Impedance measurement system and method
The impedance of ionic solutions may be determined using a bridge circuit where the ionic solution forms one leg of the bridge circuit and a reference impedance characterized by a reference time constant forms a second leg of the bridge circuit. The bridge is driven by a switched DC voltage waveform. Measurement of the ionic solution is delayed after switching to allow the reference impedance to reach its asymptotic value. The reference impedance may be varied to reduce the reference time constant.
1. An impedance measurement device comprising:
a fluid chamber comprising first and second electrodes in contact with an ionic solution contained within the chamber;
a reference impedance characterized by a reference time constant;
a switched DC voltage source characterized by a voltage source period, the switched DC voltage source driving a bridge circuit, the bridge circuit having a first leg comprising the fluid chamber and a second leg comprising the reference impedance; and
a controller receiving a signal corresponding to a voltage output of the bridge circuit excited by the switched DC voltage and calculating an impedance of the ionic solution based on the received signal, wherein the controller receives the signal after a measurement time period.
2. The impedance measurement device of claim 1 wherein the reference impedance is a resistor in parallel with a capacitor.
3. The impedance measurement device of claim 1 wherein the reference impedance is a variable impedance controlled by the controller.
4. The impedance measurement device of claim 1 wherein the reference impedance is a resistor in series with an inductor.
5. The impedance measurement device of claim 1 wherein the reference impedance is a variable impedance controlled by the controller.
6. The impedance measurement device of claim 1 wherein the voltage source period corresponds to a switching frequency between 200 Hz and 2000 Hz.
7. The impedance measurement device of claim 1 wherein the measurement time period is determined by the controller.
8. The impedance measurement device of claim 7 wherein the measurement time period is greater than the reference time constant by a factor between 3 and 10.
9. The impedance measurement device of claim 7 wherein the measurement time period is greater than the reference time constant by a factor of about 5.
10. The impedance measurement device of claim 7 wherein the controller reduces the measurement time period by varying the variable impedance.
11. A microfluidic device comprising the impedance measurement device of claim 1 .
12. The impedance measurement device of claim 1 wherein the measurement time period is greater than the reference time constant.