Measuring currents with extreme dynamics using auto-ranging technology

The high-resolution ADC technology used across the board in measurement technology already allows considerable dynamic ranges to be covered with a fixed scale design. Despite that, sensors, signals and the physical processes to be examined often encompass such wide spans that an adapted range selection (for example through pre-amplification) is necessary in order to get the maximum use from the technology.

Whereas the range is fixed before the test in classic methods, auto-ranging methods enable dynamic adaptation during the running measurement. This is useful if one doesn't know the expected course or if the test covers the entire dynamic range and should also completely record it.

If one wishes to attain range dynamics that extend far beyond the ADC resolution (24-bit, for example), one can dynamically increase the gain, at least in the case of voltage measurements. For current sensing with a shunt resistor however, the task is more demanding: it is necessary here to adapt the shunt during the measurement – without interrupting the measuring circuit or adversely affecting the continuous measurement in the transition area.

A typical application scenario is, for instance, a single continuous test measurement that analyses a complete power-up cycle: From tiny leakage current in sleep mode, through surge and inrush at the start, to high-power work areas – and back again. In addition, the measurement technology must be non-reactive. Therefore, it may not affect the test object and should provide optimum data resolution and accuracy in all work areas. There is of course a great need for this in the automotive sector, for instance in a test field that examines a large variety of components and control devices from a vehicle and their complex interplay in the on-board electrical system. Moreover, the development and testing of mobile devices with battery and wireless technology and even energy harvesting are faced with similar tasks.

The heart of the measuring module from imc is formed by the measuring current path, in which a low-impedance 2 mΩ shunt designed for the maximum current is always active. A second, high-impedance 2 Ω shunt in series can still cleanly capture even the smallest currents, but is dynamically bypassed by means of fast switches as soon as the present working current exceeds a threshold of around 100 mA. The voltages across both shunts are measured by a 30 kS/s ADC with 24-bit resolution and correctly selected, scaled and calibrated by a processor. Output data is delivered via CAN-bus with selectable data rates of between 1 Hz and 1 kHz. Output variables, apart from mean values, are also minimum and maximum values within the selected output interval. These are derived on the basis of the internal data rate of 30 kHz.

The switching of the shunt must take place much faster than is necessary for the actual measurement: Suddenly increasing current must instantaneously short-circuit the high-impedance shunt in order not only to prevent it from burning up, but also to avoid a load-voltage peak in the load circuit. As an (albeit transient) voltage drop in the load circuit, this would influence or even disconnect the load components to the tested.
Therefore, the activation of the shunt bypass is controlled by a fast comparator, but renewed activation of the high-impedance shunt is only permitted with a delay (fig. 2). The combination of level hysteresis (switching threshold) with the temporal hysteresis of the fast activation (< 1 µs) and slower deactivation (< 1 ms) of the bypass guarantees safe and stable state transitions.

Thus, it is also possible in the case of increases in current (inrush, surge) of 10 A/µs to realize virtually non-reactive switching that limits voltage drops across the load to about 400 mV. An appropriate capacitance across the shunt also helps to flatten the voltage transients in the transition zone until the comparator reacts (fig. 3). The concept is thus also suitable for measuring test objects with a highly dynamic behavior, such as switching regulators, LED drivers, etc.

The hysteresis dead times, as well as the settling times of the parallel measurement paths (ADC) that have to be masked out, limit the meaningful measuring data output rates to 100 Hz. However, it is also possible to disable the auto-range shunt switching in a "fixed mode", and then to use a maximum data rate of 1 kHz.

A rapid and reversible MOSFET-based electronic fuse guarantees safety in all cases and disconnects the current path in case of overload.