Traditional bridge circuit solutions measure capacitance by including the measured capacitor in a bridge where the balancing elements are known and accurate. Because alternating current is used, this approach is inapplicable to certain capacitor types (e.g., tantalum and aluminum electrolyte) in which a thin layer of metal oxide serves as the dielectric. These oxides exhibit certain semiconductor properties that naturally affect the accuracy when AC is flowing.
When designing measurement equipment, it’s important to assert all known sources of error and design ways to eliminate or at least minimize them. It will be worth your time to plan ahead for possible problems.
This project involves extremely sensitive circuitry, so a component parameter gone astray can throw off the measurements. Fortunately, there are ways to account for the sources of system error. The current source, for example, may be rock steady but at the wrong value because of an incorrect value of the setting resistor. Thus, the introduced error will be linearly proportional to the deviation. You can compensate for it by adjusting the value of the current source resistor.
Another possible source of error is a nonlinear response of the current source within the required operational voltage range. If the current source isn’t up to the task, it will cause inaccuracies. To prevent this from happening, we designed our current source to operate well within a safe voltage range. We implemented the current source in the CY8C27443 after reading Dave Van Ess’s “AN2089: Programmable Bipolar Analog Current Source. PSoC Style.” Everything remained under the same roof as the rest of the device.
Some implementations of a current source are frequency dependent. With such circuits, charging the capacitor too quickly (fast slew rate) may lead to significant errors in the current supplied. Because we charge large capacitors with a low current, our device operates well outside this speed range and isn’t susceptible to this type of error. It’s possible that the DAC reacts differently to positive and negative settings. Because the DAC sets the current source operating voltage, such a difference would introduce an error. As a safety precaution, we measure both the charging and discharging time of CX and average the two readings to obtain a final result.
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