Linear in dB, sub 0.2 dB gain-step CMOS programmable gain amplifier for ultrasound applications

Abstract

In this paper a new approach for controlling the gain of a programmable gain amplifier (PGA) using an eight-bit digital word has been proposed. Eight stages of R–2R ladder attenuators have been employed for attenuating the input signal. The output of the ladder network feeds the nine g_m stages as a part of the PGA. Making use of three MSB bits of the control word, at each instance just the two successive g_m stages are active whose tail currents are precisely determined to set the PGA linear gain in dB. A current bank composed of 32 current sources has been used for driving the tail currents of the active g_m stages. The current sources that bias the active g_m stages are controlled by the five LSB bits of the digital control word. Thanks to the proposed technique, the values of the current sources are determined so that the gain of the circuit is based on a ratio-metric parameter making the circuit less sensitive to process variations. For verification of the proposed idea, a differential PGA is design in a 0.18 µm 1.8 V CMOS technology. The gain of the designed circuit changes from 20.8 to −27.4 dB in 256 steps linearly in dB so that the maximum gain error respect to its ideal value is less than 0.03 dB in different corner cases. At the maximum gain, the bandwidth and the input referred noise voltage of the PGA are 150 MHz and 3.5 nv/√Hz, respectively, while the overall circuit draws 6 mA from a 1.8 V power supply.