Test & measurement
hand, a filtered LTC6655 (SFG) achieves on average a 7dB increment in dynamic range compared to LTC6655 for up to
Figure 11. SFG LPF.
20V/µs, and gain bandwidth of 100MHz, which makes it suitable for driving an ADC. In order to correctly evaluate the performance when using an LTC6655 and an LTC6655LN with an AD7177-2, a dc source with overall noise lower than the ADC voltage reference and the ADC noise is required. Therefore, an ideal source was used, namely a 9V battery supply as can be seen in Figure 12.
Figure 14. ODR vs. ENOB.
rise of the total integrated noise (rms) prevents the signal chain from achieving 25-bit measurement resolution.
Region B: Figure 12. Low noise dc source. CIRCUIT PERFORMANCE
Figure 13 displays spectral noise density and Figure 14 displays output data rate (ODR) vs. ENOB, depicting the performance of AD7177-2 with its VREF input connected to a LTC6655/LTC6655LN with 10µF NR capacitor or a filtered LTC6655 (SFG). To provide perspective of spectral noise density comparison at 1kHz, see Table 4. Both Figure 13 and Figure 14 have two important regions.
Table 4. Spectral Noise Density Comparison at 1kHz
LTC6655
Spectral Noise Density at 1kHz
(nV/√Hz) 96
LTC6655LN with 10µF NR Capaci- tance
32
LTC6655 SFG Filter
2.4
ADC Input DC Source
6.7
In this region, the spectral noise density plot (Figure 13) shows that the flicker noise of the three voltage reference options and the dc source increase and overall system noise are dominated by the dc source noise. This increase in flicker noise within region B explains the rise in deviation of ENOB between the measured performance and maximum achievable by the ADC (Figure 14).
According to ODR vs. ENOB plot, filtered LTC6655 (SFG) achieves 25-bit resolution at 20SPS and lower while the LTC6655LN-5 with 10µF NR cap and the LTC6655 cannot achieve better than 24.6-bit resolution. Table 5 below is a summary of the AD7177-2 ADC performance with the VREF input either connected to a LTC6655/LTC6655LN with 10µF NR capacitance or filtered LTC6655 (SFG). With ADC inputs tied together and the VREF input connected to LTC6655, the zero-scale column establishes the best dynamic range AD7177-2 can achieve. With the ADC inputs nearly set to full-scale range, LTC6655LN-5 with 10µF NR cap increases on average 4dB dynamic range for up to 59.96SPS compared to LTC6655. On the other
ODR
ADC Dynamic Range Zero Scale (dB)
10000 5000 2500 1000 500 Figure 13. Spectral noise density. Region A:
Spectral noise density plot Figure 13 shows that at ODR of 500SPS and higher, both the filtered LTC6655 (SFG) and ADC dc input source noise are significantly lower noise than the ADC. This results in the least amount of deviation from the maximum performance achievable by the ADC as shown in region A in Figure 14. The key takeaway based on ODR vs. ENOB and spectral noise density plot is that, within region A, the
Instrumentation Monthly May 2025 200 100 59.96 49.96 20 10 5 135.40 138.41 140.82 144.43 148.65 152.86 156.47 157.08 159.48 162.49 163.70 165.50
LTC6655 Dynamic Range (dB)
126.88 129.14 132.91 136.50 137.55 139.83 143.32 143.66 146.58 149.51 149.58 150.07
59.96SPS. The dynamic range delta does not vary much below 59.96SPS and the variance is mainly due to the dominated low frequency flicker noise induced by the ADC input dc source.
Compared to LTC6655/LTC6655LN with 10µF connected to NR pin reduces the broadband noise at 1kHz by 62 per cent and filtered LTC6655 (SFG) reduces broadband noise by 97 per cent.
CONCLUSION
A precision system that is attempting to achieve a 25-bit resolution or higher must account for the significance of voltage reference noise. As shown in Figure 2, the contribution of VREF noise to system noise is proportional to the utilisation of the ADC’s full-scale range. This article shows that adding a filter to a precision voltage reference attenuates VREF noise, which leads to reducing overall system noise. An LTC6655 voltage reference followed by an SFG filter can reduce broadband noise by 97 per cent of the LTC6655 with no filter. This comes at a cost of additional BOM, more PCB area, more power consumption, a few PPM of dc accuracy degradation, and can vary precision reference output with temperature. Considering SFG LPF trade-offs, LTC6655LN has leverage in terms of simple design, low power, only requires a single capacitor to reduce broadband noise, and eliminates the need for an external buffer to drive an ADC. LTC6655LN with 10µF NR capacitor does reduce broadband noise by 62 per cent of the LTC6655 with no filter. Hence, users can now take advantage of the built-in LTC6655LN low-pass filter to enable precision systems to achieve their desired resolution.
Analog Devices
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Table 5. Dynamic Range Comparison
LTC6655LN 10µF Dynamic Range (dB)
132.22 135.08 137.23 140.11 141.95 144.15 145.82 147.31 148.43 149.56 149.72 150.25
LTC6655 (SFG) Dynamic Range (dB)
134.65 137.37 139.86 142.42 144.37 147.40 150.49 151.71 151.72 152.26 152.26 152.26
Dynamic Range Delta (LTC6655LN 10µF - LTC6655) (dB)
5.33 5.94 4.32 3.61 4.40 4.32 2.49 3.65 1.85 0.06 0.14 0.18
Dynamic Range Delta (LTC6655 (SFG) - LTC6655) (dB)
7.77 8.23 6.95 5.92 6.83 7.57 7.17 8.05 5.14 2.76 2.68 2.19
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