DC1257B Analog Devices

DEMO MANUAL DC1257B LTC6416 2 GHz Low Noise Differential 16-Bit ADC Buffer DESCRIPTION Demonstration circuit 1257B features the LTC 6416, a developed from the DC996B-B, used to characterize the 2GHz low noise differential 16-Bit ADC buffer driving the LTC2208 family of ADCs. LTC2208, a 16-bit 130Msps ADC. The DC1257B is sup- Use the DC1257B with a DC890 FastDAACS and PScope plied with a bandpass filter centered at 140MHz between software to collect time and frequency data. the buffer and the ADC. The filter center frequency can be Design files for this circuit board are available at changed to optimize performance at different analog input DEMO MANUAL DC1257B HARDWARE SETUP Apply Encode Clock to the DC1257B on the SMA connector Collect Data by clicking on the Collect button. Time and marked (J4) CL. This transformer coupled input is frequency plots will be displayed in the PScope window. terminated with a 100 at the secondary and a 100 at Consult the DC890 Quick Start Guide for additional the ADC clock inputs. For best noise performance the clock information. input must be driven with a very low jitter source. The Buffer ADC Interface amplitude of the sinusoidal generator should be as large as possible, up to 13dBm. Bandpass filters on the clock The LTC6416 has been specifically designed to interface and the analog input will improve the noise performance directly with high speed A/D converters. It is possible by reducing the wideband noise power of the signals. Data to drive the ADC directly from the LTC6416. In practice, sheet FFT plots were taken with 10 pole LC filters made however, better SFDR may be obtained by adding a few by TTE (Los Angeles, CA) to suppress signal generator external components at the output of the LTC6416. harmonics, non-harmonically related spurs and broad band Figure 2 shows the LTC6416 being driven by a 1:4 trans- noise. Low phase noise (jitter) Agilent 8644B generators former which provides 6 dB of voltage gain while also are used with TTE band pass filters for the CLK input and performing single-ended to differential conversion. The Analog input. differential outputs of the LTC6416 are lowpass filtered Apply the Analog Input to the DC1257B on the SMA to drive the differential inputs of the LTC2208 ADC. In + connector marked(J2) A. This input is capacitively many applications, an anti-alias filter like this is desir- IN coupled to a 1:4 Balun transformer TCM4-19+. able to limit the wideband noise of the amplifier. This is especially true in high performance 16-bit designs. The Start and Configure the PScope data collection software minimum recommended network between the LTC6416 for the FastDAACS DC890 by selecting AutoConfigure. and the ADC is simply two 5 series resistors, which are If the board is not detected, up-date PScope for latest used to help eliminate resonances associated with the software and device list, and then select LTC2208 from the stray capacitances of PCB traces and the stray inductance ConfigureDevice menu. You can also manually configure of the internal bond wires at the ADC input, and the driver PScope for the LTC2208 by setting the parameters listed output pins. Table 3 suggests filter components for dif- in Table 2. ferent input frequencies. Table 2: PScope User Configuration for LTC2208 Table 3. Suggested Components for the Filter USER CONFIGURE INPUT FREQUENCY LTC6416 OUTPUT FILTERING CAPACITORS Bits 16 RESISTORS C20/C22/C24 Channels 1 R13 = R15 Alignment 16 30MHz 50 5.6pF/6.8pF/5.6pF FPGA Ld LVDS 70MHz 25 5.6pF/6.8pF/5.6pF Bipolar x 140MHz 25 1.5pF/1pF/1.5pF Positive Egde Clk x 250MHz 5 -/-/- dc1257bf 2