#active Substrate Noise Suppression In Mixed-signal Circuits Using On-chip Driven Guard Rings

Active Substrate Noise Suppression in Mixed-Signal Circuits Using On-Chip Driven Guard Rings Wolfgang Winkler and Frank Herzel Institute for Semiconductor Physics (IHP) Walter-Korsing-Str. 2 15230Frankfurt (Oder), Germany Tel: +49 335 5625 150; email: [email protected] 1. Abstract

This paper presents an active substrate noise suppression circuit using a pair of concentric guard rings. The outer guard ring senses the substrate noise, which is inverted and amplified by a SiGe circuit. This on-chip amplifier drives the inner guard ring such that efficient noise cancellation is achieved. A ring oscillator is used to sense the residual substrate noise. The measured noise suppression bandwidth is as high as 400 MHz. 2. Introduction

Modem wireless and high-speed applications require the integration of millions of digital gates with noise sensitive analog circuitry on the same chip. The fast switching of digital signals can couple into nearby analog circuits. Especially, system-on-a-chip ( S o C ) designs are prone to substrate coupling problems. Substrate noise is particularly acute with the highly doped substrate materials used in modem silicon technologies. Different methods for reducing substrate coupling have been proposed (1). They include: Guard ring structures around the noisy and/or sensitive circuitry connected to a fixed potential (2); N-well trenches between the noisy and the sensitive circuitry ; Active guard-band filters (3). The last method is promising, because it allows effective noise suppression without additional technological effort, such as deep trenches or SOI. In (3), the noise suppression effect was observed at frequencies up to 20 MHz, where an external operational amplifier was used.

As mentioned in (3), an active guard-band filter with an on-chip noise cancellation circuit is likely to be be more effective for high frequencies, because it eliminates parasitic impedances due to external components. 3. Active Guard Rings

In the active guard-band filter approach, a noise cancellation signal is coupled into the silicon substrate. In contrast to (3), our solution is fully integrated (Fig. 1). It consists of an outer guard ring acting as a receiver for substrate noise and an inner guard ring acting as a transmitter of the inverted noise. The input of the inverting amplifier is connected to the outer guard ring, and the output to the inner. The guard rings are capacitively coupled to the silicon substrate. The noise generated from, e.g., digital circuits spreads over the chip, and is sensed by the outer guard ring. The amplifier amplifies and inverts this signal. The inverted noise is coupled into the substrate by the inner guard ring. In this manner a quiter region is generated inside the inner ring. To test and demonstrate this approach, two types of test chips were fabricated. The first chip includes a bondpad inside the inner guard ring to sense the residual noise by a spectrum analyzer. Sinusoidal “noise” generated by a signal generator is coupled into the substrate through a bondpad outside the guard rings connected to the substrate. In the second chip, the bond pad inside the rings is replaced with a ring oscillator (RO) as a noise sensor. The amplifier is a symmetric arrangement of an emitter follower and a differential stage. The schematic of the

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inverting amplifier is shown in Fig. 2. noise source

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Fig. 1 Schematic view of the noise cancellation circuit. The ring oscillator (RO) detects the residual noise inside the guard rings.

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Fig:. 3 Photograph of the test chip with bondpad as noise sensor.

Fig. 4 Photograph of the test chip with ring oscillator as noise sensor.

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Fig. 2 Schematic of the inverting amplifier.

The photographs of the Figures 3 and 4.

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4. Simulation Results

one and two are shown in

A SPICE-like simulator was used to simulate the operation of the circuit. To model the substrate coupling, a simple lumped circuit similar to that in (3) was used. The ac simulation showed a noise reduction by up to 20 dB (Fig. 5). At low frequencies, the capacitive coupling of the guard rings to the amplifier limits the noise suppression. At high frequencies, the bandwidth of the amplifier with low-impedance load limits the perfor-

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The chips were fabricated in a 0.8 pm SiGe HBT technology (4).Bipolar technology provides a higher transconductance than CMOS, allowing a higher bandwidth to be obtained. Fig. 5 shows the noise suppression measured on chip one. In this measurement, the peak height of the spectrum was measured with the amplifier switched on and off, respectively. The difference between the two values is plotted as a function of the noise frequency. The bias voltage was 3 V and the amplifier current was 4.5 mA throughout the measurements. The measured 3 dB bandwidth is as high as 400 MHz. A further increase in bandwidth could be achieved with higher current. In order to test our active guard rings with an actual circuit, we measured the spectrum of the ring oscillator sensor (chip two) with the amplifier switched on and off, respectively. Fig. 6 shows these spectra for an external noise power of -4 dBm and a frequency of 100 MHz. As shown in (9,jitter and phase noise due to supply and substrate noise can be well interpreted in the framework of frequency modulation. Therefore, the spectra show sidebands at 100 MHz frequency offset, as expected. Our approach reduces the height of sidebands by more than one order of magnitude (Fig. 6). Fig. 7 shows the frequency dependence of the sideband suppression, which changes from 20 dB at low noise fre-

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(b) Fig. 6 Oscillator spectrum of the ring oscillator: (a) without noise cancellation, (b) with noise cancellation. Incident noise is -4 dBm, 100 MHz. This corresponds well to the measurements on chip one (Fig. 5). The measurement results show the noise suppression capability of our arrangement in mixed-signal

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integrated circuits. This makes the arrangement suitable for suppression of noise spikes at typical clock frequencies and its subharmonics. With higher amplifier currents it is possible to further increase the bandwidth for high-speed applications.

both sensors gave a 3 dB noise suppression bandwidth of 400 MHz. The current consumption is 4.5 mA at 3 V supply voltage. By using this approach, substrate noise in mixed-signal circuits can be effectively suppressed. Possible applications include analog-to-digital converters, low-noise amplifiers and oscillators in a highly integrated environment. 7. Acknowledgment

The authors thank the technology team at IHP for the fabrication of the test chips. 8. References (1) R. Singh, “A review of substrate coupling issues and modeling strategies,” Proc. IEEE CICC (1998), pp. 4910’

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Fig. 7 Measured sideband suppression versus frequency of the noise signal.

6. Conclusions

We have presented a noise cancellation circuit containing two guard rings and a SiGe HBT on-chip amplifier. Using a signal generator, a sinusoidal perturbation was injected into the substrate through a bondpad outside the guard rings. As noise sensing elements a bondpad and a ring oscillator were used. The measurements with

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498. (2) J. P. Z. Lee, E Wang, A. Phanse, L. C. Smith, “Substrate cross talk noise characterization and prevention in 0.35 pm CMOS technology,” Proc. ZEEE CICC (1999), pp. 479-482. (3) K. Makie-Fukuda, S. Maeda, T. Tsukada, T. Matsuura, “Substrate noise reduction using active guard band filters in mixed-signal integrated circuits,” IEICE Trans. Fundamentals, vol. E80-A, February 1997, pp. 313-320. (4) D. Knoll, B. Heinemann, R. Barth, K. Blum, J. Drews, A. Wolff, P. Schley, D. Bolze, B. Tillack, G. Kissinger, W. Winkler, H. J. Osten: “Low cost, 50 GHz fmax Si/SiGe heterojunction bipolar transistors technology with epifree collector wells,” Proc. ofthe 30th ESSDERC, 1998, pp. 140-143. (5) E Herzel, B. Razavi, “A study of oscillator jitter due to supply and substrate noise,” IEEE Trans. on Circuits and Systems-11:Analog and Digital Signal Processing, vol. 46, pp. 56-62, Jan. 1999.

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