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Dynamically Biased 1 MHz Low-pass Filter
with 61 dB peak SNR
and 112 dB Input Range
Nagendra Krishnapura1 and Yannis Tsividis2
1Celight Inc., Springfield, NJ, 07081, USA.2Columbia University, New York, NY, 10027, USA.
Motivation
N
S S/N
N
S S/N
max. inputmax. input
dynamic biasconstant bias
input level (dB) input level (dB)
outp
ut le
vel (
dB)
outp
ut le
vel (
dB)
� Dynamically biased current mode filter:– 112 dB input range (THD � -40 dB).– Quiescent power consumption of 575µW.
Outline� Principle of distortionless dynamic biasing.
� Third-order Butterworth filter.
� Measurement results.
� Comparison.
� Conclusions.
First order log-domain filter
Itune
Ibias
Ifb
core
C1
iin
+-
Io+(s)
Ii+(s)
Ig /Itune
1 + sC1Vt /Itune
[Punzenberger & Enz ’97]
Linear from ii+ to io+
Ifb
ii+ io+
Ig
� Output noise � Ibias kI2bias; Power diss. � Ibias.
� Ibias Output noise & Power dissipation .
� But, filtered Ibias � t � appears at the output.
Pseudo-differential operation
Ibias
iin
iout = io+ - io-= 2iin * h(t)
io+ = (iin + Ibias(t)) * h(t)
Ibias
-iin
io- = (-iin + Ibias(t)) * h(t)
h(t) h(t)
io+ io-
� Large signal linearity cancellation of Ibias.
Dynamic biasing
0iin
Ibias
0
ii+
= iin
+ Ibias
� Decrease Ibias for small input amplitudes.
� SNR improvement for small signals whilemaintaining linearity and time-invariance.
� Output noise depends on total input signal.
Third-order Butterworth filter
1 + s/ωp
12s/ωp
11 + s/ωp
1ΣΣ+ +
-
-in out
Ibias
iin
Ibias
-iin
Itune
C2
Ibias
-iin
+-
coreio+ io-
iout = io+ - io-
io-
Minimizing distortion due to Early effect
Ifb
Ibias
iin
Ibias
iin
Qf
MfCc
ii+ ii+
� Large gm for Ifb to reduce distortion due to Earlyeffect.
� Cc: pole-zero compensation.
Feedback paths in the filter
1 + s/ωp
12s/ωp
11 + s/ωp
1ΣΣ+ +-
-
Qa QoutQfb
QaQfb
QoutQa
� Separate emitter follower for the feedback pathensures frequency response integrity.
Chip photograph
0.52 mm2active circuitcapacitor (0.19 mm2)
Measurement conditions� Vdd = 2.5 V.
� Itune = 5µA.
� Ibias: 3µA to 2.5 mA.
� “Dynamic” bias: Ibias = 2� the single-ended inputpeak, subject to a minimum of 3µA.
� 400 kHz input for THD measurements.
� 1 MHz 20 kHz inputs for IM3 measurements.
Measured results: Frequency response
0 1 2 3 4 5−60
−50
−40
−30
−20
−10
0
10
f / MHz
dB
signal freq.response
bias leakage
Ibias
from 3 µA to 2.5 mA
0 0.25 0.5 0.75 1
−4
−2
0 930 kHz
f / MHzdB
Passband detail
Measured results: Noise
10−9
10−8
10−7
10−6
10−5
10−4
10−3
10−210
−9
10−8
10−7
10−6
10−5
10−4
10−3
10−2
differential peak input / A
rms
outp
ut (
A)
noise floor= 4.4 nA
rms
diff. rms noise(0−2MHz)
diff. rms output
Measured results: Distortion
10−9
10−8
10−7
10−6
10−5
10−4
10−3
10−2
0
20
40
60
80
differential peak input / A
dB S / N
S / THD
S / IM3
2.5 mApk
6.2 nApk
Input range = 112 dB
Output with dynamically changing bias
114µAdynamic bias
differential0
1µs
40µA
24µA0
output iout
Ibias � t �
� 20µA single-ended peak input @ 600 kHz.
� Ibias switched from 24µA to 114µA.
� Barely perceptible change in the output.
Measured results: Power consumption
10−9
10−8
10−7
10−6
10−5
10−4
10−3
10−210
−4
10−3
10−2
10−1
575 µW
differential input peak / A
Cur
rent
con
sum
ptio
n / A
0.25
2.5
25
250
Pow
er c
onsu
mpt
ion
/ mW
Summary
Technology 0.25µm BiCMOSArea (excl. pads) 0.52 mm2
Supply voltage 2.5 V-3 dB BW 930 kHzIbias 3µA 2.5 mAPower diss. (Pd) 575µW 26.1 mWOutput noise 4.4 nA 1.5µATHD (input peak = 0.5Ibias) -64.3 dB -41 dBIM3 (input peak = 0.5Ibias) -39.5 dB -30 dBInput range (THD 40 dB) 112 dBMaximum Pd / Order � BW 9.35 nJ
Comparison
50 60 70 80 90 100 11010
−14
10−13
10−12
10−11
10−10
10−9
10−8
10−7
This work
Input range [dB]
Nor
mal
ized
pow
er d
issi
patio
n / J
Previouslypublished filters
first−orderpassive RClow−pass
Conclusions� A technique for dynamic biasing without
disturbing the output.
� Noise and power reduction for small inputsignals.
� A log-domain filter with 112 dB input range in a0.25µm BiCMOS technology.
� Quiescent power: 40� smaller than maximum.
� Over an order of magnitude improvement inpower efficiency of analog filters.
