5.8.2 Voltage Noise
Voltage noise is an estimation of the amount of noise internally added by the amplifier in series with the non-inverting input signal, as shown below.
This noise source is characterized in two frequency ranges - a low frequency range and a high frequency range. Input noise voltage is the equivalent voltage noise within the low-frequency range (commonly 0.1 to 10 Hz). Outside of this range, the noise can be calculated using the bandwidth of the circuit and the input noise voltage density.
For example, if a unity-gain operational amplifier had an input noise voltage of 100µV (pk-pk), input noise voltage density of 100 nV/rt(Hz) and a circuit bandwidth of 1 MHz, then the noise from the operational amplifier can be approximated (pk-pk):
- For non-unity gain circuits, take the computed value and multiply by the noise gain of the circuit.
- Input voltage noise density is usually specified in Root Mean Square (RMS) values, not peak-to-peak.
- Circuit bandwidth must not be confused with the GBWP. Circuit bandwidth is specific to the maximum bandwidth the operational amplifier has in circuit.
One way to reduce the amount of noise is to lower the bandwidth of the operational amplifier. This can be done in multiple ways. The first way is to put a low-pass filter on the output of the operational amplifier. This method increases the output impedance and may cause stability issues depending on the R and C used in the filter. The figure below shows this implementation:
The second way is to add a capacitor across the operational amplifier’s feedback. This will cause the impedance of the feedback resistor to drop at high frequencies, thus reducing the amount of gain at higher frequencies, as shown in the figure below.
