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EDN Access -- 12.8.94 CMOS buffer delivers precise current pulse

-December 08, 1994

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Design Ideas:December 8, 1994

CMOS buffer delivers precise current pulses


Hartmut Henkel,
Consultant, Kriftel, Germany

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, VP, connect to IC3B, a 74AC240 buffer. The 5.00V reference, IC1, constrains IC3B's output, VR, to either VREF or 0V. IC3B's rise and fall times are &3 nsec. Resistor R1 converts the voltage at the output of IC3B to a current: IR=VR/R1. IR flows into integrator IC4's virtual ground.


An analog control loop comprising IC2 and IC3A stabilizes VREF. The loop compensates for the 20 Ohm output resistance of buffer IC3B. 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)


Reference

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


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