SNIU028D February 2016 – September 2020 UCD3138 , UCD3138064 , UCD3138064A , UCD3138128 , UCD3138A , UCD3138A64
There are two options for the use of Xn in the Integrator (I) input. The current Xn can be used by itself, or the current Xn can be added with the previous Xn-1. Doing the addition provides a trapezoidal approximation, which is sometimes considered theoretically superior to a single point. However, since the output of the adder is also 9 bits, if the Xn and Xn-1 values add up to more than 255 or less than -256, the output of the adder will be clamped at those values.
This will only occur at the lowest gain setting of the Error ADC. So if the lowest gain setting of the EADC is to be used, trapezoidal approximation may cause premature saturation on the input.
The Xn value, or the Xn + Xn-1 sum is multiplied by the Ki coefficient. Note that the trapezoidal mode will normally have double the output of the simple Xn value. This will change the overall gain of the I stage by a factor of two. This must be taken into account if the Xn addition mode is changed.
The output of this multiplication will always fit within 24 bits. This value is then added to the existing I value. The hardware will automatically clamp it at a 24 bit signed number, and there are high and low clamp registers available which can be used to clamp it to lower values. There are also status register bits that will be set if the input value exceeds the clamp.
The clamped value is fed to the next stage of the filter, and is also fed back to be added to the next Xn * Ki value when it is calculated.