EDN Access — 09.02.96 Limiting amplifier makes efficient A/D-converter drive

-September 02, 1996

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Design Ideas:September 2, 1996

Limiting amplifier makes efficient A/D-converter driver

Mark Amarandos,
Harris Semiconductor, Costa Mesa, CA

Limiting amplifiers are useful for protecting circuitry that has a limited input range. A classic example is an input buffer for an A/D converter. Many A/D converters are susceptible to damage if the input significantly exceeds the specified range. In addition to providing the necessary input-voltage clamping, a limiting amplifier can provide the peak currents required to charge the A/D converter's input capacitance and remain stable with step changes in input voltage. The limiting amplifier also provides the gain and level-shifting frequently required at the front end of an A/D converter.

In Figure 1a, the HFA1135 limiting amplifier drives the HI1175 8-bit, 20-M samples/sec A/D converter. The HFA-1135 is configured as a level-shifting amplifier with a 0.5V offset and a gain of two. This configuration allows a ground-referenced, 1V-maximum signal to span the full 2V input range of the HI1175. The HI1175 is typical of many single-supply A/D converters whose input range does not include ground. The VRT and VRB voltages of 2.5 and 0.5V, respectively, set the limits of the HI1175's input range. The 0V clamp voltage keeps the lower limit of the A/D converter's input voltage within its absolute maximum range.

The 0.5V difference between the nominal minimum input voltage and the clamp voltage ensures that the clamp circuitry does not affect the linearity of the circuit. A voltage divider sets the VH voltage to approximately 3V. In Figure 1b , the HFA1130 is used in a gain-of-two circuit to drive the input of the HI1166 8-bit, 250 Msamples/sec A/D converter. Typical of A/D converters that operate above 100 Msamples/sec, the HI1166 is an ECL device that operates from a -5.2V supply. The HFA1130 allows a 0 to 1V input to span the 0 to -2V range of the A/D converter. Resistor dividers set the VH and VL limit levels to 0.5 and -2.5V, respectively. (DI #1913)

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