Circuit for measuring ion concentrations in solutions

Details Circuit for measuring ion concentrations in solutions Circuit for measuring ion concentrations in solutions

1995-04-24

1997-02-11

Regan, Maura K.

Electricity: measuring and testing

Determining nonelectric properties by measuring electric...

327307, G01R 2726

Patent

active

056024673

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a circuit layout for measuring ion concentrations in solutions using ion-sensitive field effect transistors (ISFETs).
There is an increasing need for circuit layouts for determining ion concentrations and, in particular in the biomedical field, circuit layouts which can be integrated into a very small space are required, for example, for performing measurements on blood or urine.
The function of chemical sensors, on the basis of ion-sensitive field effect transistors, is based on the change to the threshold voltage of an ISFET as a function of the ion concentration in liquid electrolytes. The function of a circuit layout for determining the ion concentration with ISFETs is therefore limited to a threshold voltage measurement. The difficulty encountered is, inter alia, that with real ISFETs the desired sensor signal also has a strong dependence on different ambient influences and is also influenced by threshold voltage drifts caused by age.
A widely used method for determining the ion concentration in solutions is to measure the threshold voltage difference between an ISFET and a reference MOSFET, which operate in a negative feedback differential amplifier configuration. However, with this method it is only possible to compensate the parasitic threshold voltage influences, which are caused by the MOSFET configuration, so that, for example, external influences acting differently on the MOSFET and ISFET lead to erroneous measurements (operating point dependence of the threshold voltage). It is also disadvantageous that the known circuit layout can only operate in a narrow range around the prescribed operating point, so that it is not possible to compensate technological and operation- ally caused parameter fluctuations. Much better results can be obtained if the threshold voltage difference of two ISFETs having different sensitivities is evaluated. The use of ISFETs having different sensitivities presupposes the technical control of differently sensitive layers during the manufacturing process (cf. e.g. Isemi Igarashi et al: Multiple Ion Sensor Array, Sensors and Actuators, B1 (1990), pp.8-11). The measurement of this threshold voltage difference is not possible in the above-described manner in a simple differential amplifier, because the two ISFETs have a common gate connection in the electrolytic solution.
The subtract amplifier configuration described by A. Sibbald: A Chemical-Sensitive Integrated Circuit: The Operational Transducer, Sensors and Actuators, 7, 1985, pp.23-28 and specifically described on page 27 therein, suffers from the disadvantage that further circuit-caused quantities enter into the measured result and that it is necessary to calculate back from the output signal of the circuit, via the operating point current, to the actual sensor signal. Another disadvantage is that the two FETs are necessarily operated at different operating points, so that ambient influences cannot be compensated with the circuit.
German Patent document DE-A-3 216 791 discloses a circuit layout for measuring the ion concentration using ISFETs with different sensitivities. In this reference, the drain current of the ISFETs is kept constant by readjusting the gate voltage and the necessary voltage change for this is evaluated. However, the proposed circuit layout suffers from the disadvantage that disturbance variables, which can, for example, be caused by the amplifier or by operationally caused or technological tolerances, cannot be compensated. The prerequisite necessary for the error-free function of the circuit layout, i.e., that the difference in the transconductances of the ISFETs must be constant, cannot be ensured in the case of real ISFETs, so that this gives rise to an additional error source.
There is therefore needed a circuit layout with which it is possible to make available the threshold voltage difference of two ISFETs, having either the same or different sensitivities, for the ionic species to be measured in a direct manner independent

REFERENCES:
patent: 4385274 (1983-05-01), Shimada et al.
patent: 4754169 (1988-06-01), Morris

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