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I need to attenuate 0-5V to 0-1 mV.

The output of simple voltage dividers (normal op amps or resistor divider) can't be distinguished from baseline noise (baseline noise is 10-20mV.)

How can I attenuate the input voltage (0-5V) to get a 0-1mV low noise output?

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    \$\begingroup\$ The baseline noise depends on the impedance of the circuit. Noise across a 1 Ω output resistor would be far less than across a 1 MΩ resistor. Noise across a wide bandwidth will be greater than across a circuit of lower bandwidth. \$\endgroup\$ Jun 7 at 21:57
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    \$\begingroup\$ If using an oscilloscope to view the output from the divider try one thing; connect the oscilloscope probe tip to 0 volts and see what noise your o-scope inherently picks up. \$\endgroup\$
    – Andy aka
    Jun 7 at 21:57

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A resistive divider is low noise compared to 1mV for reasonable resistor values and bandwidth.

For example, 49.9K\$\Omega\$/10\$\Omega\$ divider will have J-N noise of less than 0.5nV/\$\sqrt{\text {Hz}}\$ at room temperature. 1mV of signal is huge compared to that.

If you can't distinguish the signal from noise, given my above assumptions, then your measurement method may be the issue.

Most oscilloscopes have a fair bit of noise, as Andy has suggested. Try using a 1:1 probe setting rather than 10:1, and limit the bandwidth if your scope has such a setting- and if that's not enough then you can use a low-noise op-amp as a preamplifier.

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