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# Diode Current Switch Multiplexer

IP.com Disclosure Number: IPCOM000081286D
Original Publication Date: 1974-May-01
Included in the Prior Art Database: 2005-Feb-27
Document File: 3 page(s) / 40K

IBM

## Related People

Schulz, RA: AUTHOR

## Abstract

A multiplexer converts analog voltage into analog currents, then switches these currents with a very fast and simple diode circuit.

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Diode Current Switch Multiplexer

A multiplexer converts analog voltage into analog currents, then switches these currents with a very fast and simple diode circuit.

Input signals eIN 1, eIN 2, etc. lie within range -E to +E, where absolute value of E is a fixed full-scale reference voltage. Assume all switching diodes except D1 are held nonconducting by their respective driver circuits (that is, D2, D3, ... Dn are all off); and driver circuit DR1 associated with the first channel is cut off so no current flows in its output 1. Then current in D1 is equal to: E-eINl- 2VD1/R, where VD1 is the voltage drop across D1 at iD1. Also assume operational amplifier 10 is ideal and therefore has no input current. Under these conditions, current iDf through feedback diode Df is identical to iD1 and equal to: E+eOUT-2VDf/R, where VDf = forward voltage drop of Df at iDf. Simultaneously solving from in1 and iDf yields: eOUT = eIN1 + 2(VDf-VD1).

Accordingly, D1 and Df are forward biased by identical currents, and are constructed exactly alike to have identical voltage drops, i.e. VDf = VD1. All diodes Df, D1 to Dn in the multiplexer are thus fabricated on the same monolithic integrated circuit (IC) chip to insure identicalness, resulting in accuracies commensurate with analog signal processing.

Resistors 11, 12, are connected to references E and -E, and provide bipolar input signal capability. The arrangement is such that if -E< or - eIN< or - E, the diodes will always be forward biased. In addition, if eIN < or - o, resistors 11, 12 may be obviated.

Identical driver circuits DR1 to DRn translate TTL (or DTL) logic levels into appropriate signals that either pass or shut off the currents through D1 to Dn. Using driver circui...