DEMO MANUAL DC2414A
LTC6268 and LTC6268-10
3-Channel SOT-23
Transimpedance Amplifier
Description
Demonstration Circuit 2414A has layouts for three channels 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
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 voltage, 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
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
Figure 1. Topside Bare PCB DC2414A, for SOT-23 LTC®6268 and LTC6268-10 Op Amps
dc2414af
1
DEMO MANUAL DC2414A
Operating Principles
The LTC6268 and LTC6268-10 have gain bandwidth products of 500MHz and 4GHz respectively. At low transimpedance 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 feedback 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 capacitance. 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 channel, U1, has a rather extreme layout under a 1206 resistor. 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
2
dc2414af
DEMO MANUAL DC2414A
Operating Principles
Split Supply vs Single Supply
Shutdown (Floats “On”)
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 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.
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.
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 increase 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 become 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 bottom side copper void thoroughly quenched the oscillations.
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3
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)
4
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E4
4
2
V-
C19
10uF
1206
C18
10uF
1206
V+
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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
-B-
-A-
*
C37
0.1uF
0805
LTC6268-10
LTC6268
U1,U2,U3
R9
V-
1Meg 100K
10K
2K
C8
1nF
2
5
R3
C1
R2
R35
33
U1
R10 49.9
R22
J1
OPT
C30
10nF
2
1. ALL CAPACITORS AND RESISTORS ARE 0603
5
R7
METAL1
C38
R12
SMA
JP1
C3
V+
R8
R6
V-
SHDN
1
C5
10Meg 1Meg
V+
C36
0.1uF
0805
ASSEMBLY TYPE
E3
E2
JP4
HD2X2-079
3
1
R1
R31
C2
4
*
6
C7
1nF
V+
C31
10nF
VERY LOW C
D1
C4
R4
R5
R9
1Meg
1206
C6
VSMA
JP2
R13
METAL2
R36
33
U2
R32
C11
*
6
C10
1nF
LOW C
D2
C14
V+
R17
CUSTOMER NOTICE
R11 49.9
C9
1nF
2
5
R19
C12
R18
C34
10nF
3
THIS CIRCUIT IS PROPRIETARY TO LINEAR TECHNOLOGY AND
SUPPLIED FOR USE WITH LINEAR TECHNOLOGY PARTS.
V-
R12
100K
C15
2
SCALE = NONE
GLEN B.
AK
J3
DATE:
N/A
SIZE
R29
C24
R27
R37
33
U3
R33
C23
*
6
R34
D3
C26
R28
1
R26
C35
10nF
R22
20K
C27
C29
0.1pF
0402
Thursday, June 09, 2016
IC NO.
1
SHEET 1
3
REV.
OF 1
LTC626X TRANSIMPEDANCE AMPLIFIER
LTC626X
DEMO BOARD 2414A
1630 McCarthy Blvd.
Milpitas, CA 95035
Phone: (408)432-1900 www.linear.com
Fax: (408)434-0507
LTC Confidential-For Customer Use Only
C22
1nF
V+
HIGH SPEED
0402
TECHNOLOGY
R21 49.9
C21
1nF
2
5
TITLE: SCHEMATIC
SMA
OPT
C33
10nF
2
METAL3
C40
R24
V+
C25
R30
R23
R25
V-
SHDN
1
C28
JP3
2
APPROVALS
R16
LINEAR TECHNOLOGY HAS MADE A BEST EFFORT TO DESIGN A
CIRCUIT THAT MEETS CUSTOMER-SUPPLIED SPECIFICATIONS;
HOWEVER, IT REMAINS THE CUSTOMER'S RESPONSIBILITY TO
PCB DES.
VERIFY PROPER AND RELIABLE OPERATION IN THE ACTUAL
APPLICATION. COMPONENT SUBSTITUTION AND PRINTED
APP ENG.
CIRCUIT BOARD LAYOUT MAY SIGNIFICANTLY AFFECT CIRCUIT
PERFORMANCE OR RELIABILITY. CONTACT LINEAR
TECHNOLOGY APPLICATIONS ENGINEERING FOR ASSISTANCE.
J2
OPT
C32
10nF
2
R14
V+
C39
R15
V-
SHDN
1
C20
C13
R20
OPT COMPONENTS
3
-
NOTES: UNLESS OTHERWISE SPECIFIED,
V-
GND
V+
V-
VBB
4
-
A
B
C
D
3
3
+
1
E1
6
Vbb/GND
5
Vbb/GND
4
Vbb/GND
CATHODE/ANODE
2
4
1
Vbb/GND
6
+
1
Vbb/GND
3
3
Vbb/GND
5
Vbb/GND
2
4
1
Vbb/GND
4
Vbb/GND
CATHODE/ANODE
3
+
1
Vbb/GND
6
Vbb/GND
3
5
Vbb/GND
2
4
Vbb/GND
4
Vbb/GND
CATHODE/ANODE
1
Vbb/GND
5
A
B
C
D
DEMO MANUAL DC2414A
Schematic Diagram
-
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DEMO MANUAL DC2414A
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
OF ALL OTHER WARRANTIES, EXPRESSED, IMPLIED, OR STATUTORY, INCLUDING ANY WARRANTY OF MERCHANTABILITY OR FITNESS
FOR ANY PARTICULAR PURPOSE. EXCEPT TO THE EXTENT OF THIS INDEMNITY, NEITHER PARTY SHALL BE LIABLE TO THE OTHER FOR
ANY INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES.
The user assumes all responsibility and liability for proper and safe handling of the goods. Further, the user releases LTC from all claims
arising from the handling or use of the goods. Due to the open construction of the product, it is the user’s responsibility to take any and all
appropriate precautions with regard to electrostatic discharge. Also be aware that the products herein may not be regulatory compliant or
agency certified (FCC, UL, CE, etc.).
No License is granted under any patent right or other intellectual property whatsoever. LTC assumes no liability for applications assistance,
customer product design, software performance, or infringement of patents or any other intellectual property rights of any kind.
LTC currently services a variety of customers for products around the world, and therefore this transaction is not exclusive.
Please read the DEMO BOARD manual prior to handling the product. Persons handling this product must have electronics training and
observe good laboratory practice standards. Common sense is encouraged.
This notice contains important safety information about temperatures and voltages. For further safety concerns, please contact a LTC application
engineer.
Mailing Address:
Linear Technology
1630 McCarthy Blvd.
Milpitas, CA 95035
Copyright © 2004, Linear Technology Corporation
6
dc2414af
Linear Technology Corporation
LT 1216 • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
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