The circuit in Fig 1 delivers stable, fast-settling reference-current pulses. You can use the circuit in systems employing A/D, D/A, or V/F converters.

Digital input pulses, V_P, connect to IC_3B, a 74AC240 buffer. The 5.00V reference, IC₁, constrains IC_3B's output, V_R, to either V_REF or 0V. IC_3B's rise and fall times are <3 nsec. Resistor R₁ converts the voltage at the output of IC_3B to a current: I_R=V_R/R₁. I_R flows into integrator IC₄'s virtual ground.

An analog control loop comprising IC₂ and IC_3A stabilizes V_REF. The loop compensates for the 20 Ohm output resistance of buffer IC_3B. And, the loop reduces high-frequency noise from reference IC₁ (Ref 1). Diode D₁ prevents reverse polarity during power-up.

You should use tantalum capacitors for C₁ and C₂. Locate C₂ as close as possible to IC₃'s supply pins. If you use a low-temperature-coefficient Vishay (Malvern, PA, (610) 644-1300) resistor for R₁ and select 100 k Ohm as the values for R₁, R₂, and R₃, IC₁ is the source of most of your current drift. (DI #1635)

1. Application note AB-003, Burr-Brown Corp, Tucson, AZ. (602) 746-1111.