A low-power, compact, adaptive logarithmic transimpedance amplifier operating over seven decades of current

This paper presents a detailed insight into the design space of wide-range transimpedance amplifiers enabling the design of micro-power, adaptive circuits for integrated current sensing applications. The analysis proves that the power dissipation of the nonadaptive structures varies linearly with dynamic range and quadratically with bandwidth. We present two adaptation techniques, modifying the bias current or output resistance, both of which alleviate this strong dependence on dynamic range. It is shown that adapting the bias current is most suitable for our application which requires a modest bandwidth but very wide dynamic range. Measurements demonstrate operation with currents ranging seven orders of magnitude from 200 fA to 2 μA with an average error of 0.8% and maximum error of 3.4%. The power consumption averaged over this entire range of currents is 3.45 μ W. Either signal-to-noise ratio (SNR) or bandwidth can be made to tradeoff with the input current magnitude depending on the application. If the bandwidth is limited to 5 kHz, it achieves an average SNR of 65 dB. © 2007 IEEE.