DEMO MANUAL DC961B LT1994 Low Noise, Low Distortion, Fully Differential Amplifier/Driver Description Demonstration circuit 961 features an LT 1994, low noise, Connection to the differential input and output of a DC961 low distortion, fully differential amplifier. The LT1994 is a is through SMA connectors. Onboard jumpers configure high precision, very low noise, low distortion, fully differ- the DC961 for dual or single power supply operation. The ential input/output amplifier (see Table 1). The LT1994s differential input of a DC961 is AC coupled with 1F capac- output common mode voltage is independent of the input itors for ease of use as a dual or a single supply circuit. common mode voltage, and is adjustable by applying a DC coupling to the DC961 input is provided through the voltage on the V pin. The DC961 board contains an shorting of the input capacitors with 0 surfacemount OCM LT1994 amplifier configured as a unity gain differential resistor jumpers. amplifier with 499 feedback and input resistors. Gains Table 1. LT1994 Noise and Distortion greater than one require changing the input resistors to Differential Input Referred Voltage Noise a value lower than 499 (refer to Figure 2). In addition, Density 3nV/ Hz RT DC961 has surface mount pads and traces for resistors Distortion, and capacitors for building first and second order fully 2V Differential Input, P-P V = 3V, f = 1MHz, R = 800 S IN LOAD differential filter circuits. The differential outputs of DC961 2nd Harmonic 99dBc can be configured with a first order RC network for driv- 3rd Harmonic 96dBc ing the differential inputs of an analog-to-digital converter Design files for this circuit board are available at (ADC). DEMO MANUAL DC961B Quick p D A. Single-Ended Input to Differential Output Note 1: The LT1994 can directly drive at least a 25pF capacitive load at each output. However the LT1994 can 1. Connect to a DC961 a dual power supply, a function drive directly a low frequency sinewave (100kHz or less) generator and an oscilloscope as shown in Figure 3 into a capacitive load of up to 100pF. In this Quick Test (the 50 termination on J2 input is used to balance Procedure, the output signal is a sinewave and each the 50 generator impedance on J1 input). LT1994 output drives the capacitance of a 24 inch or 2. Set the function generator for a 1V , 100kHz sin- less cable plus the input capacitance of the oscilloscope P-P ewave and turn on the power supply. input, a capacitive load of 70pF (30pF per foot for the coax cable and 10pF for the oscilloscope input). For testing the 3. The channel 2 input of the oscilloscope is in phase with transient response of the LT1994 to a squarewave or a the DC961 input and the channel 1 input is 180 degrees pulse, use a 10x low capacitance oscilloscope probe to out of phase with the DC961 input. The single-ended monitor the DC961 output at J3 or J4. output shown on channel 1 or 2 is a 0.5V sinewave P-P (a 1V differential output). P-P COAX CABLE DUAL POWER SUPPLY 5V 5V FUNCTION GENERATOR DC961 OSCILLOSCOPE BNC T 50 V + OUTPUT V TO JP2 + CH2 V IMPEDANCE IN C8, 1F R5 R2 Z1 R9 J1 CH1 0 0 499 499 BNC + V CH1 CH2 EXT TO 0.1F SMA V IN SMA TO 3 SET FUNCTION BNC 8 R12, 50 J3 GENERATOR 5 SHDN + TO A 1V , PWR ON 7 P-P V 100kHz OUT JP1 V LT1994 2 OCM SINEWAVE R15, 50 SET THE OSCILLOSCOPE J4 1 TO EXTERNAL TRIGGER 0.1F + 4 + V OUT SMA 6 0.1F TO 24 INCHES OR BNC LESS COAX CABLE V IN C12, 1F R6 R8 R4 J2 Z2 50 TERMINATION 0 0 499 499 + BNC V DC961 JUMPERS SETTINGS: TO JP1 LT1994, SHDN PIN TO GND OR OPEN SMA JP2 LT1994, V PIN TO V OR GND 10k + JP2 JP3 LT1994, V PIN TO V /2 OR OCM SINGLE AN EXTERNAL CONNECTION EXT V JP3 DUAL OCM 100 EXT 10k INT V DC961b Figure 3. Single-Ended Input to Differential Output Quick Test Setup dc961bf 2 ure roce test