3.1.20 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, or C].
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.
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 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.
The ADC input voltage is, then, converted by comparing it to the reference voltage, VREF. This comparison creates an additional ADC internal input conversion gain, 1.0V/VREF, due to ratio-metric effect of the reference voltage, typically 1.0/1.2, which is also part of the ADC conversion factor, and is taken care of internal to the DSP and is separate from the voltage channel calibration factor, CAL_M_Vx.
For example, it is preferable to have a max measurement voltage of 288VRMS (240VRMS + 20% over-voltage). The following three steps must be taken:
- 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.
- Next, determine 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 uQ22.10 format.
- Finally, to compute an equivalent
external voltage, multiply an internal Qformat number, VINTERNAL, in
the following fashion:
Refer to “PIC32CXMTx Metrology User Guide” for a more detailed description.
| Name: | K_Vx |
| Offset: | Metrology_Reg_In[25,27,29] |
| 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 | |||||||||
| 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 | |||||||||
| 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 | |||||||||
| 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 | |||||||||