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Stability Issues and Resolutions for High Speed Voltage Feedback Op Amps, Insight #6

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Figure 4
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Response at output pin with direct capacitive load.

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Response at output pin with direct capacitive load.

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4/9/2019 5:14:08 AM
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Be one step ahead
We might view stability issues as something that aren't that great of a deal to address, but once we have one in our lap, the problem could take ages to rectify. This is why we need to know exactly which direction we're headed to in order to prepare the necessary solution even before the problem arrives.

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The most recent VFA FDAs focus on lower power, precision, solutions more aimed at Successive Approximation Register (SAR) and delta sigma ADC support. Those will be focus in this FDA stability discussion - but the concepts are backward compatible to the earliest VFA FDAs.
Here, an updated Loop Gain (LG) simulation approach will be first detailed and then used to show paths into, and out of, low Phase Margin (PM) conditions for a current feedback amplifier.
Probably no single issue faces more high-speed signal path designers (and supplier support teams) than the risk of instability in higher speed op amps and fully differential amplifiers (FDA).
The more recent Fully Differential Amplifiers (FDA) offer both CFA and VFA versionsThe CFA based versions will also have poor DC accuracy while more recent “precision” VFA based FDAs can provide much improved DC accuracy. Those VFA-based FDAs do come with some added DC error sources beyond the typical op amp terms that will be described here
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