2022-10-29 17:30 Tags: # Offset Compensation Offset compensated amplifier. If we can disconnect the input signal and connect it in negative feedback, we can compensate for the offset. Ck1 and Ck2 are on and Ck3 is off. This means that the offset voltage is placed across the capacitor (since the left hand side of the capacitor is grounded). In the use phase (Ck3 goes high) then the capacitor works as a floating voltage source - subtracting the offset voltage from the input signal. Due to the presence of charge rejection - the switch connecting to the sampling (high Z) node, is the one that goes O/C first (i.e. ck1). If this doesn't happen, there might be more or less charge injection.  Input should only switch when the ground switch is off (else you'll short the input to gnd - not a good idea!). Can't use the amplifier all the time, need to use it in the use phase. (don't have to be symmetrical). Only limiting factor is the leakage current in the cap, which means eventually the cap will discharge through the reverse biased diodes in the transistors. How long depends on the process and how big the caps are (us to ms).  ## Dummy Switch for Charge Injection Can also use the dummy transistor for reducing charge injection. Should always be connected to the high-Z sampling node (on the capacitor side). Slightly delayed and inverted version of (Clk1) for dummy switch clock (Clk4). The onset of smaplingn is not as important, when sampling is finished is important. Dummy switch **must go close circuit before gnd switch goes open** so the impedance seen by the dummy transistor is the same as the main sampling switch.  ## Capacitor Choice/Design  Bottom plate (lowest plate in the capacitor) is on the input side. Typically, there is a parasitic capacitance to ground attached to this bottom plate. This is better on the input side as it is either driven to Vin or Gnd. It will influence the circuit on the other side (sample voltage will be atttenured - creating an error in sampling). ## No effective charge injection On the ground node - there is no effective charge injection. It is always associated with the high-Z load wehre the sampling happens. ## Noise Reduction Sample reduces the 1/f noise (which can be thought of as a slowly changing offset voltage). ## Pre-amplifier Offset Cancelling  We can place the offset cancelling sampling capacitor at the output. This is since there isn't alot of gain, so the signal at the output will be larger. The offset will sit as an output referred differential offset. Then open the sampling caps (high Z node), then connect the preamp input from gnd to signal then enable the latching function and take a sample. Because this is a differential signal, charge injection (to a first order) is the same for both transistors on the High-Z node so the differential charge injection is zero. Thus no need for dummy transistors in differential circuits, might make charge injection worse since the dummys have to be matched. [^1] --- # References [^1]: [vr-4602-wk06-sc03-offsetcomp](../../Spaces/University/ELEC4602%20–%20Microelectronics%20Design%20and%20Technology/Lectures/W5/vr-4602-wk06-sc03-offsetcomp.mp4)