The circuit in Fig 1 overdrives its output to 32 times the input signal. Typical recovery times measure 2.8 nsec for a 0 to 2.2V step at the output. The delay measures 2.5 nsec.
Design Ideas: June 23, 1994
Using current-feedback amplifiers to convert signals from sine waves to square waves for DSP confers advantages over the more common comparator approaches.
Current-feedback amplifiers have wide bandwidths and relatively small and constant propagation delays. These small, constant delays help meet the setup-and-hold requirements of digital logic. Typically, current-feedback amplifiers have delays from 1.1 to 5 nsec.
The circuit in Fig 1 overdrives its output to 32 times the input signal. Typical recovery times measure 2.8 nsec for a 0 to 2.2V step at the output. The delay measures 2.5 nsec.
Because current-feedback amplifiers' input stages operate in the linear mode, you can minimize the amplifiers' noise terms when sensing low input-signal levels. For the values in Fig 1, resistors R1, R2, R3, and R4 clamp the output's voltage swing from zero to a positive voltage.
The accuracy of the clamps depends upon the load. For loads greater than 1 k(ohm), the accuracy depends on the tolerance of the resistors and the power supplies. The clamps provide a minimal overshoot and preshoot of 0%, with rise and fall times of less than 4.7 nsec. The recovery time varies sightly, depending upon how far the input drives IC1 into saturation. For capacitive loads, you can adjust R5 to improve the output square wave.
RG selects the circuit's gain, and RF is the recommended feedback resistor. A resistor, RX, tied to the appropriate supply, forces any desired initial condition at the output VO. (DI #1450)