In this way, the slew rates of the

input and output signals are reduced, limiting the

noise injection from the digital interface.

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