DEM-OPA-SOT-1A

User's Guide SBOU010B – October 1999 – Revised April 2012 DEM-OPA-SOT-1A Demonstration Fixture 1 Description The DEM-OPA-SOT-1A demonstration fixture is a generic, unpopulated printed circuit board (PCB) designed to provide an optimized layout when evaluating wideband operational amplifiers in the tiny SOT23 package. Figure 1 shows the package pinout for this PCB. For more information on these op amps, as well as good PCB layout techniques, see the individual amplifier data sheets. Only a few components are necessary to be assembled onto the board to set up the operational amplifier in either an inverting or noninverting configuration. In order to avoid parasitic component inductance the board only accepts surface mount resistors and capacitors. The actual values of the components, especially the resistors, depend on the type of operational amplifier used and the intended mode of operation. The board was designed to accommodate a variety of amplifiers: voltage feedback types or current feedback types. Furthermore, the evaluation board not only handles op amps in the existing 5-lead SOT-23 package but also the evaluation platform for 6-lead SOT-23 package. Therefore, the layout includes a sixth pad for the IC (U1) located on the opposite side of pad 2. The sixth lead may be an optional function pin for those amplifiers featuring a disable function. As shown in Figure 3a (top layer view), the user has the option to either hardwire this function pin to the positive supply by shortening the solder switch A, or connecting it to connector J3 soldering the switch in position B. For a detailed discussion on the individual op amp and the appropriate selection of the recommended feedback and gain resistors, see the respective data sheets. As a reminder, the frequency response performance of a current feedback amplifier is determined by the feedback resistor value. Also note that the feedback resistor compensates for the package parasitics, and therefore its value usually varies for different packages. All high-frequency devices have a common sensitivity to component and layout parasitics that may alter the AC performance. Therefore, slight adjustments of the feedback resistor (R2) and/or the gain resistor (R6) might be necessary to obtain the final desired frequency response. For current feedback amplifiers, increasing RF from its nominal value will reduce the achievable bandwidth, while decreasing RF will cause the frequency response to peak more. However, at higher gains, it might be advantageous to decrease RF in order to optimize for bandwidth. For voltage feedback amplifiers, changing the impedance level (increasing or decreasing RF and RG) can also control the amount of peaking. Output 1 -VS 2 +Input 3 5 4 Output 1 6 +VS -VS 2 5 DIS Noninverting Input 3 4 Inverting Input +VS -Input SOT23-6 SOT23-5 Figure 1. SOT23 Package Pinout, Top View SBOU010B – October 1999 – Revised April 2012 Submit Documentation Feedback DEM-OPA-SOT-1A Demonstration Fixture Copyright © 1999–2012, Texas Instruments Incorporated 1 Circuit 2 www.ti.com Circuit The circuit schematic in Figure 2 shows the connections for all possible components. Each configuration uses only some of the components. The power-supply bypassing capacitors are crucial components for high-frequency amplifiers and should not be omitted during evaluation. Typically, each supply should be bypassed by a parallel combination of a ceramic 0.1μF capacitor and a tantalum 2.2μF capacitor. Those bypass capacitors are located on the board such that the return path to the power connector is very short. Any larger transient currents will have a short loop, and will not reflect to the input and cause an increase in distortion. +5V C1 + 2.2mF -5V C2 0.1mF C3 0.1mF C4 + 2.2mF 5 J1 +In 2 3 R1 49.9W J3 U1 J2 Out 6 4 JP1 R6 332W R4 R2 49.9W 1 J4 Disable B -In R7 49.9W A R5 332W R3 C6 (Optional) +5V NOTE: Values for G = +2, OPA658. R3, R4, R7, C6 not assembled. Figure 2. Schematic for DEM-OPA-SOT-1A 3 Components Components that have RF performance similar to the ones in Table 1 may be substituted. Table 1. Component Descriptions PART DESCRIPTION C1, C4 2.2μF, 16V, Size 3548 C2, C3 0.1μF, 50V, Size 1206 Feedback capacitor (optional); depends on application (not used on current feedback op amps). C6 R1, R2, R7 Typically 50Ω R4, R5, R6 Depends on application Power Connector (On-Shore Technology ED555/3DS) JP1 J1 – J4 SMA or SMB Connectors Set to get R3 ∥ R4 = desired input impedance for inverting operation. R3 2 DEM-OPA-SOT-1A Demonstration Fixture SBOU010B – October 1999 – Revised April 2012 Submit Documentation Feedback Copyright © 1999–2012, Texas Instruments Incorporated Board Layout www.ti.com 4 Board Layout This demonstration fixture is a two-layer PCB with the power traces on the bottom layer. Even though both sides have a ground plane, a window has been opened up around the DUT and its surrounding components. The purpose of this window is to reduce the parasitic capacitances between sensitive nodes and the ground planes. The footprint of the SMA connectors were designed to use straight connectors in either a vertical or horizontal mounting position. Note that the center conductor of the SMA must be on the top side of the board when mounted horizontally. 1400 (mil) P1 +Vs -Vs C1 C4 C3 J1 R1 OUT J2 U1 R2 1 R5 -In JP1 C2 J3 J4 R7 Disable 1604 (mil) +In R4 R3 (a) Top Layout with Silkscreen (b) Bottom Layout with Silkscreen Figure 3. DEM-OPA-SOT-1A Demonstration Board Layout 5 Measurement Tips This demonstration fixture, with the component values shown, is designed to operate in a 50Ω environment; most data sheet plots are obtained this way. It is easy to change the component values for different input and output impedance levels. However, do not use high-impedance probes; they represent a heavy capacitive load to the op amp, and will alter the amplifier response. Instead, use low-impedance (≤ 500Ω) probes with adequate bandwidth. The probe input capacitance and resistance set an upper limit on the measurement bandwidth. If a high-impedance probe must be used, place a 100Ω resistor on the probe tip to isolate its capacitance from the circuit. SBOU010B – October 1999 – Revised April 2012 Submit Documentation Feedback DEM-OPA-SOT-1A Demonstration Fixture Copyright © 1999–2012, Texas Instruments Incorporated 3 FCC Warning This evaluation board/kit is intended for use for ENGINEERING DEVELOPMENT, DEMONSTRATION, OR EVALUATION PURPOSES ONLY and is not considered by TI to be a finished end-product fit for general consumer use. It generates, uses, and can radiate radio frequency energy and has not been tested for compliance with the limits of computing devices pursuant to part 15 of FCC rules, which are designed to provide reasonable protection against radio frequency interference. Operation of this equipment in other environments may cause interference with radio communications, in which case the user at his own expense will be required to take whatever measures may be required to correct this interference. 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