Cirrus Logic AN366 - Manual (Page 2)

AN366
2 AN366REV2
3.1 System Scaling Overview
The maximum voltage, current, and power measurements are unique in each meter design and dependent on
the sensors used in the measurement of these parameters. The CS5480/84/90 solves this problem using scal-
ing. Instead of recording the actual voltage, current, or power sensed by the power meter, the IC records a ratio
of each measurement that is proportional to the meter’s full-scale. Using this ratio, the actual voltage, current,
and power can be calculated based on the values of the AFE registers.
There are two methods of obtaining the most recent power measurement readings:
- Voltage, current and power measurements are read directly from registers using the serial port.
- Power measurements are accumulated using the pulses on the DO pin(s).
Both methods are dependent on full-scale calibration to accurately scale the most recent power measurement.
Traditional power meters typically use the pulse accumulation method. Since calibration constants are recorded
in registers and power measurements are reported by register reads/writes, this document will focus on the reg-
ister read/write method.
To use the built-in calibration functions, an understanding of the scaling factors due to the different system com-
ponents within a typical meter is required. Below are three general scale factors in the signal path:
- Hardware Scale: The real voltage and currents are provided to the meter using sensors that must be
attenuated on the meter board or by the sensor before applying the sensed signal to the input of the
CS5480/84/90.
- AFE Register Scale: The device stores information for each voltage, current, and power parameter to
internal registers. Each register value is scaled to a range of ±1 or 0 to 1 and stored in a 24-bit register.
The values measured at the input (for example, 500 mVpp) are stored as a scaled version of input signal
amplitudes. Refer to the CS5480/84/90 data sheet for register formats. The gain and offset registers
are scaled to be within the range of 0 to 4 and ±1, respectively. Therefore, the MCU does not read the
sensor output voltage and current; instead, it reads the scaled values recorded in the registers.
- MCU Scale: The MCU is typically used to rescale the real voltage, current, and power values for display.