Speaker: Vadodariya Vivekkumar Ramanlal (EE14S059)
In certain industrial automation applications, resistive DACs (R-DACs) are the de-facto choice since the DAC may have to hold a particular voltage output for very large duration (sometimes in years). R-DACs require amplifier based output buffer to drive load. To address increasing accuracy requirements in R-DACs, precision amplifiers are used. Precision amplifiers are not only limited to R-DACs, but also find applications in sensor front end, precision measurement instruments, medical instruments, etc. Low offset and low flicker noise are critical careabouts for these amplifiers, which can be obtained with chopper stabilization technique. However, this architecture suffers from high frequency tones at multiples of chopping frequency. These tones may be reduced by switched capacitor notch filter (SCNF). Further reduction of high frequency tones can be achieved with the basics of spread spectrum clocking (SSC). Time domain response is sluggish for such an amplifier because of high settling time of notch filter. Hence, a reset phase is introduced to improve settling time.
A high loop gain, SSC based chopper stabilized SCNF precision amplifier with reduced setting time has been designed in this work. It drives 20-bit R-DAC having 200pF of capacitive load and 5kΩ of resistive load with rail-to-rail input and output. Silicon measurement shows 23dB improvement of high frequency chopping tones due to spread spectrum clocking. The fabricated chip is validated with unity gain bandwidth of 8MHz, large step settling time (0.01% settling error) of 5µS, integrated flicker noise of 180nVPP (0.1Hz to 10Hz), maximum input referred offset of ±10µV, THD of 98dB @ 1kHz and RMS thermal noise of 9nV/√Hz.