Dynamic Offset Compensated Operational Amplifiers

Typical sensor output signals are in the microvolt range and have bandwidths ranging from DC up to a few kilohertz. Boosting such signals to levels compatible with typical analog-to-digital converters requires low-offset operational amplifiers with gain-bandwidth (GBW) products of a few megahertz. For example, implementing an amplification of 40 dB with a gain accuracy of 1% and a bandwidth of 1 kHz calls for a low-offset operational amplifier with a GBW of at least 10 MHz. Achieving such GBW products in combination with a microvolt-level input offset voltage is not straightforward. In the previous chapter, two dynamic offset compensation techniques were discussed: chopping and auto-zeroing. Chopping is a frequency modulation technique, which requires low-pass filters in the signal path to filter chopper ripple out. Therefore, chopping alone is not suitable for broadband applications. Auto-zeroing is a time domain technique in which the offset is measured and afterwards subtracted from the signal. Therefore, this technique alone is not suitable for continuous-time operation. However, in this chapter it will be shown that, by using these techniques in multi-path topologies, broadband continuous-time amplifiers can be realized.