5.1 A 10MHz-bandwidth 4µs-large-signal-settling 6.5nV/√Hz-noise 2µV-offset chopper operational amplifier

Low-offset and low-noise operational amplifiers (OpAmps) are essential for precision measurement systems. Applications such as precision weigh scales, sensor front-ends, bridge transducers, interfaces for thermocouple sensors, and medical instrumentation require amplification of low-level signals with high closed-loop gain and low noise. An OpAmp with high precision and fast response enables more frequent measurement of sensor signals leading to more accurate control. Offset and its temperature drift in a traditional CMOS OpAmp cannot be made less than 100μV/0.5μV/°C even with multi-temperature trimming, with flicker noise corner at 100's of Hz. Reduction of the offset and flicker noise below these limitations can be achieved by chopping (correlated double sampling) of the input signal [1]. Disadvantages of chopping are: increase in OpAmp input current, caused by charge injection through input chopping switches, large ripples and noise at the chopping frequency, and long settling time limited by the chopping frequency. Existing chopper OpAmps show good DC and low-frequency accuracy but cannot combine it with bandwidth higher than 2 to 4MHz or settling time comparable not only to high speed OpAmps, but even OpAmps in the most popular 10MHz market segment [1-3]. This results in low usable bandwidth when the OpAmp is configured in high closed-loop gain configurations, where output error needs to be minimized. The described OpAmp achieves a high unity gain-bandwidth (10MHz), fast large-signal settling (4μs to 0.01%) and low noise at both high and low frequencies (6.5nV/√Hz), while having low input current (<;100pA). This is made possible by a combination of new topology and circuit decisions based on the structural design methodology [4].