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SUSS'97: Noninvasive diagnostics with electrical impedance.
M.Min: Lock-in EBI measurement techniques.
APPENDIX A1
CURRENT MODE SIGNAL PROCESSING AS A CHALLENGE
FOR IMPROVEMENT OF LOCK-IN MEASURING INSTRUMENTS
Mart Min and Toomas Parve
Institute of Electronics, Tallinn Technical University, Tallinn, Estonia
Abstract: Modern lock-in analysers enable to measure low level AC signals in the presence
of non-coherent noise, exceeding the useful signal 100,000 times and even more within the
input dynamic range of 140 dB (24 effective bits) at the frequencies up to 100 kHz. But the
operating frequencies up to 50 MHz can be reached using mixed analog and digital signal
processing and current mode operation. The latter approach enables also to reduce current
consumption and to use low voltage power supply in mobile and micropower instruments like
an on-chip lock-in bioimpedance analyser for implantable pacemaker.
Keywords: lock-in measurements, current mode signal processing, phase locking.
1 INTRODUCTION
Lock-in instruments (synchronous or phase sensitive amplifiers, and signal or network
analysers) are commonly used for measurement of low level AC signals (nV and V) in the
presence of non-coherent noises. In most of applications lock-in measurement systems consist
of an excitation signal Sexc source and a synchronous detector SD (Fig. 1), driven by the
inphase (0) or quadrature (90) reference signal Vref from the same excitation oscillator. As
the interference can far exceed the useful signal (e.g. 100,000 times), the dynamic range of the
input signal reaches even 140 dB (24 effective bits).
Vin
Sexc
Excitation
oscillator
System
under
test
SYNCHRONOUS DETECTOR (SD)
Vout
LPF
Amp
V ref
Digital
Post
Processing
Data
out
0/90
Figure 1. A lock-in measurement system.
Actually this value is achievable at the frequencies up to 100 kHz and a bit higher [1]. The
problem is how to maintain the input dynamic range at much higher frequencies. Direct
analog-to-digital conversion and following digital signal processing (DSP) is not an effective
solution in this case, though the DSP has given quite good results [2].
In conditions of limited space and power, the well-balanced mixed analog and digital
signal processing will certainly give far better results [3], in particular, if the current mode
signal processing is used, i.e. currents, not voltages, are used to carry information.
2 CURRENT MODE SIGNAL PROCESSING AND LOCK-IN INSTRUMENTS
Current mode signal processing is not a new technique concerning the high frequency lockin amplifiers [4]. It has been used also in low frequency lock-in amplifier with unprecedented
success [5] already in 70-ies, but is coming into more wider use only nowadays after appearance of commercially available current mode microcircuits.
______________________________________
This work was supported by the Estonian Science Foundation under the grants 573 and 2123.
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SUSS'97: Noninvasive diagnostics with electrical impedance.
M.Min: Lock-in EBI measurement techniques.
SYNCHRONOUS DETECTOR (SD)
Transconductance
Polarity Switch
Amplifier
+
Vin
I+
Sw1
I
VCC
+
I–
–
–
Gain
control
Vref
Vout
CCC
Iout
S1
S1
to Digital
Post
Processing
LPF
Reference
Channel
Figure 2. Circuit diagram of a current mode lock-in amplifier.
An example of the current mode lock-in amplifier in Fig. 2 contains a transconductance
amplifier based voltage-to-current converter VCC at the input, a polarity current switch Sw1,
driven from the reference channel, and a differential current-to-current converter CCC, which
is also based on transconductance amplifiers. The rectified current Iout is smoothed and
converted into the output voltage Vout by a low pass filter LPF.
The current mode circuits are not the best ones everywhere and in all qualities. But the
superiority of current mode circuits over the voltage mode ones is evident at higher operating
frequencies. The linear current mode ICs have enabled to achieve the frequency limit of some
hundreds MHz [6, 7] but the limit for synchronous detection (some tens of MHz) is
determined mainly by the current switching process, which takes a few nanoseconds.
The transconductance amplifiers or "diamond transistors" are the main building blocks of
current mode circuits. These complementary symmetrical bipolar or CMOS voltage controlled
current sources (Fig. 3, a and b) have almost ideally symmetrical transfer characteristic.
V+
a)
V+
b)
J
J
E
B
C
S
G
D
J
J
V–
V–
C
c)
B
V–
B, G
E
D
C, D
V+
V+
G
E, S
V–
S
Figure 3. Circuit diagrams of basic bipolar (a) and CMOS (b) diamond transistors, and usable symbols (c).
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SUSS'97: Noninvasive diagnostics with electrical impedance.
M.Min: Lock-in EBI measurement techniques.
