LTM9002 datasheet(26/28 Pages) LINER

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LTM9002

9002f

APPLICATIONS INFORMATION

the output busses are swapped and channel A comes

out on DBx; channel B comes out on DAx. To multiplex

both channels onto a single output bus, connect MUX,

CLKA and CLKB together (see the Timing Diagram for

the multiplexed mode.) The multiplexed data is available

on either data bus – the unused data bus can be disabled

with its OE pin.

Supply Sequencing

The VCC pin provides the supply to the ampliﬁer and the

auxiliary DAC while the VDD pin provides the supply to the

ADC. The ampliﬁer, ADC and the DAC are separate integrated

circuits within the LTM9002; however, there are no supply

sequencing considerations beyond standard practice. It is

recommended that the ampliﬁer, ADC and DAC all use the

same low noise, 3.0V supply, but VCCmaybeoperatedfrom

a different voltage level if desired. Both rails can operate

from the same 3.0V linear regulator but place a ferrite bead

between the VCC and VDD pins. Separate linear regulators

can be used without additional supply sequencing circuitry

if they have common input supplies.

Grounding and Bypassing

The LTM9002 requires a printed circuit board with a

clean unbroken ground plane; a multilayer board with an

internal ground plane is recommended. The pinout of the

LTM9002 has been optimized for a ﬂow-through layout

so that the interaction between inputs and digital outputs

is minimized. A continuous row of ground pads facilitate

a layout that ensures that digital and analog signal lines

are separated as much as possible.

The LTM9002 is internally bypassed with the ADC, (VDD)and

ampliﬁer and DAC (VCC) supplies returning to a common

ground (GND). The digital output supply (OVDD)isreturned

to OGND. Additional bypass capacitance is optional and

may be required if power supply noise is signiﬁcant.

The differential inputs should run parallel and close to each

other. The input traces should be as short as possible to

minimize capacitance and to minimize noise pickup.

Heat Transfer

Most of the heat generated by the LTM9002 is transferred

through the bottom-side ground pads. For good electrical

and thermal performance, it is critical that all ground pins

are connected to a ground plane of sufﬁcient area with as

many vias as possible.

Recommended Layout

The high integration of the LTM9002 makes the PC board

layout very simple and easy. However, to optimize its electri-

cal and thermal performance, some layout considerations

are still necessary.

• Use large PCB copper areas for ground. This helps

to dissipate heat in the package through the board

and also helps to shield sensitive on-board analog

signals. Common ground (GND) and output ground

(OGND) are electrically isolated on the LTM9002, but

can be connected on the PCB underneath the part to

provide a common return path.

• Use multiple ground vias. Using as many vias as

possible helps to improve the thermal performance

of the board and creates necessary barriers separat-

ing analog and digital traces on the board at high

frequencies.

• Separate analog and digital traces as much as pos-

sible, using vias to create high-frequency barriers.

This will reduce digital feedback that can reduce the

signal-to-noise ratio (SNR) and dynamic range of the

LTM9002.

The quality of the paste print is an important factor in

producing high yield assemblies. It is recommended to

use a type 3 or 4 printing no-clean solder paste. The solder

stencil design should follow the guidelines outlined in

Application Note 100.

The LTM9002 employs gold-ﬁnished pads for use with

Pb-based or tin-based solder paste. It is inherently Pb-free

and complies with the JEDEC (e4) standard. The materi-

als declaration is available online at http://www.linear.

com/leadfree/mat_dec.jsp.

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