Binary-Weighted Current Source for Precise Nanoamp DAC Division

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Solution Overview

Problem

Existing methods for dividing currents in the range of microamperes or nanoamperes, such as those used in current-controlled analog-to-digital converters, are imprecise for currents below hundreds of nanoamps, particularly when dividing by powers of two, as they fail to maintain precise binary weighted current division.

Innovation Solution

A binary weighted current source comprising a driving voltage generating circuit, a current dividing circuit using resistors in series, and a current steering circuit, which employs an exponential relationship between voltage and current in transistors to achieve precise binary weighted current division by successively reducing control voltage, ensuring accurate division by powers of two in low current ranges.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of manufacture

If differential transistors with equal control voltages are used to divide current by 2, then the circuit is simple and easy to manufacture, but the measurement precision deteriorates for currents in the range of hundreds of nanoamps or less

Engineering Contradiction:

Improvecircuit simplicityVSAvoidcurrent division precision

Core Design Contradiction:

Ease of manufactureVSMeasurement precision

Solution Approach 1:

The patent changes the control parameter from equal control voltages to successively reduced control voltages (Vref, Vref/2, Vref/4, etc.) applied to current output transistors. This parameter change enables precise binary-weighted current division (Iref, Iref/2, Iref/4, etc.) even in the sub-nanoampere range, resolving the precision limitation of conventional differential transistor methods.

Inventive Principle:

Principle #35Parameter changes

2Measurement precision

If R-2R ladder network is used to divide current by 2, then current division can be achieved, but the device complexity increases due to multiple resistors and switches

Engineering Contradiction:

Improvecurrent division capabilityVSAvoidcircuit structure complexity

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The patent extracts and eliminates the complex R-2R ladder network structure (multiple resistors and switches) by replacing it with a simplified configuration using current output transistors controlled by successively reduced voltages. This extraction maintains precise current division capability while dramatically reducing circuit complexity.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent substitutes the passive resistor-based R-2R ladder network with an active transistor-based current control system. By using transistors with exponentially related currents controlled by linearly reduced voltages, the system achieves the same current division function with simpler, more controllable active components rather than complex passive networks.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Use of energy by moving object

If conventional current division methods are used for microampere or nanoampere range currents, then the circuit operates in low current mode, but the measurement precision deteriorates for currents below hundreds of nanoamps

Engineering Contradiction:

Improvelow current operationVSAvoidcurrent division accuracy

Core Design Contradiction:

Use of energy by moving objectVSMeasurement precision

Solution Approach 1:

The patent introduces dynamic control where current output transistors are controlled by successively reduced control voltages (Vref, Vref/2, Vref/4, etc.) rather than static equal voltages. This dynamic approach enables the circuit to maintain precise binary-weighted current division across the full dynamic range including sub-nanoampere levels, overcoming the precision limitations of conventional low-current operation.

Inventive Principle:

Principle #15Dynamics

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

The solution enables precise division of currents by powers of two in low current ranges, improving the accuracy of digital-to-analog conversion and binary weighted current generation, particularly for currents in the submicron ampere or nanoampere levels, addressing the imprecision of prior art methods.

Implementation Method 1

The current dividing circuit divides the driving voltage through resistors in series

Methodology Applied

Scientific EffectVoltage division: Ohm's Law

Implementation Method 2

Each of the binary weighted currents and a voltage cross the corresponding current output transistor have an exponential relationship between them

Methodology Applied

Scientific EffectExponential current-voltage relationship in transistors:

Data Source

PatentUS11251806B2Binary weighted current source and digital-to-analog converter

Publication Date: 2022.02.15 MICROTERA SEMICON (GUANGZHOU) CO LTD

AI summary

The present disclosure provides a binary weighted current source and a digital-to-analog converter, which include: a driving voltage generating circuit, generating a driving voltage based on a preset current; a current dividing circuit, connected to an output terminal of the driving voltage generating circuit; a current steering circuit, connected to the current dividing circuit. The current dividing circuit divides the driving voltage through resistors in series, and drives each of a plurality of current output transistors to output a current in response to a voltage across the current output transistor. Currents output by the plurality of current output transistor are binary weighted currents, each two of the binary weighted currents have a binary relationship, and the binary weighted currents are produced by successive binary divisions of the preset current.