AN61-1 Application Note 61 August 1994 Practical Circuitry for ...

+Application Note 61Figure 37’s similar circuit substitutes a standard zener forthe noise source but is more complex and requires a trim.A1, biased from the LT1004 reference, provides optimumdrive for D1, the noise source. AC coupled A2 takes abroadband gain of 100. A2’s output feeds a gain-controlstage via a simple selectable lowpass filter. The filter’soutput is applied to LT1228 A3, an operational transconductanceamplifier. A3’s output feeds LT1228 A4, a currentfeedbacks amplifier. A4’s output, the circuit’s output,is sampled by the A5-based gain control configuration.This closes a gain control loop back at A3. A3’s set inputcurrent controls its gain, allowing overall output levelcontrol.To adjust this circuit, place the filter in the 1kHz positionand trim the 5k potentiometer for maximum negative biasat A3, pin 5.100k23A11/2 LT1013–+1M15V84–15V15k6.2k1µF50k1µFD11N7531k32–+A2LT12261k1k1.6k0.1µF0.01µF0.001µF100pF1kHz10kHz100kHz1MHz10Ω10pF5MHz15V32+A31/2 LT1228–57SET3k0.1µF900Ω18+A41/2 LT1228–4–15V10k6510Ω10Ω1µFNONPOLAR+22µF10k0.05µF22µF+1V P-P OUTPUT15V7–A51/2 LT1013+651M1N4148sCOUPLE THERMALLY4.7kLT10041.2V–15V–15VAN61 F37Figure 37. A Similar Circuit Uses a Standard Zener Diode, But Is More Complex and Requires TrimmingAN61-26