3 CURRENT MODE SIGNAL PREAMPLIFIER
3.1 Gain and errors
The current mode amplifiers are very similar to the old two-transistor (pnp and npn) noninverting amplifier units known from 60-ies. But thanks to full symmetry of nowadays integral
circuits (Fig. 3a and 3b) they can be considered as the almost ideal nonpolar diamond
transistors (Fig. 3c). The gain (transconductance) of the diamond transistor circuit can be
determined by only one emitter (or source) resistor RE, which causes the current depending
negative feedback (Fig. 5a).
The special feature of lock-in instruments containing the synchronous or phase-sensitive
detector is importance of the phase stability besides of the amplitude accuracy, especially in
the case of vector components (inphase and/or quadrature) measurement.
Nearly in every measurement instrument there is a need to change the preamplifier gain
correspondingly to the input conditions (signal level, signal-to-noise ratio, character of
interference). For the lock-in instruments there is an urgent need to maintain the input and
output phase relationship independent on gain settings in a wide operating frequency range.
This demand can be very hardly answered using conventional voltage mode amplifiers, which
have the constant gain bandwidth product (G  BW = const). But in current mode circuits this
product has essentially non-constant value, rather the bandwidth is almostly constant and
independent of gain settings [6, 7]. This is a very important advantage of current mode
technique, which has made possible to widen more than ten times the higher frequency limits
in lock-in instruments [3].
3.2 Dynamic range and noises
The noise level determines the minimal discernible signal at a given system bandwidth, which
can be 1 Hz in the highly selective lock-in instruments. Between the minimal discernible
signal value and the overload level lies the input dynamic range of the instrument (Fig. 4).
Input overload
1 mA (1 V)
Presumable
input range
max. 200 dB
(36 bits)
150 dB
current mode
(PAR186A)
1 A (1 mV)
100 dB
18-bit DSP
(SR850)
1 nA (1 V)
180 dB
practical limit
for nowadays
current mode
signal
processing
(32-bit DSP)
120 dB
dynamic
reserve
(20 bits)
Full scale
4060 dB
812 bits
incl. +/– bit
10 pA (10 nV)
1 pA (1 nV)
100 fA
Minimal discernible/detectable level
Figure 4. Dynamic ranges of lock-in amplifiers.
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SUSS'97: Noninvasive diagnostics with electrical impedance.
M.Min: Lock-in EBI measurement techniques.
The thermal Johnson noise having about 1 nV level in the 1 Hz bandwidth is unavoidable
in voltage mode circuits, and near to 10 pA Schottky shot noise is characteristic to the current
mode circuits with the same bandwidth. The maximal signal level is limited with some volts
and some milliamperes. Therefore the presumable input dynamic range can extend principally
up to 200 dB or 1010 times maximal signal-to-noise ratio value, which corresponds to the 36bit digital range, including +/- sign and overload indication bits.
In practice the best commercially available fully digital lock-in instrument SR850 [2]
covers only 100 dB range, and the dynamic range of the best fully analog current mode
instrument PAR 186A [5] has 150 dB value within the 100 kHz range.
Experiments and practical considerations make it possible to assume that balanced
combination of current mode analog and digital signal processing [3, 8] can give us a
possibility to cover 180 dB (32-bit) dynamic range. After using the 40 to 60 dB area (from 8
to 12 bits) for the full scale range, wholly 120 dB remains for the dynamic reserve, which
corresponds to the capability to suffer up to 106 times overloading of the full scale signal level
by non-coherent noises and disturbances.
4 CURRENT MODE SYNCHRONOUS DETECTOR
There are several reasons for implementation of the current mode technique in synchronous
detectors (SD), but the main one is connected with the obtained high speed operating, which
enables to reduce the frequency response errors of the SD.
The best diamond transistors like OPA660 (Burr-Brown) are able to work normally up to
several hundreds of MHz [6]. But the SD has about ten times lower frequency limit because of
the time delays in the switches (Fig. 5, 6, and 7). Despite of the fact that transient process lasts
only a pair of nanoseconds in current switches the driving delays cause the most essential
errors at the frequencies higher than 10 MHz (Fig. 8).
The simplest switching type SD in Fig. 5a has an unadvised response to the higher
harmonics of the input signal (see the upper drawing in Fig. 5b). Using of two-value (Fig. 6a)
or three-value (Fig. 6b) code controllable switched resistors instead of the constant value
emitter resistor RE in Fig. 5a, one can get the far better harmonic response of the SD, see the
lower drawings in Fig. 5b and Fig. 6c. An analogous, but fully current mode solution with the
three constant value weighting resistors and the three switched currents, is given in Fig. 7.
The discrete approximation levels can be calculated as follows [8]:
gj = G0sin[(/4m)(2j - 1)]
(1)
where j = 1, 2, 3, … m is the number of the level to be calculated, and m is the total number
of approximation levels.
The harmonic content of the SD sensitivity is exactly the same as the spectrum of the SD
transconductance variation waveform, and can be calculated [3] using a simple equation
h = (4m) i  1
(2)
where h is the number of existing harmonics, m is the number of approximation levels, and
i = 1, 2, 3, … is the current integer.
