Test & measurement Figure 5. LTC6655 voltage reference noise peaking density.
4 shows the sigma-delta AD7177-2 and SAR AD7980 ADC dynamic reference current response. The user can choose the value of the C1 capacitor to meet the LPF cutoff frequency requirement, but some SAR ADCs require 10µF minimum capacitor on the reference input in order to operate correctly. This minimum 10µF C1 capacitor reduces the phase margin of the reference buffer. As the phase margin reduces, the buffer feedback is no longer negative.1 The signals near the unity-gain crossover frequency are fed back in-phase with the incoming signals. This causes the closed-loop response to introduce noise peak near the crossover frequency.1 Since the bandwidth from the cutoff frequency (–3dB point) reaches up to 16MHz, the total integrated noise (rms) is dominated by the noise peak. Even though voltage reference reservoir capacitor C1 operates as a noise filter and compensates for voltage spikes, the caveat is the noise peak. Figure 5 shows the noise peak of the LTC6655 voltage reference introduced by reservoir capacitor C1. The noise peak magnitude is determined by the value of the reservoir capacitor and its ESR rating.
One of the other solutions to filter voltage reference noise, remove the noise peak, and properly drive the ADC is to add a passive RC LPF followed by a buffer. By adding a buffer, we separate the design constraints of the LPF and the ADC reference input capacitor. See Figure 6. Setting the passive RC LPF cutoff frequency well below the unity-gain crossover frequency will not only reduce broadband and low frequency noise but also avoid noise peaking. For example, Figure 7 shows the LTC6655 noise response with C1 = 100µF (ESR = 0Ω), followed by a passive LPF where R = 10kΩ and C2 = 10µF (ESR = 0Ω), creating a pole at 1.59Hz.
Figure 6. Passive RC LPF followed by a buffer.
Most voltage references are designed with a complex output stage to drive a large load capacitance suitable for ADC reference circuitry. For example, the LTC6655 output stage is designed to be critically damped with a reservoir capacitance set to 10µF. When the LTC6655’s reservoir capacitance is set to a minimum of 2.7µF and a maximum of 100µF, noise peaking is introduced. The equivalent series resistance of the VREF output reservoir capacitance does mitigate the primary noise peak but introduces a secondary noise peak at 100kHz and above. This can be explained by the fact that the ESR of the cap introduces a zero, which leads to improving phase margin and reducing primary noise peak. However, this zero combines with the inherent zero of the LTC6655 and creates secondary noise peaking. Note, the noise response in Figure 5 is only valid for the LTC6655 voltage reference.
Instrumentation Monthly May 2025
Increasing the low-pass filter resistor R can help achieve a low cutoff frequency, but can also result in dc accuracy degradation of precision voltage reference. When adding a passive RC LPF, the user must also consider the impact on the load regulation and the impact on the VREF buffer response ( = RC), which affects its transient performance when driving an ADC. To achieve the required transient performance, it is suggested to use a buffer as shown in Figure 6. Critical specifications to consider in terms of selecting buffer includes ultralow noise, capability to support high load capacitance, low distortion, excellent slew rate, and wide gain bandwidth. Recommendations for reference buffers are the ADA4805-1 and ADA4807-1.
NOISE REDUCTION USING AN ACTIVE LPF
Table 1 states the required dynamic range and maximum allowable system noise that must be met in order to achieve the desired ENOB ADC
resolution. Depending on the ADC bandwidth, a single-pole, low-pass filter attenuating at 20dB/decade may not achieve the desired wideband noise reduction. Cascading passive low-pass filters creates a ladder structure that can generate a higher order filter, but each section’s input impedance will be a load on the previous section. This can degrade the dc accuracy of the precision voltage reference. However, designing a higher order LPF based on active components will provide excellent isolation between input to output, minimising voltage reference dc accuracy degradation, and provide low output impedance to drive the reference circuitry of the ADC.
There are several different types of active low-pass filters - for example, Bessel, Butterworth, Chebyshev, and elliptic - as shown in Figure 8. Having a band- pass that is flat or does not exhibit ripple will keep the precision voltage reference’s dc accuracy degradation to a minimum. Out of all filter types,
Table 1. Condition: VREF = 5V and ADC Input Set to Full-Scale Range
ENOB 20 21 22 23 24 25 26 27 28 29 30 31 32 SNR (dB) 122.16 128.18 134.2 140.22 146.24 152.26 158.28 164.3 170.32 176.34 182.36 188.38 194.4 Noise (µVrms) 7.798301 3.89942 1.949845 0.97499 0.487528 0.243781 0.121899 0.060954 0.030479 0.015241 0.007621 0.003811 0.001905
Figure 7. LTC6655-5 followed by passive RC LPF noise response. Continued on page 44... 43
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