DEMO CIRCUIT DC1108 QUICK START GUIDE DEMO CIRCUIT 1108 650MHZ DIFF. ADC DRIVER / DUAL SELECTABLE-GAIN AMPLIFIER QUICK START GUIDE LT6411 650MHz Differential ADC Driver / Dual Selectable Gain Video Amplifier DESCRIPTION Demonstration circuit 1108 features the LT6411 Dual Amplifier and ADC Driver. It incorporates a variety of passive components to support configurations for varied applications, including large AC-coupling capacitors for video applications. Single-ended or differential input and output configurations are possible with on-board component changes. The LT6411 supports both a single +5V power supply as well as split ±5V supplies. The LT6411 is a high-speed differential amplifier with superior distortion and noise performance, perfect for demanding video distribution, communications, and other high- speed signal chain applications. The LT6411 accepts singleended or differential inputs with almost no difference in distortion performance. The topology of the LT6411 allows the flexibility of selecting various gains without the use of extra resistors. The LT6411 consumes only 16mA of supply current (80mW) when enabled, and includes a low-power shutdown mode for additional power savings. Design files for this circuit board are available. Call the LTC factory. , LTC and LT are registered trademarks of Linear Technology Corporation. QUICK START PROCEDURE Table 1 shows the function of each SMA connector on the board. Refer to Figure 1 for the connection diagram and follow the procedure below: 1. Connect the power supply as shown. The power labels of VCC and GND are used to apply a single +5V to +10V supply; by default, VEE is connected to GND through resistor R21. Jumper JP1 enables or shuts down the LT6411. 2. Apply an input signal to J1. A low-distortion, low noise signal source with an external high-order filter will yield the best performance. DC1108’s input is impedance-matched to 75Ω; no external termination is necessary. To use a 50Ω source instead of a 75Ω source, replace impedance-matching resistor R6 with a 57.6Ω 0603 resistor. 3. Observe the output via J2 and/or J3. The default gain is +2, relative to the single-ended input. The outputs have 75Ω series resistors. Table 1: DC1108 SMA Connector Descriptions CONNECTOR FUNCTION J1 (IN1) Board Input. Use this connector to supply an input to the DC1108. Drive from a 75Ω signal source, no external termination necessary. Resistor R6 provides a 75Ω impedance match. Input is AC coupled by default. J4 (IN2) Board Input. Connected to GND through resistor R20 by default. Resistor can be removed to drive the DC1108 differentially. Input is AC coupled by default. J2 (OUT1) J3 (OUT2) Output. By default, connected by a 75Ω series resistor for video cable driving. Output. By default, connected by a 75Ω series resistor for video cable driving. 1 DEMO CIRCUIT DC1108 QUICK START GUIDE 650MHZ DIFF. ADC DRIVER / DUAL SELECTABLE-GAIN AMPLIFIER Power Supply (+5V) Optional Supply (-5V) (Remove R21) Input (Z = 75Ω) Differential Output with 75Ω series resistors for cable driving Figure 1. DC1108 Default Board Evaluation Setup – Single Ended Input, AC-Coupled. Gain = +2 Figure 2. Block Diagram of LT6411 Internal Amplifiers and Resistors 2 DEMO CIRCUIT DC1108 QUICK START GUIDE 650MHZ DIFF. ADC DRIVER / DUAL SELECTABLE-GAIN AMPLIFIER Figure 3. Input and Output Networks of the DC1108 Demo Board ADDITIONAL INFORMATION Although the DC1108 demo board is ready to use out of the box, it has features that you can access by adding, removing or changing components on the board. These are described below. ENABLE (SHUTDOWN) The enable (shutdown) function of the LT6411 is controlled by JP1. To enable the part, move the on-board JP1 jumper to the lower (EN) position. To place the driver in a lowpower shutdown mode, move the jumper into the upper (DIS) position. NOTE. The input bias resistors R2/R3 and R17/R18 enable current flow from VCC to GND, and must be accounted for when measuring the current of the LT6411, especially in shutdown. GROUND AND VEE CONNECTIONS DC1108’s GND connector is connected to the board’s ground planes. By default, VEE is also connected to