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Analog Switch with Gating Spike Suppression

IP.com Disclosure Number: IPCOM000076408D
Original Publication Date: 1972-Feb-01
Included in the Prior Art Database: 2005-Feb-24
Document File: 2 page(s) / 27K

Publishing Venue

IBM

Related People

Rossero, H: AUTHOR

Abstract

A problem commonly encountered in digital-to-analog converters where field-effect transistors (FET) switches couple an analog voltage to a voltage-summing point, is the spikes generated during the "turn on" and "turn off" transitions of the FET gate driving pulse. Such spikes can be neglected if the analog voltage is greater than about 2 volts, but become objectionable for lesser values of analog voltage approaching zero volts.

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Analog Switch with Gating Spike Suppression

A problem commonly encountered in digital-to-analog converters where field-effect transistors (FET) switches couple an analog voltage to a voltage- summing point, is the spikes generated during the "turn on" and "turn off" transitions of the FET gate driving pulse. Such spikes can be neglected if the analog voltage is greater than about 2 volts, but become objectionable for lesser values of analog voltage approaching zero volts.

The circuit in the drawing is arranged to suppress the aforementioned gating spikes, by the provision of low conduction pathways to ground during the intervals when the spikes are generated. This technique avoids the need for spike-suppression filters with their attendant penalty in reduced circuit settling time. A first analog voltage V(in) is applied to terminal 1 and via series FET 2 (when conducting) and isolation resistor 3 to voltage summation point 4. Terminal 1 also is coupled to ground by small capacitor 5 (of the order of about
0.001 microfarad). Shunt FET 6 is connected between ground and the junction between FET 2 and resistor 3. A plurality of additional analog voltage switching circuits may be connected to summing point 4.

FET 7 and resistor 8 in the feedback loop of the operational amplifier control the ratio of output to input voltage magnitude. The effect of resistance of FET 7 is closely matched to that of FET 2. This provides good accuracy in the feedback loop, assures a prope...