Engineers' Experience: How to Measure High-Precision DC Voltage

　　Traditional optimization techniques have used high-precision amplifier circuits and faster measurement devices. These two are still necessary conditions for achieving the best measurement in the shortest time, but they are not sufficient. The inverse relationship between settling delay time and signal noise depends on the equivalent noise bandwidth of the measurement device drive circuit. The device under test (DUT) and the measurement instrument define the system characteristics, tightly linking the settling delay time and broadband noise.

　　If the circuit bandwidth is zero, the noise will also be zero, and we may be able to make a measurement with one sample, but the circuit will never settle and the DC error will be 100%. Therefore, too low a bandwidth will cause the measurement time to be too long. On the other hand, if the circuit bandwidth is infinite, the settling delay time will be zero, but the broadband noise will also be infinite, and we will not have enough measurements to average. Therefore, the faster the amplifier, the longer it takes to make a voltage measurement with high accuracy.

　　Let's explore this relationship. In a test sequence, the output of the DUT must settle to within a predefined error range after a voltage step. Assuming a unipolar step response, the settling time will be directly dependent on the bandwidth.

　　Every voltage measurement includes broadband noise from the DUT, amplifier, and resistors. Amplifiers contribute voltage and current noise; resistors contribute Johnson noise. Since the filter roll-off is not infinitely steep, the noise becomes less important in the region after the –3dB cutoff. The effective noise bandwidth takes this region into account.

　　For a given broadband noise and effective noise bandwidth, the number of samples required is determined by the measurement tolerance. Basic statistics give the average number of samples required to achieve a 98% confidence level for a given amount of noise. This deviation from the average reflects the repeatability of the single DC voltage measurement. The issues involved in achieving high resolution measurements are numerous and cannot be addressed in this article. Below we will discuss the importance of addressing the overall issues.

　　Settling delay time. If a component in the circuit has settling time issues, it will increase the overall measurement time. Limited slew rate is a common cause. Small signal settling time is always used for calculations. Dielectric absorption is a detrimental issue, so filter capacitors must be selected carefully.

　　Stability targets. These targets can easily be set too small, such as 0.0001%, resulting in a dramatic increase in measurement time. Since the targets are affected by the step size, larger targets should be used when the step size is a small fraction of the measurement dynamic range. It may be necessary to set bandwidths separately for different measurement sequences.

　　Error voltage. The permissible error voltage is often set too small for all measured values. Statistics show that if a Student T table value of 1.6 is used, the measurement deviation will be within the permissible error range 98% of the time.

　　Voltage reference. This can introduce noise. In the case of a D/A converter, this noise can be code dependent.

　　Broadband Noise. Use a high-quality spectrum analyzer to directly measure the broadband noise of a circuit. With the same number of typical circuit noise sources, making precise calculations on paper is tedious and prone to error.

　　Measurement accuracy and resolution. Test engineering practice generally requires that the resolution of the measuring device be an order of magnitude greater than the allowable error, but in fact it is always assumed that the accuracy and resolution of the measuring device are much smaller than the allowable error in actual measurement.

　　Amplifiers. Use low noise op amps in the signal chain. This is a good way to keep the resistor values ​​low, but not so low that the amplifier creates current drive and thermal issues.

　　Cost of test requirements require optimization of traditional slow, high-precision measurements. This technology allows us to minimize measurement time, saving money and also an experiment in test design.

　　The semiconductor industry is at the turning point of 20-bit DC circuit production. The next issue is the need for test engineers with good professional capabilities.