DEMO CIRCUIT 1251 QUICK START GUIDE LTC6601-1, LTC6601-2 nd 5MHz to 27MHz, Pin Configurable, Fully Differential, 2 Order Lowpass Filter DESCRIPTION Figure 1. An LTC6601-X Block Diagram. Demonstration Circuit DC1251A features the LTC6601-X nd series family of pin configurable, fully differential, 2 or- der lowpass filter and amplifier. There are two DC1251A boards: a DC1251A-A with an LTC6601-1 and a DC1251A-B with an LTC6601-2. Typically an LTC6601-1 is the choice for most lowpass filter applications in a 5MHz to 27MHz cutoff frequency range and the LTC6601-2 is optimized for lower distor- tion and lower power applications. The LTC6601-X is designed to make the implementation of high frequency fully-differential filtering functions very easy. A very low noise amplifier is surrounded by 8 preci- sion matched resistors and 12 precision matched capaci- nd tors so that many 2 order lowpass filter transfer func- tions can be configured by hard wiring pins. Figure 1 shows the LTC6601-X block diagram with all the internal resistors and capacitors. On a DC1251A the LTC6601-X resistor and capacitor pins can be connected through 0603 resistor jumpers to configure the cutoff frequency and the passband gain. The LTC6601-1 or LTC6601-2 data sheet contains tables and circuits for configuring the pins for a variety of cutoff frequencies and passband gains (The cutoff or -3dB frequency is a function of the fo frequency and Q value, refer to the Fig- ure 3 filter equations in the LTC6601-1 data sheet). Figure 2 shows an example of an LTC6601-1 configured Figure 2. The LTC6601-X Circuit as configured in a nd as a 2 order lowpass filter with a 10MHz -3dB frequency DC1251A assembly. and a passband gain equal to 1 (0dB). The DC1251A-A/B demonstration circuit default configuration is the circuit of Figure 2. The DC1251A has SMA connectors for the differential input and output, a jumper with shunts to set the LTC6601-X to high power (HP), low power (LP) or shut- down (SHDN) operation and turrets to connect an exter- nal VOCM. A DC1251A board operates with a single 2.7V to 5.5V power supply. Design files for these circuit boards are available. Call the LTC factory. , LTC and LT are registered trademarks of Linear Technology Corporation. 1 DEMO CIRCUIT 1251 QUICK START GUIDE ND 5MHZ TO 27MHZ, PIN CONFIGURABLE, FULLY DIFFERENTIAL, 2 ORDER LOWPASS FILTER QUICK TEST SET-UP Figure 3. A Quick Test Set-Up. Quick Test Procedure 5. Channel 1 and channel 2 on the oscilloscope should each show a 1MHz, 0.5Vp-p sine- Test Equipment: wave. A Single Power Supply (3V 5V). A Sinewave Generator Note 1: For accurate gain testing the DC1251 in- An Oscilloscope (100MHz or greater). puts must be ideally driven by a zero ohm source (for example, the outputs of a high speed differ- 1. Connect a DC1251 board as shown on Figure ential amplifier). However, the 50 ohm imped- 1 (set the oscilloscope input impedance to ance of the generators output and terminator is 1M). in series with the LTC6601-X input resistors. A 2. Set the power supply to 3V. 50 ohm resistor in series with the input lowers 3. Set the signal source output for a 1MHz, the expected passband gain and changes the fil- 1Vp-p sinewave. ters Q value (refer to the Figure 3 GAIN and Q 4. Turn on the signal source and power supply. equations in the LTC6601-1 data sheet). 2