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I need to attenuate a signal which outputs from a function generator. The minimum amplitude of the signal from the function generator is 1mV. What would be the best method to attenuate the signal from 1mV to 1uV without adding noise to the system? Are there any attenuators available? The signal of interest is sine wave, frequency being 1Hz to 100Hz.

I am new to this and would be obliged if someone helps me for the same.

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You can buy ultra-low noise metal film resistors (0.1% Tol) from many vendors. Also important is to keep their values to under 100 K ohms. You need a 1000:1 divider, and for precision you may need a trim pot. 100 K and 49.9 ohms is 2000:1. Insert a 100 ohm trim pot between them to get an exact 1000:1 ratio.

Adding op-amps will add noise and offset, and power supply issues, so if passive resistors will work then go simple on this.

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A simple resistive divider or attenuator is the way to go.

First, you need to specify your output impedance. Most function generators have an output impedance of 50 Ω. It's a practical impedance for reasonably low noise, and it will keep calibration with any other testgear you have.

As you're only going to 100 Hz, a simple single stage pi attenuator like this is sufficient.

schematic

simulate this circuit – Schematic created using CircuitLab

If your function generator is specified to work into 50 Ω, then it's worth having R1. R2 is needed to establish the output impedance. R3 gives you an approximately 1000:1 reduction in output level when the output is loaded with 50 Ω.

For higher frequencies, a large resistor like 25 k is not a good idea, and a multistage attenuator is flatter.

If you don't want to load your function generator, or want a lower or higher output impedance for some reason, then you can scale the resistors appropriately.

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