When simultaneously measuring large numbers of very low current levels (<1 µA) in parallel at relatively low speeds (<100 kSPS) but with high accuracy (>16 bits), two solutions available to engineers include the Texas Instruments (TI) AFE family of X-ray analog front ends and the DDC family of analog-to-digital converters (ADCs). Although originally designed for medical imaging applications, these devices also work in instrumentation, in-vitro diagnostic applications, and any other applications with large numbers of photodiodes or a large parallel number of voltage measurements.
In both device families, the measured currents are not floating (shunt) but sinking or pulling from a fixed DC level (see Figure 2-1 and Figure 3-1) because the input is virtually shorted to a voltage through the feedback action of an amplifier, like in the case of a discrete transimpedance amplifier. Unlike transimpedance amplifiers, however, which convert current into voltage by multiplying by a resistor (in the feedback of the input amplifier), both families use an integrator as the first stage.
Both device families integrate most of the required components into a single IC, enabling functionality from current input to digital output (a serial stream in both cases). This functionality includes the ability to adjust the full scale (the input gain) to optimize the signal-to-noise ratio for a particular application or even to use the same solution for different applications, creating a platform design. Besides the board-space savings, this level of integration – with many of the system-level specifications set and backed by the data sheet of these devices – enormously simplifies design work and testing strategies during production, which then results in reduced time to market and lower development costs.
Although many applications can use either device, their differences are significant enough that one or the other will be a better fit for a given application. In this article, we will explain why.