US 5,180,932 AGranted

Current mode multiplexed sample and hold circuit

OwnerDavid W. Bengelinventor

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Abstract

A sample and hold circuit is disclosed in which differentially coupled input stages are multiplexed to drive a common output stage. In this way, a plurality of input stages can be employed wherein the transition between sample and hold modes produces greatly reduced switching transients. The circuit has very high overall gain so that sampling accuracy is improved and a very low current input stage configuration permits the use of small hold capacitors without introducing excessive droop in the hold mode. The differential balance is completed by a dummy hold capacitor which is switched along with the hold capacitor. Both of these capacitors are switched in a virtual ground configuration. Also, disclosed are means for producing an auto zero input condition, means for adding a correlation feature for improved digital performance and means for injecting controlled offset potentials so that the sample and hold means can be employed as either a DA converter or an AD converter in either a pipelined or recirculating converter state.

Claims22

Dependent claims

A sample and hold circuit having a signal input, an output and a control logic input, said control logic input being responsive to logic signals that command said circuit to operate in a sample mode in which said output replicates said signal input or in a hold mode in which said output holds the signal input value present at the instant of the hold logic command, said circuit comprising:

an output stage having an output suitable for providing said circuit output and an input;
first and second differential input stages each having outputs commonly connected to drive said input of said output stage and each having inverting and noninverting differential inputs, said first and second differential input stages having substantially equal operating characteristics including substantial signal gain;
means for coupling said noninverting input of said first differential input stage to said signal input whereby said circuit responds to said input;
means, responsive to said logic signals, for alternately turning said first and second differential input stages on whereby when said sample mode is commanded said first differential input stage is on and when said hold mode is commanded said second differential input stage is on;
hold capacitor means coupled between said output of said output stage and the inverting input of said second differential input stage to produce negative feedback in said hold mode;
and
first switch means coupled between said inverting input of said second differential input stage and a source of reference potential and means for turning said first switch means on when said sample mode is commanded whereby the charge on said hold capacitor means follows said output signal in said sample mode.

4 more elements

A sample and hold circuit capable of responding to an analog input signal to produce a related output in response to a sample mode command and to provide a constant output in response to a hold mode command wherein said constant output is a reproduction of said analog input signal at the instant of said hold command, said circuit comprising:

first and second differential amplifier means having differential inputs and outputs;
means for coupling at least one of the differential inputs of said first differential amplifier means to said analog input signal;
control means responsive to said hold mode command for selectively operating said first and second differential amplifier means in a complementary fashion whereby said first differential amplifier means is off in said hold mode and said second differential amplifier means is off in said sample mode;
an operational amplifier having a differential input and a single ended output that comprises said related output;
means for commonly coupling the outputs of said first and second differential amplifier means to said differential input of said operational amplifier;
a hold capacitor coupled between said output of said operational amplifier and one input of said second differential amplifier means differential inputs to create a negative feedback in response to said hold mode command;
a dummy hold capacitor coupled between the other input of said second differential amplifier means and a source of reference potential;
and
first switch means coupled between said inputs of said second differential amplifier means and said source of reference potential and operated by said control means so that said first switch means are off when said second differential amplifier means is on whereby said second differential amplifier means inputs operate at a virtual ground in said hold mode.

6 more elements

Dependencies are inferred from the claim text.