A CRITICAL ANALYSIS OF AN INSTRUMENTATION CURRENT SOURCE

摘要

In many measurement systems for instrumentation and control applications, the assumption that an analog circuit configuration is a current source is necessary for the system to meet its performance specifications. There are standard operational amplifier-based configurations easily implemented that will meet this need for most applications. For those instances where the current source assumption must be very precise, careful analysis and testing must be performed to validate the design. Montana Tech and the Idaho National Laboratory (INL) have been collaborating together for many years to develop an advanced technology Impedance Measurement Box (IMB) that is portable and can perform rapid impedance spectroscopy measurements on test batteries. Montana Tech has performed the design and development of electronics of all the various generations of the IMB (1, 2, & 3). The measurement concept for all generations of IMB must use a current source for the excitation signal. For IMB systems that were designed to work with low voltage batteries (12 V or less), the design implementation with a quality power op-amp and 1% resistors worked just fine. The degree to which the design must be extremely close to a perfect current source came to light during the development of the latest generation IMB as the current source interfaces to batteries that are up to 50 V. The issue was source impedance and bias current. Gain accuracy is never a problem as the IMB will simply calibrate that variation away. Bias current and/or source impedance induced by the high voltage test batteries has been shown to corrupt the measurements and could cause excessive thermal stress on the power op-amps used for the current source. The analytical techniques, the computer simulations, and the testing that were done to make the design more robust are presented.