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Technical note Low-cost i.c. transducer for medical pressure measurements* Keywo rd--lntegrated- circuit pressure transducer A MANUFACTURER of integrated devices recently made an absolute pressure transducer commercially available; it was originally designed for automotive, air-conditioning and meteorology purposes (ZIAs and HARE, 1972). AS the characteristics of this device were also promising for medical research, we tested and adopted it for pressure measurements in the living body. The size, weight, voltage output and configuration of the instrument, as well as the results obtained with it (comparable with those of transducers several times more expensive), suggest applications impossible with conven- tional transducers. The transducer The National Semiconductor type LX1600A is an accurate, completely field-interchangeable temperature- compensated absolute-pressure transducer. Fig. 2a Transducer LXI 6OOA mounted into a Plexiglass body and connected to a luer-lock fitting Fig. 1 Transducer mounted in plastic housing "Received 20th March 1973 364 It contains, built on a 4 x 3 mm silicon chip, a pressure diaphragm and vacuum reference, a piezoresistive strain sensor in a Wheatstone-bridge configuration, a signal discriminator and conditioner (type 747 at unity gain) and an amplifier and signal processor (type 741 opera- tional amplifier which raises the balanced and compen- sated sensor signal to a nominal full-scale value of 7.5 V d.c.). The pressure diaphragm measures only 2.3 x 1 '6 mm, is 0-025 mm thick and is etched out of one wall of a vacuum reference cavity. The bridge power supply is Zener regulated and temperature compensated with diodes and has internal self-compensation. A gauge factor of about 90 is used (to be compared with 2-5, typical for metals). Medical and Biological Engineering May 1974

Low-cost i.c. transducer for medical pressure measurements

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Page 1: Low-cost i.c. transducer for medical pressure measurements

Technical note

Low-cost i.c. t ransducer for medical pressure measurements*

Keywo rd--lntegrated- circuit pressure transducer

A MANUFACTURER of integrated devices recently made an absolute pressure transducer commercially available; it was originally designed for automotive, air-conditioning and meteorology purposes (ZIAs and HARE, 1972).

AS the characteristics of this device were also promising for medical research, we tested and adopted it for pressure measurements in the living body.

The size, weight, voltage output and configuration of the instrument, as well as the results obtained with it (comparable with those of transducers several times more expensive), suggest applications impossible with conven- tional transducers.

The transducer The National Semiconductor type LX1600A is an

accurate, completely field-interchangeable temperature- compensated absolute-pressure transducer.

Fig. 2a Transducer LXI 6OOA mounted into a Plexiglass body and connected to a luer-lock fitting

Fig. 1 Transducer mounted in plastic housing

"Received 20th March 1973

364

It contains, built on a 4 x 3 mm silicon chip, a pressure diaphragm and vacuum reference, a piezoresistive strain sensor in a Wheatstone-bridge configuration, a signal discriminator and conditioner (type 747 at unity gain) and an amplifier and signal processor (type 741 opera- tional amplifier which raises the balanced and compen- sated sensor signal to a nominal full-scale value of 7.5 V d.c.). The pressure diaphragm measures only 2.3 x 1 '6 mm, is 0-025 mm thick and is etched out of one wall of a vacuum reference cavity. The bridge power supply is Zener regulated and temperature compensated with diodes and has internal self-compensation. A gauge factor of about 90 is used (to be compared with 2-5, typical for metals).

Medical and Biological Engineering May 1974

Page 2: Low-cost i.c. transducer for medical pressure measurements

The piezoresistive elements are the four arms of a Wheatstone bridge and consist of four p-doped (boron) regions diffused into the edged chip of the n-type silicon.

The transducer has a nominal range of 0-1 atm., with the silicon stressed less than 18 ~ of its rupture point at l arm. It will survive 3 atm. excursions.

LX 1600A

4

3

+12V D.C. regulated

- I 4 zero setting " ~

II out

Fig. 2b Connections of power supply, zero setting and output

Calibrations performed with the LX1600A in the physiological range (0-300 Hg) show perfect linearity, an output of --0.75 V per 100 mm Hg, and no detectable hysteresis. This high output permits the use of this trans- ducer with practically any d.c. recorder. The light weight

0~I O0

0~I00

2oo 13~176 STATHAM P23AA

j 2 0 0 J 3 0 0

l

L X 1 6 0 0 A

Fig. 3a Uncorrected calibration of Statham P23AA and NS LX 1600,4

(only 2.8 g) and the extremely small mass of moving liquid make the transducer particularly suitable for implantation, when incapsulated in silicone rubber or something similar.

Some of the electrical characteristics as given by the manufacturer show: Maximum voltage deviation between transducers at zero and full scale: • Temperature coefficient: • mV/deg. C. Hysteresis: • ~ full scale. Deadband error at 1 atm. and25 deg. C: +0.1 ~ full scale.

Application (Fig. 1) In using the LXI600A as a pressure transducer for

medical experiments, we found it advisable to mount the transducer in a plastic housing with a cable inlet and luer-lock pressure inlet. This protects the transducer against rough handling and allows the use of a 3-way stopcock for flushing and zero setting (Fig. 2). The trans- ducer and the pressure inlet are filled with neutral paraffin oil, which stays very well in the pressure cavity. Without oil filling, the measured pressure is the mean value with no or little systolic-diastolic variations. The transducer is insensitive to vibrations and shocks.

Fig. 3b Recording of blood pressure in cat. At t injection of 10 jug of noredrenaline Top curve recording with Statham P23AA. Bottom curve: same blood-pressure recording with LXI6OOA

The output voltage of the transducer at zero pressure is • V. This value has to be backed off and a zero- setting potentiometer has to be provided. We use a 22 kf2 10-turn potentiometer for this purpose (Fig. 2B). Fig. 3, which shows a recording from a cat, compares the Statham P23AA transducer with the LX1600A. There is no obvious detectable difference between the two recordings.

Its low cost and its resistance to relatively rough handling make this item ideal for student laboratories and routine work.

Reference ZIAS, A. R. and HARE, W. F. J. (1972) Integration brings

a generation of low cost transducers. Electronics 45, 83-88.

A. L. DELAUNOIS J. F. & C. Heymans Institute

Ghent, Belgium

Medical and Biological Engineering

H

May 1974 365