DAC-BBF Common-Mode Interface for Differential Signal Linearity
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Solution Overview
Problem
High-speed digital-to-analog converters (DACs) with complementary current signals pose a challenge as common-mode currents can flow into the baseband filter (BBF), causing deviations in input common-mode voltage and potentially leading to high voltage at the BBF input, which affects operational amplifier input bias voltage and linearity.
Innovation Solution
A common-mode current removing unit is implemented, comprising a generator unit, current removing units, and a measurement unit with an operational amplifier to measure and sink the common-mode current from the complementary DAC currents, ensuring only differential-mode currents flow into the BBF.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If complementary current signals are used in high-speed DACs, then conversion speed and signal range are improved, but common-mode current flows into the BBF causing voltage deviations and affecting operational amplifier performance
Solution Approach 1:
The patent extracts and removes the common-mode current component from the complementary DAC current signals using a dedicated common-mode current removing unit. This unit measures the common-mode voltage and generates compensating currents to cancel out the harmful common-mode current before it reaches the BBF, while preserving the useful differential-mode signal.
Solution Approach 2:
The patent introduces an intermediary common-mode current removing unit between the DAC and BBF. This intermediary component includes a measurement unit that detects common-mode voltage and a generator unit that produces compensating currents, acting as a mediator to eliminate harmful common-mode current while allowing the differential signal to pass through to the BBF.
2Device complexity
If common-mode current is allowed to flow into the BBF, then the circuit structure remains simple, but input bias voltage and linearity of the operational amplifier are affected
Solution Approach 1:
The patent implements a feedback mechanism where the measurement unit continuously monitors the common-mode voltage at the BBF input and feeds this information back to the generator unit. The generator unit uses this feedback to dynamically adjust and produce compensating currents that precisely cancel the common-mode current, ensuring stable operational amplifier input bias voltage.
Solution Approach 2:
The common-mode current removing unit performs self-adjustment by measuring its own operating conditions (common-mode voltage) and automatically generating the appropriate compensating currents without external intervention. This self-service capability maintains reliable operational amplifier biasing while keeping the control mechanism relatively simple.
Data Source
Figure 1
Figure 2
Figure 3A~3B
AI summary
Removing common-mode current from a pair of complementary current signals, including: generating a common-mode voltage of the pair of complementary current signals including at least a first current signal and a second current signal; measuring and outputting a difference voltage between the generated common-mode voltage and a common-mode reference voltage; and removing at least a portion of the common-mode current from the first current signal and the second current signal based on the difference voltage.