How to Reduce Noise in Low-Voltage Amplifier Designs
This article compares the noise performance of two different amplifiers and optimizes noise performance through the use of passive filtering.
Depending on the application, you may need to consider the effects of noise in your circuit. Unfortunately, noise is a complex subject. While various online resources do cover this topic in detail, knowledge of the fundamental approaches to noise reduction can also be helpful. In this article, I will compare the noise performance of two different amplifiers and optimize noise performance through the use of passive filtering. To verify the approach, I will use SPICE simulations and a noise calculator. Finally, I will cover the inclusion of footprints for a feedback capacitor and isolation resistor to help fine-tune noise performance.
Consider an analog input signal of 50 to 450 mV at 100 kHz. This signal can be amplified to a range of 500 mV to 4.5 V using a noninverting circuit in a gain configuration of +10 V/V (Figure 1). For this application, let’s use the TLV6741, a low-noise (5 nV/√Hz at 1 kHz) general-purpose operational amplifier (op-amp) with a gain bandwidth of 10 MHz. Remember, it is important to pick resistor values with consideration for thermal noise performance. You don’t want to pick a low-noise amplifier only to have the thermal noise of large resistors dominate the overall noise performance.
Figure 1. TLV6741 noninverting, G = 10 V/V noise simulation
Figure 2. TLV6741 noninverting, G = 10 V/V noise calculation
Given that the expected output voltage is 4 Vpp, a simulated output noise of approximately 55 µVRMS (or 330 µVpp) is relatively small. The calculated noise of 64 µVRMS is similar to the simulated value, though it may underestimate the total noise. This discrepancy is likely caused by the input of a conservative value of 5 nV/√Hz of broadband noise into the calculator. At higher frequencies, the broadband noise level actually is lower. The current flicker noise is considered negligible and not included in the calculated estimate.
Let’s now consider the same circuit, but this time using the LMP7731. The LMP7731 is also a low-noise op-amp, but with a greater bandwidth of 22 MHz and a lower broadband voltage noise of 2.9 nV/√Hz. Given that the voltage noise of this device is significantly lower than the TLV6741, you might expect the overall output noise to be lower as well for the LMP7731 in the same configuration.
However, the LMP7731 circuit actually has a slightly higher simulated output noise level of 63 µVRMS, or 378 µVpp, as shown in Figure 3, which is confirmed by the value from the noise calculator for 56.9 µVRMS. So why does the lower-noise LMP7731 circuit have greater overall output noise when compared to the TLV6741 circuit, even when they have the same configuration? Remember that the total output noise of a circuit depends on the noise density integrated across frequency. Since the TLV6741 has a lower bandwidth than the LMP7731, the TLV6741 will not have the same high-frequency noise contributions as the LMP7731. Thus, the overall noise of the TLV6741 circuit is lower in this case.
Figure 3. LMP7731 noninverting, G = 10 V/V noise simulation
To further optimize noise performance, you can add a filter in the feedback path. Including a capacitor in parallel with the feedback resistor lowers the gain at higher frequencies, thereby reducing output-referred noise. In this manner, you can combine the high performance of a low-noise device with the noise-reduction technique of bandwidth limiting described earlier. This feedback capacitor method is more commonly used to cut the bandwidth of high gain configurations.
Let’s simulate the TLV6741 and LMP7731 circuits with the addition of a feedback capacitor that sets the cutoff frequency at 500 kHz. Figures 4 and 5 show the new circuits and their simulated noise.
Figure 4. TLV6741 noninverting, G = 10 V/V with feedback capacitor noise simulation
Figure 5. LMP7731 noninverting, G = 10 V/V with feedback capacitor noise simulation
The simulation results show that adding a feedback capacitor to the circuits diminished the overall noise from 55 µVRMS (330 µVpp) to 41 µVRMS (246 µVpp) for the TLV6741. The noise of the LMP7731 circuit decreased from 63 µVRMS (378 µVpp) to 31 µVRMS (186 µVpp). In light of these results, it’s a good idea to leave a feedback capacitor footprint on the layout for noise-reduction purposes. If you don’t need it, you can leave the footprint unpopulated. The primary shortcoming of this method is that a reduction in the voltage gain also reduces the feedback capacitor’s noise attenuation, even to the point of becoming negligible.
For low-gain amplifier configurations, the addition of a low-pass resistor-capacitor (RC) filter at the amplifier’s output can be a more effective method for reducing noise. The idea behind this technique is again to attenuate higher frequency ranges that only contribute noise, while continuing to pass the signal frequency. Creating an RC filter at the output involves the addition of two components – a resistor and a capacitor. If you don’t need it, you can short out the resistor and leave the capacitor unpopulated. It is also possible to use this output resistor to stabilize the amplifier circuit in a technique known as the “isolation resistor” technique.
Because of the different properties of the two op amps, you’ll need slightly different isolation resistors to set the same cutoff frequency for both circuits. Figures 6 and 7 show the TLV6741 and LMP7731 circuits, respectively – using output RC filters with 500-kHz cutoffs instead of feedback capacitors – along with their TINA-TI software noise simulation results. Note the additional noise improvement down to 35 µVRMS (210 µVpp) for the TLV6741 and 26 µVRMS (156 µVpp) for the LMP7731. The results of all of these noise simulations are shown in Table 1, which summarizes the effectiveness of different noise reduction techniques.
Figure 6. TLV6741 noninverting, G = 10 V/V with output RC filter noise simulation
Figure 7. LMP7731 noninverting, G = 10 V/V with output RC filter noise simulation
Table 1. Comparing the noise performance of the TLV6741 and LMP7731
|Circuit||No filter||CF filter||Output RC filter|
|TLV6741||55 µVRMS (330 µVpp)||41 µVRMS (246 µVpp)||35 µVRMS (210 µVpp)|
|LMP7731||63 µVRMS (378 µVpp)||31 µVRMS (186 µVpp)||26 µVRMS (156 µVpp)|
As I mentioned at the beginning of this article, noise is a complex subject, but dealing with it can be straightforward. I recommend including footprints for a feedback capacitor and isolation resistor in your design for evaluating noise performance. If you don’t need them, you can always leave these components unpopulated or shorted. Ultimately, the proper implementation of the techniques discussed here should make you feel confident about minimizing the impact of noise in your system.
- TI Precision Labs – Op Amps: Noise (training series)
- Analog Engineer’s Pocket Reference (e-book)
- The Signal: A compendium of blog posts on op-amp design topics (e-book)
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