Switched Current Source Error Correction for DAC Accuracy
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Solution Overview
Problem
Conventional current sources in digital-to-analog converters (DACs) face errors due to finite output impedance and impact ionization, leading to inaccuracies in output currents.
Innovation Solution
A switched current source apparatus is designed with a specific configuration of transistors and resistors, including an error correction transistor and a resistor, to sense and correct error currents caused by transistor current gains, thereby isolating the DAC output current from output voltage variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional current sources are used in DACs, then the device complexity is low, but the output impedance is finite and error currents are generated due to impact ionization
Solution Approach 1:
The patent implements a feedback mechanism where error currents generated by impact ionization in the current source are sensed and fed back through the bias transistor to cancel the errors. The feedback path includes the error correction transistor and resistor that detect and compensate for the voltage-dependent error currents, thereby achieving high output impedance and improved accuracy without requiring a completely different current source topology.
Solution Approach 2:
The patent introduces intermediate components including error correction transistors and resistors that act as mediators between the current source and the DAC output. These intermediaries sense the error currents and facilitate their cancellation through the feedback path, enabling high output impedance while maintaining a relatively simple overall structure.
2Measurement precision
If conventional current sources are used, then the device complexity is low, but accuracy deteriorates due to error currents from transistor current gains and impact ionization
Solution Approach 1:
The feedback mechanism senses error currents caused by transistor current gains and impact ionization, then feeds them back through the bias transistor to cancel these errors. This feedback loop continuously compensates for accuracy-degrading effects, achieving high measurement precision while adding only moderate complexity through the error correction transistor and resistor network.
Solution Approach 2:
The patent converts the harmful error currents generated by impact ionization and transistor current gains into a beneficial correction mechanism. By sensing these error currents and feeding them back through the bias transistor, the harmful effects are transformed into a self-correcting system that actively cancels the errors, thereby improving accuracy without requiring completely different transistor characteristics.
3Reliability
If error correction components are added to the current source, then output impedance improves tenfold, but device complexity increases
Solution Approach 1:
The feedback path using the error correction transistor and resistor provides a straightforward mechanism to achieve tenfold improvement in output impedance. The feedback loop directly counteracts the voltage-dependent error currents, and its simplicity relative to alternative high-impedance current source topologies justifies the moderate increase in device complexity.
Solution Approach 2:
The current source with error correction components becomes self-correcting, automatically compensating for its own errors through the feedback mechanism. The error correction transistor and resistor enable the circuit to self-regulate and maintain high output impedance without requiring external calibration or complex control systems, making the added complexity worthwhile.
Data Source
AI summary
A switched current source is provided. The switched current source is generally comprised of transistors and resistors, and the source has a high output impedance. Included with the switched current source is an error correction transistor and a resistor that cooperate to feed a current back through a bias transistor to correct an error that generally results from the current gains or β's of transistors within the switched current source. To accomplish this, however, the resistor is selected to have a value that is sufficiently large such that current from the error correction transistor flows back through the bias transistor.