The same current switching technique can be used for generation of the excitation signal
(Fig. 1). In this case it is important to underline that approximation of the harmonic waves in
excitation generator and the SD should be different. The errors will be caused only by those
higher harmonics hC, which are coinciding in the both waveforms:
hC = (4mn) i  1
(3)
where m and n are the numbers of approximation levels in the different waveforms, e.g. for
the case m = 2 and n = 3, hC = 23 and 25, 47 and 49, etc., see Fig. 7b.
SUSS'97: Noninvasive diagnostics with electrical impedance.
M.Min: Lock-in EBI measurement techniques.
Figure 5. Circuit diagram of a simple current mode switching SD (a), and the spectra (b) of its
transconductance variation (m=1), and of a two-level waveform (n=2).
Figure 6. Circuit diagram of the 2-value (a) and 3-value (b) code controllable resistors, and the
spectra of waveforms (b) approximated by two and three levels (m=2, and n=3).
Figure 7. Circuit diagram of a multiple current switching SD (a), and the spectra (b) of its
transconductance variation (m=3), and of a two-level waveform (n=2).
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SUSS'97: Noninvasive diagnostics with electrical impedance.
M.Min: Lock-in EBI measurement techniques.
Errors, % of full scale
3
Total error
2
Offset error
1
0
10 k
100 k
1M
10 M
100 M
Frequency, Hz
Figure 8. Frequency response of total and output offset errors of a current mode synchronous detector
with OPA660A diamond transistors, enhancement n-MOS switches, and ACT MOS logic.
CONCLUSIONS
The current mode signal processing enables to accomplish different lock-in instruments for
measurement of low level AC signal in the presence of non-coherent noise, which can exceed
the useful signal even more than 100,000 times. The input dynamic range reaches 140 dB (24
effective bits) at low frequencies, and maintains at high level up to 30 MHz thanks to use of
balanced analog and digital co-processing of measurement signals.
Using a current, not a voltage, as an information carrier, one can obtain very fast and
adequate frequency responses, what is extremely important for phase-sensitive measurements.
This current mode approach is very suitable for low voltage (2.5V power supply) and low
power units for mobile and other specific instruments like an on-chip micropower lock-in bioimpedance analyser for implantable pacemakers.
The frequency limit of 50 MHz is achievable with some loss in accuracy. This gives a
possibility to cover the whole - and -dispersion regions in bio-impedance measurement [9].
Finally one can conclude that current mode signal processing offers an exciting challenge
for development of lock-in and phase-lock measurement technique.
REFERENCES
[1] M.L. Meade, Lock-In Amplifiers: Principles and Applications, 2nd ed., Peregrinus, London, 1989, 232 p.
[2] Model SR850 Digital Lock-In Amplifier, in: SRS Scientific and Engineering Instruments, Stanford Research
Systems, Stanford, 1994, p. 66-75.
[3] M. Min, T. Parve, A. Kuusik, Current mode signal processing in modern lock-in amplifiers, in: Proc. of the
IMEKO TC-4 8th International Symposium on New Measurements and Calibration Methods of Electrical
Quantities and Instruments, (Budapest 16-17 September 1996), Hungary, 1996, p. 49-52.
[4] High Performance, Wide Bandwidth Dual Phase Lock-in Amplifier Model 5302, in: Signal Recovery Instrumentation. Product Catalogue 1994, EG&G Instruments, P.A.R.C., Princeton, 1994, p. 28-30.
[5] Model 186 Synchro-Het Lock-In Amplifier, in: Lock-in Amplifiers, Preamplifiers, Accessories. EG&G
P.A.R.C., Princeton, 1973, p. 14-17.
[6] Burr-Brown IC Applications Handbook. Burr-Brown Corporation, Tucson, Arizona, U.S.A., 1994, 426 p.
[7] C. Toumazou, F.J. Lidgey, D.G. Haigh (ed.), Analogue IC design: the current-mode approach, Peregrinus,
London, 1990, 646 p.
[8] M. Min, T. Parve and A. Ronk, Design Concepts of Instruments for Vector Parameter Identification, IEEE
Trans. on Instrumentation and Measurement, IM-41 (1) (1992) 50-53.
[9] M. Min, T. Parve, Current Mode Signal Processing in Lock-in Instruments for Electrical Bio-impedance
Measurement, in: Proc. 1st Int. Conf. on Bioelectromagnetism (ICBEM'96), (Tampere 9-13. June 1996),
Finland, 1996, p. 167-168.
Contact point: Mart Min, Institute of Electronics, Tallinn Technical University, Ehitajate tee 5, EE-0026 Tallinn,
Estonia, Phone Int +372 6202150, Fax Int +372 6202151, E-mail: min@edu.ttu.ee.