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(R)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 0 jumper, or a 33 to 100 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. dc2414af 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 dc2414af 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 - dc2414af 5 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 (c) 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 (c) LINEAR TECHNOLOGY CORPORATION 2016