DC2414A - ANALOG DEVICES - Datasheet.Directory

dc2414af

DEMO MANUAL DC2414A

Description

LTC6268 and LTC6268-10

3-Channel SOT-23

Transimpedance Amplifier

Demonstration Circuit 2414A has layouts for three chan-

nels of SOT-23 transimpedance amplifiers. Each of the

three layouts applies different techniques achieving

various parasitic feedback capacitances (CF). The upper

channel, U3, assumes a low Transimpedance gain, has

a parasitic CF of 0.1pF, and provides a footprint for an

additional component CF in an 0402 footprint (C29). The

lower channel, U1, assumes high transimpedance gain and

was laid out to minimize CF, achieving approximately 7fF,

although the real situation is more complex. The middle

channel, U2, assumes a middle case. The outputs of the

circuit are laid out for SMA edge connectors. The inputs

consist of six pads per channel along the edge where a

through-hole photodiode can be mounted, with provision

L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear

Technology Corporation. All other trademarks are the property of their respective owners.

BoarD photo

for any photodiode pinout. The reverse bias voltage for

the photodiode can be applied at a VBB turret, or can be

taken from V+ or V– through jumper JP4. The VBB traces

were spaced according to IPC2221 for 150V working volt-

age, so high voltage APDs can be applied. Take caution

when working with high voltages to avoid contact with

any part of the VBB trace. Because of the many possible

varieties of population, the board is stocked as a bare

unpopulated board.

Design files for this circuit board are available at

http://www.linear.com/demo/DC2414A

Figure 1. Topside Bare PCB DC2414A, for SOT-23 LTC

6268 and LTC6268-10 Op Amps

dc2414af

DEMO MANUAL DC2414A

operating principles

The LTC6268 and LTC6268-10 have gain bandwidth prod-

ucts of 500MHz and 4GHz respectively. At low transimped-

ance gain with small photodiodes (say 10k gain and 1pF

photodiode) the bandwidth achieved can be quite high,

such as 200MHz. The upper channel, U3, is best suited

for these types of applications. The parasitic CF around the

RF is about 0.1pF in this channel, and an 0402 footprint

is provided so additional CF can be added. The RF and CF

in this highest speed channel are on the topside to avoid

vias, and are close to the op amp.

At very high gains, much less bandwidth is achievable both

because the gain is high and because parasitic feedback

capacitance reduces the effective impedance of the feed-

back resistor prematurely. For both the low capacitance

channels, the RF is mounted on the bottom side for the

most effective shielding from the output trace.

But the resistor and its pads also have parasitic capaci-

tance. For example, a typical 0805 resistor has about 70fF

of capacitance in air

, so a 10M resistor would already be

3dB lower impedance at 220kHz. However, the circuit is

not “in air,” but on a board; and by placing some grounded

copper underneath a resistor, the parasitic CF can be

greatly reduced.

The middle channel, U2, applies this technique rather

modestly under an 0603 resistor, while the lower chan-

nel, U1, has a rather extreme layout under a 1206 resis-

tor. These channels have parasitic CF of about 33fF and

“7fF,” respectively, with a regular Vishay CRCW resistor

installed. However, the “7fF” is a simplification, derived

from the fact that at 10M of gain, that channel achieves

rise times as low as 100ns. In fact, however, the extended

body of the resistor and the ground underneath it cause

some capacitive loading of the resistor element, perhaps

best modelled as an RC network as shown in Figure 2.

Figure 2. Capacitively Loaded Resistor Element Approximation for the U1 Channel, with RF = 10M

dc2414af

DEMO MANUAL DC2414A

operating principles

Split Supply vs Single Supply

The default configuration of the board assumes a ±1.55V

to ±2.5V split supply. To use a single supply, short V– to

ground at the GND and V– turrets. Also, in order to keep

inductance low at the op amp’s V– connection, short the

V– bypass capacitor locations near the op amp (C8/C9/

C21) depending on which op amp is installed. Valid single

supplies are 3.1V to 5V. When using a single supply, you

will probably want to change the default voltage on the

+input.

The +Input

On all 3 channels, the +input is grounded through R35/

R36/R37 and a trace called “METAL” to ground. The series

resistor may be a 0Ω jumper, or a 33Ω to 100Ω resistor

to de-Q the path. To create a voltage other than ground at

the +input, cut the “METAL” trace and create the desired

voltage with the resistor strings there (R6-8, R13-15, and

R23-25). Capacitors C38/C39/C40 are provided to filter

the resistor noise and any supply ripple.

