Extended Abstracts of the 2013 International Conference on Solid State Devices and Materials, Fukuoka, 2013, pp886-887
H-2-3
CMOS Common-Mode Filter with Gyrator-C Network
Daisuke Uchida1, Masayuki Ikebe1, Junichi Motohisa1, Eiichi Sano2 and Akira Kondou3
1
Graduate School of Information Science and Technology, 2RCIQE, Hokkaido University, Japan
and 3Japan Radio Co., Ltd.
Kita 14, Nishi 9, Kita-ku, Sapporo, Japan 060-0814
Phone: +81-11-706-7689 Fax: +81-11-706-7689
E-mail: uchida@impulse.ist.hokudai.ac.jp
Abstract
We propose an inductor less common mode filter
with a gyrator-C network. The filter was designed and
fabricated in a TSMC 0.18-m CMOS process. This
filter exhibited a CMRR of 80 dB, output noise voltage
of 103 nV/Hz1/2, cutoff frequency of under 6 MHz. The
total power consumption was 14.8 mW with a 2.5-V
supply, and the chip area was 0.7 x 0.4 mm2.
1. Introduction
We developed an inductor less high-order analog filter
with a gyrator-C network. In an analog baseband signal
converted using a mixer, interference has to be rejected
from the desired signal. Therefore, an analog baseband filter is key component of radio frequency circuits. Examples
of specifications of filters using a Butterworth structure for
various communication standards are listed in Ref.[1]. In
the standards, the cutoff frequencies are from 0.7 to 20
MHz; the order of filters is from 4 to 5; usual noise level of
the filters is from Vrms, Our filter reduced common-mode-noise by active transformer operation.
2. Structure of the proposed circuit
2.1 LC-ladder filter with gyrator-C network
Filters using OPamps such as Bi-quad filters have high
linearity. However, the filter order is limited to 4 and designing higher-order ones is difficult. Large inductance for
LC-ladder filters cannot be achieved by on-chip inductors;
therefore, we designed high-order LC ladder filters with
gyrator-C network, which generates large inductance, by
connecting Gm cells and capacitors. Figure 1 shows the
gyrator-C network which operates as a floating inductor
[2][3][4]. Using the gyrator- C circuits, we can implement
an inductor for the filter with a cutoff frequency of tens of
MHz. Figure 2 shows a differential-input LC-ladder filter
with the gyrator-C network.
2.2 Common-mode noise reduction
When using a differential signal, we should consider
normal/common-mode noise. For common-mode noise
reduction, there are differential Gm-C filters with a common-mode feedback (CMFB) scheme. However, the
CMFB scheme does not actively reduce this type of noise.
It is used to prevent saturation of amplifier output with the
common-mode signal. Therefore, the common-mode rejection ratio (CMRR) is low at about 20 dB.
3. Circuit Configuration
Figure 3 shows the proposed active transformer. By adding
Gm amplifiers, -Gm3 to the ladder structure, we can reduce
common-mode noise. Figure 4 shows the signal flow of the
proposed active transformer. Each port of the differential
signal is inverted and connected to the opposite side. This
cross coupling connection enhances the normal-mode signal and reduces the common-mode one. The equivalent
mutual conductance M is expressed as
 Gm3 
C

M n  p  G
 Gm1  Gm1Gm2 
(1)
2
Gm3
  1
Gm1Gm2
(2)
When we set a larger equivalent mutual conductance,
more noise reduction can be achieved. In Eqs. 1 to 2, when
the Gm3 increases, M increases. However, the inductance L
is related to C, Gm1 and Gm2. Therefore, we should design
the circuit by taking into account the relationship between L
and Gm3. By using this active transformer, simultaneous
LC-ladder filter operation and common-mode noise reduction is possible.
3. Simulation and Experimental result
Figure 5 shows the whole circuit configuration of our
6th order LC-ladder LPF with common-mode noise reduction. This filter based on the above methodology was designed and fabricated in a TSMC 0.18-m 1P6M CMOS
CMOS process. The die micrograph is shown in Fig. 6. The
total area, including the MIM capacitor was 0.4 x 0.7 mm2.
We obtained a simulated CMRR of 80 dB and cutoff
frequency of 6 MHz (in Fig. 7(left)) and simulated output
noise voltage of 103 nV/Hz1/2 at 100-kHz operation (Fig.
7(right)). Figure 8 shows the measured waveforms of normal/common-mode input and filter output. The input
common-mode signal with 100-mV amplitude at
low-frequency operation of only 50 Hz was rejected with
our filter. This means that just low-frequency common-mode noise can be reduced with our filter, which
works as a LPF.
4. Conclusion
We proposed an inductorless CMOS common-mode
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filter with a gyrator-C network. The filter was fabricated in
a TSMC 0.18-m CMOS process. By using the gyrator-C
network, effective high-order filtering could be brought to
the desired band. Moreover, by adding Gm amplifiers to the
ladder-filter structure, we could reduce common-mode
noise. The total power consumption was 14.8 mW with a
2.5-V supply, and the chip area was 0.7 x 0.4 mm2, which
facilitates a low cost design.
Acknowledgment
This work was partially supported by SCOPE and by
VDEC in collaboration with Cadence Design Systems, Inc.
References
[1] V. Giannini, et al., ISBN: 978-1-4020-6537-8, Springer.
[2] T-Y, Lo, et al., ISBN 978-90-481-2410-7, Springer.
[3] F. Yuan, et al., ISBN 978-0-387-76479-5, Springer.
[4] Q. T. Lai, et al., Microwave Symposium Digest (MTT),
(2010).
Fig. 1
Fig. 2
Fig. 3
Fig. 5
Circuit configuration of 6th order LC-ladder LPF with common-mode noise reduction.
Active inductor with gyrator-C network.
Fig. 6
Die photo.
Fig. 7
Simulation results of AC analysis(left) and noise analysis(right).
LC-ladder filter with gyrator-C network
Floating active transformer
Fig. 8 Measured waveforms for input (lower trace) and output (upper
Fig. 4
Signal flow of active transformer
trace).
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