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Input/source impedance and noise performance of OPamps

alpha24 (preamp/buffer, voltage DAC analog output stage, balanced ↔ unbalanced converter)

Input/source impedance and noise performance of OPamps

Postby Kabal{MSU} » October 23rd, 2019, 5:58 am

Hi Ti,

one thread about choosing best sounding OPamps on another forum triggered me to look into the datasheet(s) of OPamps (1611/1612) used in alpha24 once again. The datasheet contains the Figure 3 "Voltage Noise vs. Source Resistance". Am I right and for U1a/b the source resistance is about 500kOhm, that degrades noise performance from 1.1 nV/√Hz to approx. 80 nV/√Hz for resistor noise and 800 nV/√Hz for total output voltage noise?

Regards.
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Re: Input/source impedance and noise performance of OPamps

Postby amb » October 23rd, 2019, 9:32 am

That would be true only if the α24 is running with nothing connected to its input. In normal use, the impedance "as seen by the opamp" includes the output impedance of the connected source, which appears in parallel with those input resistors. In most cases that impedance is very low, in the order of a few ohms to a few hundred ohms.
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