VBB

The photodiode reverse bias can be applied at the VBB

turret. (Note that all “turrets” are close to the edges of the

board, so clips can be applied directly to the plated holes,

thus not requiring turrets to be populated.) VBB can also be

taken from V+ or V– though jumper selection JP4. If VBB

will be a high voltage, be careful not to touch it when

energized, as the trace runs along the entire edge of the

photodiode side of the board. Lightning bolts are placed

in the silkscreen as a reminder when high voltage is used

for VBB. Various pads are provided to RC connect VBB to

the cathode or anode of the photodiode. In practice, very

few of the passives shown around the photodiode will be

installed. Which ones will be installed will depend on the

photodiode pinout and whether the TIA will be anode or

cathode connected.

Shutdown (Floats “On”)

The LTC6268 _SHDN pin floats high, turning the op amp

on. If you want to play with the shutdown function, install

JP1/JP2/JP3. Placing the shunt on the jumper pulls the

_SHDN to ground turning off the op amp.

PCB Material

The PCB dielectric, chosen for its low dielectric constant

of 3.4, is Nelco-4000-EP-SI. If FR-4 is used with a similar

layout, remember that the parasitic capacitances will in-

crease by 30% to 40%. Note that this demo board is not

fabricated to controlled impedance. The special material

was chosen purely for low capacitance.

Checking for High Frequency Oscillations in

LTC6268-10 Designs

The LTC6268-10 has a gain bandwidth product of 4GHz.

When checking an LTC6268-10 design for any problems,

it is best to use an oscilloscope with adequate bandwidth

(>1GHz), so that any high frequency oscillations are not

hidden by limited scope bandwidth. If using a spectrum

analyzer, it should have at least 3GHz of bandwidth.

The DC2414A high speed channel was designed with

little ground copper in the photodiode region, to support

the lowest input capacitance possible. However, when

the photodiode is larger, then input inductance can be-

come an issue. For example, at 12pF of lumped element

photodiode C, and with the nominal 20k || 0.1pF feedback

network in place, ~30mVP-P oscillations were detected at

1.4GHz. Placing a grounded copper foil tape along the bot-

tom side copper void thoroughly quenched the oscillations.

dc2414af

DEMO MANUAL DC2414A

sample population

Figure 3. An Example of a Population of the Very Low Capacitance Channel Around U1.

Photodiode is OSI FCI-125G-006HRL, Anode Connected to the TIA Input, so Cathode is

Held High to VBB Shorted to V+. RF is 9.76M on the Bottom Side (Left)

dc2414af

DEMO MANUAL DC2414A

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-

tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.

schematic Diagram

D D

C C

B B

A A

NOTES: UNLESS OTHERWISE SPECIFIED,

1. ALL CAPACITORS AND RESISTORS ARE 0603

OPT COMPONENTS

ASSEMBLY TYPE

-A-

-B-

U1,U2,U3

LTC6268

LTC6268-10

R9 R12 R22

10Meg

1Meg

1Meg 2K

100K 10K

SHDN SHDN SHDN

VERY LOW C LOW C HIGH SPEED

V+

V+V-

V+

V+V-

V+

V-

V+V-

V- V- V-

V+

V-

SIZE

DATE:

IC NO. REV.

SHEET OF

TITLE:

APPROVALS

PCB DES.

APP ENG.

TECHNOLOGY Fax: (408)434-0507

Milpitas, CA 95035

Phone: (408)432-1900

1630 McCarthy Blvd.

LTC Confidential-For Customer Use Only

CUSTOMER NOTICE

LINEAR TECHNOLOGY HAS MADE A BEST EFFORT TO DESIGN A

CIRCUIT THAT MEETS CUSTOMER-SUPPLIED SPECIFICATIONS;

HOWEVER, IT REMAINS THE CUSTOMER'S RESPONSIBILITY TO

VERIFY PROPER AND RELIABLE OPERATION IN THE ACTUAL

APPLICATION. COMPONENT SUBSTITUTION AND PRINTED

CIRCUIT BOARD LAYOUT MAY SIGNIFICANTLY AFFECT CIRCUIT

PERFORMANCE OR RELIABILITY. CONTACT LINEAR

TECHNOLOGY APPLICATIONS ENGINEERING FOR ASSISTANCE.

THIS CIRCUIT IS PROPRIETARY TO LINEAR TECHNOLOGY AND

SCHEMATIC

SUPPLIED FOR USE WITH LINEAR TECHNOLOGY PARTS.

SCALE = NONE

www.linear.com

Thursday, June 09, 2016

LTC626X TRANSIMPEDANCE AMPLIFIER

GLEN B.

N/A

LTC626X

DEMO BOARD 2414A

SIZE

DATE:

IC NO. REV.

SHEET OF

TITLE:

APPROVALS

PCB DES.

APP ENG.

