The Design Of A Measurement Circuit For Small Capacitance Change

Modern capacitive sensors are widely used in daily life and provide greatconvenience to our lives. Lots of Capacitance measurement system are developedalready in order to be applied to different application environments. Furthermore，Withthe rapid development of capacitive sensor，the measurement system of capacitancesignal has put forward higher and higher requirements，good thermal stability，highresolution，good dynamic response，and can work normally under the condition ofenvironmental change. At the same time，for the purpose of realizing automaticmeasurement，and convenient carrying，digital capacitance measurement method can beused widely.Time to Digital conversion technology is to digitize the delay time of internal logicgate. The very short charge and discharge time can be measured accurately by thismethod. In the paper，the project has adopted this measure method with digitalconversion technology， to calculate the capacitance indirectly by measuring thedischarge time. Also a hardware test platform had been already developed successfullywith the core chip PS021and the MSP430microcomputer. The main conclusions are asfollows:(1) After using the Time to Digital conversion technology (TDC), we can measuresmall capacitance’s changes and get the average results after repeated measurements.The capacitance’s accuracy can reach to fF magnitude.(2) In the measurement of small capacitance, we use large discharge resistor toimprove the discharge time, that we can measure discharge time and capacitanceaccurately.(3) After using fixed reference capacitance and the measured capacitance,measured results can indicate the influence of parasitic capacitance, and themeasurement results can be corrected at the last.(4) The measured capacitance and the reference capacitance should be in the samerange, otherwise the difference of discharge time is too large to be measured accurately.