the ground planes via R21. However, the LT6411 is capable of running on up to 10V power supply voltage (or ±5V). Therefore, the LT6411 VEE pins and exposed paddle are floating, and connected to the ground plane through bypass capacitors. By removing R21 and powering the LT6411 through the VEE turret (E3), the user can provide split supplies to the LT6411. INPUT AND OUTPUT VOLTAGE RANGES The input range of the LT6411 (Pins 13 and 16) extends from 1V above VEE to 1V below VCC. The inverting inputs, Pins 14 and 15, do not have this limitation, and can be driven from rail to rail. The outputs, Pins 8 and 5, can swing to within 1.3V of either supply (less under heavy load). These ranges can be limiting factors, especially when using DC1108 without the AC-coupling capacitors. 3 DEMO CIRCUIT DC1108 QUICK START GUIDE 650MHZ DIFF. ADC DRIVER / DUAL SELECTABLE-GAIN AMPLIFIER By default, R17 and R18 set the LT6411 input/output common-mode voltage to 2.1V, which is the LT6411’s lowest-distortion bias point (see datasheet, page 10). SPARE COMPONENTS DC1108 comes with spare components for use when evaluating different circuit configurations. C6, R23, and R24 are an extra 220µF capacitor and 0Ω resistors, re- spectively. See the next section below (Flexible Configurations) for some examples of possible configurations for the LT6411 on DC1108. FLEXIBLE CONFIGURATIONS The LT6411 is a dual amplifier with internal gain and feedback-setting resistors. This simple, intuitive topology allows the user to test many different circuit configurations on DC1108 simply by changing the resistors on the board. Refer to Figure 3 for the networks at the inputs and outputs of the LT6411. Table 2 describes only some of the myriad configurations possible on DC1108. Contact the LTC factory for assistance with any of these configurations, or if your configuration needs differ from what is shown. For DC-coupled configurations, C2, C3, and C4 can be replaced by 1206-size surface mount resistors. SETTING DGND The DGND pin (Pin 12) is the reference voltage for the Enable Low pin (Pin 11). On a single +5V supply, when the EN pin is pulled to within 0.4V of the DGND pin, the part is enabled. However, there are two restrictions on the voltage compliance of the DGND pin, which is discussed in the datasheet: DGND should be tied to VEE. On the board, removing R22 and populating R26 will accomplish this adjustment. Table 2: Example DC1108 Configurations FUNCTION CONFIGURATION VIDEO AMPLIFIER, LINE OR ADC DRIVER CONFIGURATIONS No changes necessary Single Ended Input (default) Gain = +2 AC-Coupled Differential Outputs Remove R6. Single Ended Input Install 37.5Ω at R5. Gain = +1 Install 40.2Ω at R3. AC-Coupled (resistors attenuate and Z-match to 75Ω) Differential Outputs Remove R6, R20 (or use for Z-Match). Differential Input Remove R12 and C3. Gain = +1 Install R2, R3 to match R17, R18. AC-Coupled Install 0Ω at R7 and R15. Differential Outputs Remove R6, R20 (or use for Z-Match). Differential Input Remove R7. Gain = +2 Install R2, R3 to match R17, R18. AC-Coupled Install 220µF at C3. Differential Outputs Remove R6, R20 (or use for Z-Match). Differential Input Remove C3, R7 and R12. Gain = +3 Install R10 and R25 (0Ω). AC-Coupled Differential Outputs VIDEO SIGNAL SPLITTER Remove R20 and/or C4. Single Ended Input Install R11 (0Ω). Gain = +1 Remove R12 and C3. AC-Coupled Install 0Ω at R7 and R15. Redundant Outputs Remove R20 and/or C4. Single Ended Input Install R11 (0Ω). Gain = +2 Remove R7. AC-Coupled Install 220µF at C3. Redundant Outputs CASCADED AMPLIFIERS, GAIN = +4 Remove R16, R7, R12. Single Ended Input Install R10 (0Ω). Gain = +4 AC-Coupled Move R17, R18 resistors to R2, R3. Single Output at J3 Install R8, R14 (0Ω). Change R9 to 20Ω (for gain flatness). Install 220µF at C3. These two voltage compliance restrictions are easy to meet on a single +5V or split ±5V supply by tying DGND to the ground plane, which by default is done on the demo board. However, if using split supplies and VCC<3V, then the first requirement will not be met, and 4 DEMO CIRCUIT DC1108 QUICK START GUIDE 650MHZ DIFF. ADC DRIVER / DUAL SELECTABLE-GAIN AMPLIFIER 5