Current-to-frequency converter
AssigneeAnalog Technology Corporation
InventorDavid Lee Raymond
Life of this patent
Estimating the term…
Abstract
A current-to-frequency converter in which a succession of small charge feedback pulses is selectively generated to null the input current. The feedback pulses are generated by applying voltage pulses of precisely controlled magnitude to a precision capacitor to produce pulses of precisely-determined charge content. These pulses are then applied to a charge-dividing circuit including a matched pair of bipolar transistors. The charge dividing circuit bypasses the bulk of the charge in each pulse, while applying only a minute fraction of each pulse as the feedback current. The charge division ratio is maintained constant by the circuit in spite of temperature fluctuations.
Claims23
A circuit for generating a current pulse containing an accurately-determined minute quantity of electrical charge, comprising:
- a current pulse generator for generating, in response to an input signal, an output current pulse containing a precise, pre-determined quantity of electrical charge;
- and
- a current divider connected to said current pulse generator for dividing each current pulse supplied by said current pulse generator and having an output terminal at which a small constant fraction of each current pulse is presented,
- said current divider further comprising a pair of transistors connected in parallel to receive each current pulse, one of said transistors providing a conductive path to the output terminal and the other of said transistors providing a conductive shunt path for each current pulse, said transistors being biased to operate at different operating points of their characteristics, at which the conductances of the transistors are in a predetermined ratio, a small substantially constant fraction of each current pulse supplied by said current pulse generator being conducted through said one of said transistors to the output terminal.
A charge feedback current-to-frequency converter comprising:
- an integrator for generating as an output the time-integrated difference between an applied current to be converted to frequency and a feedback current;
- a threshold device connected to receive the output of said integrator, for generating an output signal when the output of said integrator exceeds a predetermined level;
- a current pulse generator connected to said threshold device for generating in response to the output signal generated by said threshold device an output current pulse containing a precise predetermined quantity of electrical charge;
- and,
- a current divider connected to said current pulse generator for dividing each current pulse supplied by said current pulse generator and having an output terminal at which a small constant fraction of each current pulse is presented,
- said current divider further comprising a pair of transistors connected in parallel to receive each current pulse, one of said transistors providing a conductive path to the output terminal and the other of said transistors providing a conductive shunt path for each current pulse, said transistors being biased to operate at different operating points of their characteristics, at which the conductances of the transistors are in a predetermined ratio to conduct a small substantially constant fraction of each current pulse supplied by said current pulse generator through said one of said transistors to the output terminal for use as the feedback current.
In a charge feedback current-to-frequency converter of the type wherein a current of known polarity but unknown magnitude is nulled by an average current of opposite polarity, said nulling current consisting of current pulses of predetermined charge content generated when the integrated difference between the unknown current and the nulling current exceeds a preset threshold causing a threshold device to generate an output signal, an improved nulling current pulse generator comprising:
- a current pulse generator connected to the threshold device and responsive to its output signal for generating each time the preset threshold is exceeded a current pulse containing a precisely determined quantity of electrical charge;
- and
- a current divider connected to said current pulse generator for dividing each current pulse supplied by said current pulse generator and having an output terminal at which a small constant fraction of each current pulse is presented,
- said current divider further comprising a pair of transistors connected in parallel to receive each current pulse, one of said transistors providing a conductive path to the output terminal and the other of said transistors providing a conductive shunt path for each current pulse, said transistors being biased to operate at different operating points of their characteristics, at which the conductances of the transistors are in a predetermined ratio, to conduct a small substantially constant fraction of each current pulse supplied by said current pulse generator through said one of said transistors to the output terminal.
In a charge-feedback current-to-frequency converter of the type wherein a current of a known polarity but unknown magnitude is nulled by an average current of opposite polarity, said nulling current consisting of current pulses of predetermined charge content generated when the integrated sum of the unknown current and the nulling current exceeds a preset threshold causing a threshold device to generate a signal, an improved nulling current pulse generator comprising:
- a precision voltage pulse generator connected to the threshold device, for generating as an output a voltage pulse whose amplitude is precisely determined in response to the signal generated when the threshold is exceeded;
- a capacitor, having first and second plates, connected in series with said precision voltage pulse generator, for generating in succession in its second plate in response to the leading and trailing edges of the voltage pulse supplied by said precision voltage pulse generator and applied to the first plate of said capacitor, two current pulses of opposite polarity and accurately predetermined charge content, one of which is of a desired polarity;
- a diode connected to the second plate of said capacitor for bypassing the current pulse of undesired polarity;
- and,
- a pair of transistors having their emitters connected to the second plate of said capacitor, said transistors biased to operate at different predetermined operating points on their characteristics, to maintain the conductances of the transistors in a substantially fixed ratio to conduct a small substantially constant fraction of the input current pulse of desired polarity through one of said transistors for use as a nulling current pulse of predetermined charge content.
A method of generating a current pulse containing an accurately determined minute quantity of electrical charge in response to an applied voltage pulse, comprising the steps of:
- (a) applying a voltage pulse of precisely controlled amplitude V to a first plate of a capacitor of capacitance C, whereby output current pulses of opposite polarity are generated in succession, each conveying an amount of electrical charge equal to CV, one pulse being of a desired polarity and the other pulse being of an undesired polarity;
- (b) bypassing the output current pulse of undesired polarity;
- (c) applying the output current pulse of desired polarity to two parallel connected conductive transistors having a predetermined ratio of conductances to obtain a small fraction of the current pulse as an output of one of said parallel-connected conductive transistors while bypassing the remainder of the current pulse through the other transistor.
A method for generating a current pulse containing an accurately determined subnanocoulomb quantity of electrical charge in response to an applied signal comprising the steps of:
- (a) generating in response to the applied signal, a voltage pulse of fixed amplitude V;
- (b) applying the generated voltage pulse through a capacitor of capacitance C, to form two successive current output pulses of opposite polarity in response to the leading and trailing edges respectively of the generated voltage pulse, whereby each of the output current pulses conveys an amount of electrical charge equal to CV;
- (c) bypassing output current pulses of undesired polarity through a diode;
- and
- (d) applying output current pulses of a desired polarity to a current-dividing circuit, including a matched pair of bipolar transistors biased to operate at different points on their characteristics and having their emitters connected, whereby a small constant fraction of each current pulse is passed by one of the transistors as an output while the remainder of each current pulse is shunted through the other transistor.
A circuit for generating a current pulse containing an accurately-determined minute quantity of electrical charge, comprising:
- a current pulse generator for generating, in response to an input signal, an output current pulse containing a precise predetermined quantity of electrical charge;
- and,
- a current divider connected to said current pulse generator for dividing each current pulse supplied by said current pulse generator and having an output terminal at which a small constant fraction of each current pulse is presented,
- said current divider further comprising a pair of semiconductor devices connected in parallel to receive each current pulse, one of said semiconductor devices providing a conductive path to the output terminal and the other of said semiconductor devices providing a conductive shunt path for each current pulse, said semiconductor devices being biased to operate at different operating points of their characteristics, at which the conductances of the semiconductor devices are in a predetermined ratio to conduct a small substantially constant fraction of each current pulse supplied by said current pulse generator through said one of said semiconductor devices to the output terminal.
Dependencies are inferred from the claim text.