Manual

71M6541 Demo Board REV 3.0 User’s Manual
18 Rev 4.0
overvoltages. This choice need not be of concern, since the ADC in the 71M6541 has enough resolution, even
when operating at 120Vrms or 240Vrms.
If a different set of voltage-dividers or an external voltage transformer (potential transformer) is to be used,
scaling techniques should be used.
In the following example we assume that the line voltage is not applied to the resistor divider for VA formed by
R15-R21, R26-R31, and R32, but to a voltage transformer with a ratio N of 20:1, followed by a simple resistor
divider. We also assume that we want to maintain the value for
VMAX at 600V to provide headroom for large
voltage excursions.
When applying
VMAX at the primary side of the transformer, the secondary voltage V
s
is:
V
s
= VMAX / N
V
s
is scaled by the resistor divider ratio R
R
. When the input voltage to the voltage channel of the 71M6541 is the
desired 177mV, V
s
is then given by:
V
s
= R
R
* 177mV
Resolving for R
R
, we get:
R
R
= (VMAX / N) / 177mV = (600V / 30) / 177mV = 170.45
This divider ratio can be implemented, for example, with a combination of one 16.95 kΩ and one 100 Ω resistor.
If potential transformers (PTs) are used instead of resistor dividers, phase shifts will be introduced that will re-
quire negative phase angle compensation. Maxim Integrated Demo Code accepts negative calibration factors
for phase.
1.9 CALIBRATION PARAMETERS
1.9.1 GENERAL CALIBRATION PROCEDURE
Any calibration method can be used with the 71M6541F chips. This Demo Board User’s Manual presents cali-
bration methods with three or five measurements as recommended methods, because they work with most
manual calibration systems based on counting "pulses" (emitted by LEDs on the meter).
Naturally, a meter in mass production will be equipped with special calibration code offering capabilities beyond
those of the 71M6541 Demo Code. It is basically possible to calibrate using voltage and current readings, with
or without pulses involved. For this purpose, the MPU Demo Code can be modified to display averaged voltage
and current values (as opposed to momentary values). Also, automated calibration equipment can communi-
cate with the Demo Boards via the serial interface and extract voltage and current readings. This is possible
even with the unmodified Demo Code.
Complete calibration procedures are given in section 2.3 of this manual.
Regardless of the calibration procedure used, parameters (calibration factors) will result that will have to be ap-
plied to the 71M6541F chip in order to make the chip apply the modified gains and phase shifts necessary for
accurate operation. Table 1-4 shows the names of the calibration factors, their function, and their location in the
CE RAM.
Again, the command line interface can be used to store the calibration factors in their respective CE RAM ad-
dresses. For example, the command
>]10=+16302
stores the decimal value 16302 in the CE RAM location controlling the gain of the current channel (
CAL_IA).
The command
>]11=4005
stores the hexadecimal value 0x4005 (decimal 16389) in the CE RAM location controlling the gain of the volt-
age channel (
CAL_VA).
The internal power supply generates a ripple on the supply and ground nets that is 90° phase shifted with re-
spect to the AC supply voltage. This affects the accuracy of the VARh measurements. If optimization of the
VARh accuracy is required, this can be done by writing a value into the QUANT_VAR register of the CE (see
section 2.3.7).