Page 64


www.us-tech.com


September, 2021


Accurately Measuring Extremely Low Capacitance Values


By Michael Obrecht, Ph.D., Siborg Systems


response and bridge methods. The first method is only applica- ble to RC while the latter two to LCR measurements. The first method is imple-


T


mented by charging and dis- charging the capacitor with a known current and measuring the rate of rise of the resulting voltage; the slower the rate of rise, the larger the capacitance. The response method to


measure capacitance is done by passing a known high-frequency alternating current through the device and recording the result- ing voltage across it. From the ratio of these the magnitude of the impedance is calculated. The phase angle between


the voltage and current is also measured in combination with the impedance, the equivalent capacitance or inductance, and resistance can be calculated. More sophisticated instruments use other techniques such as a


here are three main ways of measuring capacitance: DC charge/discharge, AC


bridge circuit where the meas- ured component is put in the bridge circuit. By varying the values of the


other leg of the bridge the value of the unknown impedance is deter- mined. This indirect method of measuring impedance ensures a


very good precision. The bridge usually can also measure parasitic resistance along with capacitance and inductance. Siborg’s LCR-Reader-MPA


offers only the DC charge/dis- charge and AC response meth- ods. The first method is more


efficient in the range from 1 mF to 1 F while the other demon- strates a superb 0.1 percent basic accuracy for capacitance values from 0.1 pF to 1 mF.


Multimeter Operation As seen in the block dia-


gram, during operation of the LCR-Reader-MPA, voltage from the voltage source through a lim- iting 100W resistor is applied to the DUT connected at points A and B. The amplitude and fre- quency of the test signal are adjustable. It is also possible to apply


either positive or negative DC voltage to the DUT. A voltage drop on the DUT is measured by DAu. The voltage drop on resistor Rj measured by DAj is proportion- al to the current flowing through the measured component. After digitizing the ADC sig-


Siborg System’s LCR-Reader-MPA, an all-in-one multimeter.


nals the impedance is calculated according to the formula DUT impedance Z = Rj* Vau/Vaj. Initial values of Impedance (off-


Continued on next page


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100