IMTC

1
IMTC 2006 Sorrento, Italia, April
24  27, 2006 A Method of Synchronous Sampling
in Multifrequency Bioimpedance Measurements Mar
t Min, Toomas Parve, Paul Annus, and Toivo
Paavle Department of Electronics, Tallinn
University of Technology, Estonia
_________________________________________________
______________________________________

2
IMTC 2006 , Sorrento 2
Mart Min, et al. A Method of Synchronous
Sampling in Multifrequency Bioimpedance
Measurement
Simultaneous multichannel and multifrequency
measurements
Why multichannel ? Why multifrequency
? Why simultaneous ? Examples of
electrical bio-impedance (EBI) measurement in
cardiology.
a) b) c) d)
noninvasive plethysmography
multielectrode invasive estimation of
the ventricular volume
intracardiac impedance based pacing control
biomodulation
3
IMTC 2006 , Sorrento 3
Mart Min, et al. A Method of Synchronous
Sampling in Multifrequency Bioimpedance
Measurement
Electrical Bioimpedance (EBI)
What is specific in the bioimpedance (EBI) ?
a) Essentials of the EBI
b) The 3-element equivalent of the static EBI
c) The phasor diagram of the static EBI
Fig. 1. The essentials of the EBI (a), the
3-element equivalent of the static EBI (b), and
the phasor diagram (c) of the static EBI for two
frequencies, low ?l and high ?h. Frequency
response of the EBI Z(f) is also called
impedance spectrum.

4
IMTC 2006 , Sorrento 4
Mart Min, et al. A Method of Synchronous
Sampling in Multifrequency Bioimpedance
Measurement
EBI measurement using synchronous sampling
The direct current component DC can be
determined as DC (Re Re) / 2 or
DC (Im Im) / 2 The real Re and
imaginary Im parts of the phasor Z are
determined as Re (Re  Re) / 2 and
Im (Im  Im) / 2
Fig. 2. Synchronous sampling of a single sine
wave response. Real part samples Re are
designated as filled red dots ?and Re as
unfilled red ones ?, imaginary part samples Im
as filled green squares , and Im as unfilled
green squares ?

5
IMTC 2006 , Sorrento 5
Mart Min, et al. A Method of Synchronous
Sampling in Multifrequency Bioimpedance
Measurement
Two typical cases of measurement
multisite multifrequency
non-uniform synchronous sampling
a)  Two different impedances are measured
b)  The same impedance is measured
       at a slightly differing frequency
at (two)
essentially different frequencies
Fig. 3. Simultaneous measurement of responses to
two excitations Note Only the Re samples are
shown for the response signal.
6
IMTC 2006 , Sorrento 6
Mart Min, et al. A Method of Synchronous
Sampling in Multifrequency Bioimpedance
Measurement
Why to use the non-uniform sampling at
simultaneous measurement?
f1 / f2  6 / 5 tmeas  6 / f1  5 / f2
Fig.3. Simultaneous measurement of the responses
to two excitations with different frequencies f1
and f2

7
IMTC 2006 , Sorrento 7
Mart Min, et al. A Method of Synchronous
Sampling in Multifrequency Bioimpedance
Measurement
Piecewise uniform synchronous sampling at
sequential measurements (time division mode)
f1 / f2  6 / 5 tmeas  12 / f1  12T1 ,
where T1 1 /  f1

8
IMTC 2006 , Sorrento 8
Mart Min, et al. A Method of Synchronous
Sampling in Multifrequency Bioimpedance
Measurement
Time division mode measurement with channel
multiplexing
Synchro is important !
9
IMTC 2006 , Sorrento 9
Mart Min, et al. A Method of Synchronous
Sampling in Multifrequency Bioimpedance
Measurement
Measurement in time division mode with channel
multiplexing
Fig.5. Re sampling moments for two input
channels and two frequencies in each
first f1 is measured in both channels one by
other (red samples), then f2 is measured (green
samples) in the same way (time division mode).

10
IMTC 2006 , Sorrento 10
Mart Min, et al. A Method of Synchronous
Sampling in Multifrequency Bioimpedance
Measurement
Digitizing and Digital Processing of Response
Signal


11
IMTC 2006 , Sorrento 11
Mart Min, et al. A Method of Synchronous
Sampling in Multifrequency Bioimpedance
Measurement
Design example
The modular arrangement is based on the Field
Programmable Gate Array (FPGA) Spartan-3
(Xilinx). The FPGA ? selects channels, ?
generates sampling pulses, ? controls gain, ?
generates excitation and compensation codes, ?
reads samples from digityzers (ADCs), ?
performs math functions, and ? provides
communication with the outside world.
Fig.6. The FPGA based DSP unit block diagram
with I/O connectionsand peripherial components

Fig.7. A photo of the prototyped DSP unit
12
IMTC 2006 , Sorrento 12
Mart Min, et al. A Method of Synchronous
Sampling in Multifrequency Bioimpedance
Measurement
The Measurement Unit
The bio-modulation istypically about 1,
frequently  0.01 (100µV peak-to-peak).
1)  The multiplexed response signal is
sampled and digitized by ADC1. 2)  The
digital samples are converted back to analog
signals using DAC1. 3)  The output of DAC1 is
subtracted from the response signal, the result
gained128 times (gives 7 additional bits), and
digitized by ADC2.

Fig. 8. Functional diagram of the bioimpedance
measurement unit
13
IMTC 2006 , Sorrento 13
Mart Min, et al. A Method of Synchronous
Sampling in Multifrequency Bioimpedance
Measurement
Experimental results
Fig.9. Output code of the 12-bit ADC1,
LSB488 µV
Fig. 9 shows variations of output code of the
ADC1. A 0.01 triangular amplitude modulation
(100µVpeak-to-peak value) of 2Vpeak-to-peak
carrier was used for testing.
0.01 of carrier
"1299"
Fig.10. Output code of the 12-bit ADC2, LSB7.7
µV
"1298"
Fig.11. Digital raw signal stored
in the FPGA
14
IMTC 2006 , Sorrento 14
Mart Min, et al. A Method of Synchronous
Sampling in Multifrequency Bioimpedance
Measurement
Conclusions
The designed bioimpedance measurement unit is
qualified for multichannel noninvasive diagnosing
of cardiovascular system (impedance
cardiograph ICG), but can find much wider use.
The method ensures fast and precise digital
signal processing, enabling to demodulate 0.01 
bio-modulation of carrier signals up to 1 MHz
and higher frequencies.
Thank you for your attention !
__________________________________________________
____ C o n t a c t Prof. Mart Min, Department
of Electronics, Tallinn University of
Technology, Phone [phone], Fax 372
6202151, Email min_at_edu.ttu.ee Ehitayate tee
5, 19086 Tallinn, Estonia