DEMO CIRCUIT 1422 QUICK START GUIDE LTC6605-7, LTC6605-10 and LTC6605-14 7MHz, 10MHz and14MHz, Dual, Matched, Filter/Amplifier DESCRIPTION Demonstration circuit 1422 features the LT6605-XX se- nd ries of a dual, matched, fully differential 2 order low- Table 1. The DC1422 Assemblies pass filter and amplifier. A DC1422 is available in three ASSEMBLY Filter IC assemblies (refer to Table 1). The typical -3dB fre- DC1422A-A LTC6605-7 quency (f3dB) of an LT6605-7, an LTC6605-10 and an LTC6605-14 is 7MHz, 10MHz and 14MHz respectively. DC1422A-B LTC6605-10 However each LTC6605-XX the f3dB frequency can be DC1422A-C LTC6605-14 set with one external resistor per channel in a small range listed on Table 2 (refer to Figure 3 and Table 2 of an LTC6605-XX data sheet). Table 2. Resistor Set f3dB Range The LTC6605-XX Internal laser-trimmed resistors and Filter IC f3dB Range capacitors implement a very well matched (in gain LTC6605-7 6.5MHz 10MHz and phase) 2nd order lowpass filter response. The LT6605-XX combines a two fully differential LTC6605-10 9.7MHz 14MHz nd matched amplifiers and 2 order lowpass filters for LTC6605-14 12.4MHz 20MHz driving the differential inputs of a dual Analog to Digital Converter (ADC) and band-limiting the base- band signal of a digital communications receiver. Figure 1. The DC1422 Top Silk Screen A DC1422 is configured as an AC coupled unity gain fil- ter/amplifier using the lowest f3dB frequency in the range of Table 2 (6.5MHz, 9.7MHz and 12.4MHz for a -7, a -10 and a -14 respectively). Surface mount pads for 0603 passive components provide options to configure a nd DC1422 for gains greater than one and as 2 order low- pass filter circuits as shown in an LTC6605-XX data sheet. Connection to a differential input and output of a DC1422 is through SMA connectors. An on-board jumper with shunts configures the LTC6605-XX for full power, low power or shut-down operation. Design files for this circuit board are available. Call the LTC factory. , LTC and LT are registered trademarks of Linear Technology Corporation. 1 DEMO CIRCUIT 1422 QUICK START GUIDE, LTC6605-XX 7MHz, 10MHz and 14MHz, Dual, Matched, Filter/Amplifier QUICK START SET UP Single Power Supply (3V - 5V) Oscilloscope (100MHz) Signal Generator with 50 Ohm Output (Set to 1MHz, 2Vp-p Sinewave) 50 Ohm Terminator Figure 2. Single Supply Test Set Up Connections (set the JP1 shunt to V+) Quick Test Procedure A DC1422 has two pairs of differential inputs, VINA+/VINA- 2. Testing the VINB to VOUTB Signal Path. and VINB+/VINB- and two pairs of differential outputs Connect signal generator to VINB+ and terminate VINB- with a VOUTA+ /VOUTA- and VOUTB+/VOUTB-. 50 ohm terminator. Set the generator for a 1MHz, 2Vp-p, and sinewave. Connect oscilloscope channel 1 to VOUTB- and 1. Testing the VINA to VOUTA Signal Path. channel 2 to VOUTB+. The oscilloscope channels connected Connect a DC1422 board as shown in Figure 2 (connect sig- to VOUTB- and VOUTB+ show a 1MHz, 1Vp-p, sinewave. nal generator to VINA+ and terminate VINA- with a 50 ohm terminator). Set the generator for a 1MHz, 2Vp-p, and sine- Note: For unity gain testing the DC1422 inputs must be ide- wave. The oscilloscope channels connected to VOUTA- and ally driven by a zero ohm source (the outputs of a high speed VOUTA+ show a 1MHz, 1Vp-p, sinewave (see Note 1). differential amplifier). However, if the 50 ohm impedance of the generators output and terminator is in series with the input resistors then the gain is less than one and the filters Q value is affected (refer to the filter equations of Figure 3 in the LTC6605-7 data sheet). 2