3.1.27 Voltage Conversion Factor
The voltage conversion factor is used when converting an internal voltage quantity to an external equivalent RMS voltage quantity. K_Vx: where x = [A, B, C, D].
For proper ADC function, voltage divider circuitry must be provided to reduce each high-voltage input channel from its peak input value to the max peak acceptable ADC differential input voltage.
In the case of the ATSENSE AFEs, all voltage channel inputs are constrained to use single-ended input voltages. While the maximum differential input voltage must not exceed 0.5V (V+ = 0. 25V, V- = -0.25V), it is recommended that the single-ended inputs be limited to ±0.25V to avoid approaching forward conduction of input protection diodes.
In case of using a MCP391x AFE, all the inputs are differential, so the channels used for measuring voltage can be used in differential or in single-ended mode. In this case, the differential input voltage supported is 0.6V and the single-ended voltage must be limited to ±0.3V. In typical applications, external scaling allows for any over-voltage factor (typically 20%). In some applications the over-voltage is increased to be able to measure the phase to phase voltage value (plus the typical 20%), to detect improper connection of the wires to the meter.
For example, it is preferable to have a max measurement voltage of 288 VRMS (240 VRMS + 20% over-voltage). Users need to consider the following three steps:
- Determine external voltage
divider ratio (single-ended input):
- Internal scaling within the DSP takes care of the multiplication factor, VREF, to renormalize the ADC gain, when the channel calibration constant, CAL_M_Vx, is used.
- Determine the voltage conversion
factor:
When using a simple resistive voltage divider, the equation for K_Vx simplifies to the equivalent short-form:
K_Vx is stored in the uQ22.10 format.
- To compute an equivalent external
voltage, multiply an internal Qformat number, VINTERNAL, as given
below:
Refer to the “PIC32CXMTx Metrology User Guide” (DS50003460) for additional information.
| Name: | K_Vx |
| Property: | Read-Write |
| Bit | 31 | 30 | 29 | 28 | 27 | 26 | 25 | 24 | |
| K_Vx[31:24] | |||||||||
| Access | R/W | R/W | R/W | R/W | R/W | R/W | R/W | R/W | |
| Reset | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | |
| Bit | 23 | 22 | 21 | 20 | 19 | 18 | 17 | 16 | |
| K_Vx[23:16] | |||||||||
| Access | R/W | R/W | R/W | R/W | R/W | R/W | R/W | R/W | |
| Reset | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | |
| Bit | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | |
| K_Vx[15:8] | |||||||||
| Access | R/W | R/W | R/W | R/W | R/W | R/W | R/W | R/W | |
| Reset | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | |
| Bit | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | |
| K_Vx[7:0] | |||||||||
| Access | R/W | R/W | R/W | R/W | R/W | R/W | R/W | R/W | |
| Reset | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | |