US 4,285,051 AGranted

Low glitch current switch

AssigneePrecision Monolithics, Inc.

InventorPaul R. Henneuse

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Abstract

An improved analog track and hold circuit has a glitch-free output as the circuit switches between the tracking and the holding of an input analog signal. The circuit is of the type having a capacitor for storing an analog voltage, a transconductance amplifier for producing a charging current for the capacitor proportional to the analog voltage, a current switch for connecting and disconnecting the charging current for the capacitor, and an output circuit to buffer the capacitor voltage to the output. The improvement includes a diode array establishing first and second reference nodes across the capacitor. The diodes in the array clamp the first and second nodes to fixed incremental voltage values greater and lesser, respectively, than the capacitor voltage as the circuit tracks the analog voltage, and to fixed incremental voltage values lesser and greater, respectively, than the capacitor voltage, as the circuit holds the analog voltage. The nodes reverse their polarities by equal amounts during the switching of the circuit between the tracking and holding. In this manner, an equal and opposite voltage magnitude change is produced at each node during the switching interval, thereby preventing any net change in the capacitor voltage, and providing a glitch-free output. A special current switch gates currents through the diode array to effect the clamping.

Claims20

Dependent claims

An improved analog signal storage circuit of the type having capacitor means for storing an analog voltage signal connected to the input of said circuit, input means for developing a charging current for said capacitor means from said analog voltage, and output means for buffering the voltage across said capacitor means to the output of said circuit, wherein said improvement prevents said voltage across said capacitor means from changing as said circuit switches between the tracking and the holding of said analog signal and comprises:

(a) circuit means, connected to said capacitor means, for establishing first and second voltage reference nodes in said circuit, said establishing means including
(i) means for holding said nodes at voltages incrementally different by a first value from said capacitor voltage as said circuit tracks said analog signal, said first incremental voltage value being of opposite polarity at each node, and
(ii) means for holding said nodes at voltages incrementally different by a second value from said capacitor voltage as said circuit holds said analog signal, said second incremental voltage value being of opposite polarity from said first incremental voltage value, said incremental voltages being disposed so as to allow said charging current to flow through said capacitor as said circuit tracks said analog voltage and to prevent said flow as said circuit holds said analog voltage;
and
(b) current switch means, connected to both of said reference nodes, for directing currents through said circuit means, as said circuit changes between said tracking and said holding of said analog voltage signal, in a manner causing said voltage at said nodes to switch between said first and second incremental voltage values, said switching producing equal and opposite voltage magnitude changes at each of said nodes, thereby preventing any net change in said voltage across said capacitor means and preventing unwanted voltage changes at said output of said circuit.

2 more elements

An improved analog signal storage circuit of the type having capacitor means for storing an analog voltage signal connected to the input of said circuit, input means for converting said analog voltage to a charging current for said capacitor means, and output means for buffering the voltage across said capacitor means to the output of said circuit, wherein said improvement prevents said voltage across said capacitor means from changing as said charging current is connected to or disconnected from said capacitor means, said improvement comprising:

circuit means, connected to said capacitor means, for establishing a first voltage reference node and a second voltage reference node in said circuit, each of said nodes being at equal and opposite incremental voltage values different from said capacitor voltage, said incremental voltages being disposed so as to allow said charging current to flow through said capacitor as said circuit tracks said analog voltage and to prevent said flow as said circuit holds said analog voltage;
and
current switch means connected to both of said reference nodes, for directing current in said circuit through said circuit means, as said circuit changes between said tracking and said holding of said analog voltage signal, in a manner causing said nodes to exchange said incremental voltage values, said exchange producing an equal and opposite voltage magnitude change at each of said nodes, thereby preventing any net change in said voltage across said capacitor means and at said circuit output.

An improved analog signal storage circuit of the type having capacitor means, including a connection terminal, for tracking an input analog signal, and switch means for changing the configuration of said circuit whereby said capacitor means holds, instead of tracks, said signal, wherein the improvement comprises:

first circuit means establishing first and second voltage nodes, with said terminal of said capacitor means connected in circuit between said nodes, said first circuit means latching said first and second nodes during a tracking mode to respective fixed voltage increments greater and lesser than the capacitor voltage;
and
second circuit means, responsive to said switch means and cooperating with said first circuit means, for reversing the polarity of said voltage increments at said nodes relative to said capacitor voltage during the transition from said tracking to said holding mode, said reversed polarity voltages being latched at said nodes by said first circuit means, whereby said capacitor voltage is constantly maintained at a value between said voltages at said nodes, said polarity reversal during said switching transition serving to substantially eliminate changes in said capacitor voltage as said circuit switches between said tracking and said holding of said analog signal.

Dependencies are inferred from the claim text.