DEMO CIRCUIT 1287 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1287A QUICK START GUIDE LTC6416 2 GHZ LOW NOISE DIFFERENTIAL 16-BIT ADC BUFFER LTC6416 2 GHz Low Noise Differential 16-Bit ADC Buffer DESCRIPTION for additional system gain. The VCLLO and VCLHI Demonstration circuit 1287 is a 2 GHz Low Noise Differ- pins set lower and upper limits on the output voltages ential 16-Bit ADC Buffer featuring the LTC6416. It incor- seen at each differential output pin. This feature helps porates a variety of passive components to support protect the ADC inputs from undervoltage and over- configurations for varied applications. Both single- voltage waveforms. The output common-mode volt- ended and differential configurations are supported age of the LTC6416 is determined by the voltage ap- at the inputs and at the outputs. plied to the VCM pin. It is internally buffered then The LTC6416 is a high-speed differential buffer am- drives the LTC6416s differential inputs through 2k plifier with superior distortion and noise perform- series resistors. For AC-Coupled operation, no exter- ance. It is intended for driving high speed 14 and 16 nal bias network is needed. DC-coupled inputs applied bit ADCs. It is perfect in applications such as de- to the LTC6416 can easily override the 2k resistance manding communications transceivers, cellular base of the internal bias network. Design files for this cir- stations, and other high-speed signal chain applica- cuit board are available. Call the LTC factory. tions. L, LTC, LTM, LT, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are registered Notable features of the LTC6416 include high input trademarks of Linear Technology Corporation. Adaptive Power, C-Load, DirectSense, Easy Drive, FilterCAD, Hot Swap, LinearView, Module, Micropower SwitcherCAD, Multimode impedance, adjustable output voltage limiting and Dimming, No Latency , No Latency Delta-Sigma, No R , Operational Filter, PanelProtect, SENSE PowerPath, PowerSOT, SmartStart, SoftSpan, Stage Shedding, SwitcherCAD, ThinSOT, simple output common-mode adjustment. The UltraFast and VLDO are trademarks of Linear Technology Corporation. Other product names LTC6416s high input impedance allows it to be used may be trademarks of the companies that manufacture the products. in 50 systems with 1:4 and 1:8 input transformers QUICK START PROCEDURE Table 1 shows the function of each SMA connector on Figure 1 shows the Top Silkscreen for the DC1287 the board. Demo Board. Table 1: DC1287 SMA Connector Descriptions Figure 1: DC1287 Top Silkscreen Single-Ended/Differential Input. By default, this is J1 (IN+) configured as a single-ended input. Use this con- nector to supply an input to the DC1287. Drive from a 50ohm signal source, no external termina- tion necessary. J2 (IN-) Differential Input. Not connected by default. Ca- pacitor C10 can be installed and C12 removed to drive the DC1287 differentially. J3 (OUT+) Single-Ended/Differential Output. By default, this is configured as a single-ended output. This output is designed to drive 50 ohms, such as a network or spectrum analyzer input. J4 (OUT-) Differential Output. Not connected by default. Ca- pacitor C9 can be installed and C11 removed for differential output drive. 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1287A LTC6416 2 GHZ LOW NOISE DIFFERENTIAL 16-BIT ADC BUFFER 3. Observe the output via J3. Although the LTC6416 Refer to Table 1 and Figure 1, and follow the proce- is intended to drive the high-impedance inputs of dure below: an ADC, the DC1287As output has been designed 1. Connect ground to both GND turrets and a 3.6V to drive 50, suitable for the input of a network or power supply to the V+ turret. VCM, CLLO and spectrum analyzer. A resistive network comprised CLHI turrets may be left floating. of R1, R3, 18 of internal resistance and the 50 2. Apply an input signal to J1. It should be >10MHz load terminates the LTC6416 in a 400 differen- because of the lower frequency limit of the 1:1 in- tial load. T2, a 1:1 balun, transforms the output put and output transformers. A good starting signal for single-ended observation. Due to volt- point for amplitude is about 0dBm (0.63V ) at the PP age division within the output network, an at- DC1287As input. A low-distortion, low noise sig- tenuation of approximately -19dB is observed at nal source with an external high-order low-pass or the output. band-pass filter will yield the best performance. The DC1287s input is impedance-matched to 50 no external termination is necessary. ADDITIONAL INFORMATION Although the DC1287A is ready to use out of the box, can be used to level-shift the DC output voltage for it has features that you can access by adding, remov- optimum system performance. Normally, capacitors ing or changing components on the board. These are C8 and C9 prevent the output common mode voltage described below. from reaching the output. However, if the common mode voltage is desired, C8 and C9 can be shorted DIFFERENTIAL INPUTS or replaced by 0 resistors to allow the DC bias to To drive the DC1287A differentially, remove capacitor reach the output. Transformer T2 is a 1:1 transmis- C12 and install capacitor C10. J1 and J2 can now be sion line type, which means DC voltages will couple driven differentially from a 50 signal source. The from input to output of the transformer. By default, input to the DC1287A will still be matched to 50 the LTC6416 will self-bias the VCM pin to 1.36V (differential) when driven in this manner. (with V+=3.6V). For a different common-mode volt- age, simply apply a voltage source to Turret E1. DIFFERENTIAL OUTPUTS ADJUSTING THE CLLO AND CLHI VOLTAGES To use the both outputs of the DC1287A, remove capacitor C11 and install capacitor C9. The output The LTC6416 features high speed voltage limiting at each now appears differentially across J3 and J4. The of the output pins. This feature is important to help pro- output of the DC1287A will still be matched to 50 tect the inputs of the ADC. The output voltage at either of (differentially) when used in this manner. the differential outputs will not exceed the voltage set at the LTC6416s CLHI pin. Similarly, it will not go below the voltage set at the LTC6416s CLLO pin. See the CHANGING THE OUTPUT COMMON-MODE LTC6416 datasheet for more information. VOLTAGE Turret E1 (labeled VCM) controls the output com- Turret E4 (labeled CLHI) drives the CLHI pin of the mon-mode voltage of the DC1287A. This function LTC6416 directly. If left floating, the CLHI pin will self- 2