High Frequency Amplifier Evaluation Board Design Note 50 Mitchell Lee Introduction grounds are still separated. For example, the termination resistors (R3 and R7) and the gain-setting resistor (R1) Demo board DC009 is designed to simplify the evaluation are grounded in the vicinity of the input connector. Supply of high speed operational amplifiers. It includes both an bypass capacitors (C1, C2, C4, C5, C7, C8, C9, and C10) inverting and non-inverting circuit, and pads are provided are returned to ground in the vicinity of the output to allow the use of board-mounted BNC or SMA connectors. connectors. The two circuits are independent, with the exception of shared power supply and ground connections. Optional Components High Speed Layout Techniques The circuit board is designed to accommodate standard 8- pin miniDIP, single operational amplifiers, such as the Layout is a primary contributor to the performance of any LT1190 and LT1220 families. Both voltage and current high speed amplifier. Poor layout techniques adversely feedback types can be used. Pins 1, 5, and 8 are outfitted affect the behavior of a finished circuit. Several important with pads for use in adjusting DC offsets, compensation layout techniques, all used in demo board DC009, are or, in the case of the LT1223 and LT1190/1/2, for shutting described below: down the amplifier. Top Side Ground Plane: The primary task of a ground If a current feedback amplifier such as the LT1223 is being plane is to lower the impedance of ground connections. evaluated, omit C3/C6. R4 and R8 are included for imped- The inductance between any two points on a uniform sheet ance matching when driving low impedance lines. If the of copper is less than the inductance of a narrow, straight amplifier is supposed to drive the line directly, or if the load trace of copper connecting the same two points. The impedance is high, R4 and R8 can be replaced by jumpers. ground plane approximates the characteristics of a copper Similarly R10 and R12 can be used to establish a load at sheet and lowers the impedance at key points in the circuit, the output of the amplifier. such as at the grounds of connectors and supply bypass capacitors. Low profile sockets may be used for the op amps to facilitate changing parts, but performance may be affected Ground Plane Voids: Certain components and circuit above 100MHz. nodes are very sensitive to stray capacitance. Two good examples are the summing node of the op amp and the Supply Bypass Capacitors feedback resistor. Voids are put in the ground plane in High speed operational amplifiers work best when their these areas to reduce stray ground capacitance. supply pins are bypassed with RF-quality capacitors. C1, Input/Output Matching: The width of the input and output C5, C8, and C10 should be 10nF disc ceramics with a self- traces is adjusted to a stripline impedance of 50W . Note resonant frequency greater than 10MHz. The polarized that the terminating resistors (R3 and R7) are connected capacitors (C2, C4, C7, and C9) should be 1m F to 10m F to the end of the input lines not at the connector. While tantalums. Most 10nF ceramics are self-resonant well stripline techniques arent absolutely necessary for the above 10MHz, and 4.7m F solid tantalums (axial leaded) demo board, they are important on larger layouts where are self-resonant at 1MHz or below. Lead lengths are line lengths are longer. The short lines on the demo board critical: the self-resonant frequency of a 4.7m F tantalum can be terminated in 50W , 75W , or 93W without adversely drops by a factor of 2 when measured through 2 inch affecting performance. leads. Although a capacitor may become inductive at high frequencies, it is still an effective bypass component Separation of Input and Output Grounds: Even though the above resonance because the impedance is low. ground plane exhibits a low impedance, input and outputDemo DC009 High Frequency Amplifier C3 +V + R2 C1 C2 1 V 2 7 J2 R4 6 U1 NON-INVERTING J1 OUTPUT 3 8 NON-INVERTING + 4 INPUT 5 R10 R3 R9 R1 C11 C5 C4 + C6 GND + C8 C7 R5 1 J3 R6 2 7 INVERTING J4 R8 INPUT 6 INVERTING U2 OUTPUT 3 + 8 4 R7 R12 5 R11 C12 C10 C9 + LT DB009 TA01 Demo Board DC009 Parts List C10 Negative Supply High Frequency Bypass (10nF) C12 Compensation Capacitor Non-inverting Amplifier: J3 Input Connector (AMP 227699-3) R1 Gain Setting Resistor J4 Output Connector (AMP 227699-3) R2 Feedback Resistor R3 Input Line Termination (51W ) High Frequency Amplifier, Demo 009A Component Side R4 Output Line Termination (51W ) R9 Shutdown Pin Pull Down R10 Output Load Resistor C1 Positive Supply High Frequency Bypass (10nF) C2 Positive Supply Low Frequency Bypass (4.7m F) C3 Feedback Capacitor C4 Negative Supply Low Frequency Bypass (4.7m F) C5 Negative Supply High Frequency Bypass (10nF) C11 Compensation Capacitor J1 Input Connector (AMP 227699-3) J2 Output Connector (AMP 227699-3) Inverting Amplifier: R5 Feedback Resistor R6 Gain Setting Resistor R7 Input Line Termination (51W ) R8 Output Line Termination (51W ) R11 Shutdown Pin Pull Down R12 Output Load Resistor C6 Feedback Capacitor C7 Positive Supply Low Frequency Bypass (4.7m F) C8 Positive Supply High Frequency Bypass (10nF) For literature on our High Speed Amplifiers, C9 Negative Supply Low Frequency Bypass (4.7m F) call (800) 637-5545. For applications help, call (408) 432-1900, Ext. 456. BA/GP 0691 10K REV 0 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7487 l l (408) 432-1900 FAX: (408) 434-0507 TELEX: 499-3977 LINEAR TECHNOLOGY CORPORATION 1991