TECHNOLOGY Fax: (408)434-0507

Milpitas, CA 95035

Phone: (408)432-1900

1630 McCarthy Blvd.

LTC Confidential-For Customer Use Only

CUSTOMER NOTICE

LINEAR TECHNOLOGY HAS MADE A BEST EFFORT TO DESIGN A

CIRCUIT THAT MEETS CUSTOMER-SUPPLIED SPECIFICATIONS;

HOWEVER, IT REMAINS THE CUSTOMER'S RESPONSIBILITY TO

VERIFY PROPER AND RELIABLE OPERATION IN THE ACTUAL

APPLICATION. COMPONENT SUBSTITUTION AND PRINTED

CIRCUIT BOARD LAYOUT MAY SIGNIFICANTLY AFFECT CIRCUIT

PERFORMANCE OR RELIABILITY. CONTACT LINEAR

TECHNOLOGY APPLICATIONS ENGINEERING FOR ASSISTANCE.

THIS CIRCUIT IS PROPRIETARY TO LINEAR TECHNOLOGY AND

SCHEMATIC

SUPPLIED FOR USE WITH LINEAR TECHNOLOGY PARTS.

SCALE = NONE

www.linear.com

Thursday, June 09, 2016

LTC626X TRANSIMPEDANCE AMPLIFIER

GLEN B.

N/A

LTC626X

DEMO BOARD 2414A

SIZE

DATE:

IC NO. REV.

SHEET OF

TITLE:

APPROVALS

PCB DES.

APP ENG.

TECHNOLOGY Fax: (408)434-0507

Milpitas, CA 95035

Phone: (408)432-1900

1630 McCarthy Blvd.

LTC Confidential-For Customer Use Only

CUSTOMER NOTICE

LINEAR TECHNOLOGY HAS MADE A BEST EFFORT TO DESIGN A

CIRCUIT THAT MEETS CUSTOMER-SUPPLIED SPECIFICATIONS;

HOWEVER, IT REMAINS THE CUSTOMER'S RESPONSIBILITY TO

VERIFY PROPER AND RELIABLE OPERATION IN THE ACTUAL

APPLICATION. COMPONENT SUBSTITUTION AND PRINTED

CIRCUIT BOARD LAYOUT MAY SIGNIFICANTLY AFFECT CIRCUIT

PERFORMANCE OR RELIABILITY. CONTACT LINEAR

TECHNOLOGY APPLICATIONS ENGINEERING FOR ASSISTANCE.

THIS CIRCUIT IS PROPRIETARY TO LINEAR TECHNOLOGY AND

SCHEMATIC

SUPPLIED FOR USE WITH LINEAR TECHNOLOGY PARTS.

SCALE = NONE

www.linear.com

Thursday, June 09, 2016

LTC626X TRANSIMPEDANCE AMPLIFIER

GLEN B.

N/A

LTC626X

DEMO BOARD 2414A

R26

C20 C15

C1 C11

R21 49.9

1nF

C37

0.1uF

0805

R24

R27

C14

METAL3

R19

R31

C39

R29

Vbb/GND

CATHODE/ANODE

Vbb/GND

R12

100K

C30

10nF

JP2 OPT

1 2

SMA

C21

1nF

C25

VBB

C28

C19

10uF

1206 R23

C35

10nF

C31

10nF

R33

R10 49.9

R15

R30

V-

C10

1nF

V+

C23C24

R16

R35

R25

Vbb/GND

CATHODE/ANODE

Vbb/GND

R34

0402

METAL2

SMA

R18

C34

10nF

R28

R20

R32

C29

0.1pF

0402

C36

0.1uF

0805

JP4

HD2X2-079

3 4

C33

10nF

JP3 OPT

1 2

C32

10nF

C38

METAL1

R11 49.9

JP1 OPT

1 2

R36

SMA

R13

R14

C22

1nF

C27

1Meg

1206

R37

C13

C40

R22

20K

1nF

Vbb/GND

CATHODE/ANODE

Vbb/GND

GND

C18

10uF

1206

C12

C26

R17

1nF

dc2414af

DEMO MANUAL DC2414A

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com

LT 1216 • PRINTED IN USA

DEMONSTRATION BOARD IMPORTANT NOTICE

Linear Technology Corporation (LTC) provides the enclosed product(s) under the following AS IS conditions:

This demonstration board (DEMO BOARD) kit being sold or provided by Linear Technology is intended for use for ENGINEERING DEVELOPMENT

OR EVALUATION PURPOSES ONLY and is not provided by LTC for commercial use. As such, the DEMO BOARD herein may not be complete

in terms of required design-, marketing-, and/or manufacturing-related protective considerations, including but not limited to product safety

measures typically found in finished commercial goods. As a prototype, this product does not fall within the scope of the European Union

directive on electromagnetic compatibility and therefore may or may not meet the technical requirements of the directive, or other regulations.

If this evaluation kit does not meet the specifications recited in the DEMO BOARD manual the kit may be returned within 30 days from the date

of delivery for a full refund. THE FOREGOING WARRANTY IS THE EXCLUSIVE WARRANTY MADE BY THE SELLER TO BUYER AND IS IN LIEU

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observe good laboratory practice standards. Common sense is encouraged.

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