CS5460A
DS284PP4
27
rent inputs are set at V
Vnom
and V
Inom
, we want the
pulse rate to be at ‘IR’ = 100 pulses per second. IR
will be some percentage of PR. The percentage is
defined by the ratios of V
Vnom
/250 mV and
V
Inom
/250 mV with the following formula:
We can rearrange the above equation and solve for
PR.
This is the value that we put into the
Pulse-Rate Register.
Therefore we set the Pulse-Rate Register to
~420.875 Hz. Therefore, the Pulse-Rate Register
would be set to 0x00349C.
The above equation is valid when current channel
is set to x10 gain. If current channel gain is set to
x50, then the equation becomes:
where it is assumed that the current channel has
been calibrated such that the Instantaneous Current
Register will read at full-scale when the input volt-
age across the IIN+ and IIN- inputs is 50 mV (DC).
EXAMPLE #2: Suppose that instead of being giv-
en a desired frequency of pulses per second to be is-
sued at a specific voltage/current level, we are
given a desired number of pulses per unit energy to
be present at EOUT, given that the maximum
line-voltage is at 250 V (RMS) and the maximum
line-current is at 20 A (RMS). For example, sup-
pose that the required number of pulses per kW-hr
is specified to be 500 pulses/kW-hr. In such a situ-
ation, the nominal line voltage and current do not
determine the appropriate pulse-rate setting. In-
stead, the maximum line-voltage and line-current
levels must be considered. We use the given maxi-
mum line-voltage and line-current levels to deter-
mine K
V
and K
I
as previously described to get:
K
V
= 150 mV / 250 V = 0.0006
K
I
= 150 mV / 20 A = 0.0075
Ω
where we again have calculated our sensor gains
such that the maximum line-voltage and line-cur-
rent levels will measure as 0.6 in the RMS Voltage
Register and RMS Current Register.
We can now calculate the required Pulse-Rate Reg-
ister setting by using the following equation:
Therefore PR = ~1.929 Hz.
Note that the Pulse-Rate Register cannot be set to a
frequency of exact 1.929 Hz. The closest setting
that the Pulse-Rate Register can obtain is
0x00003E = 1.9375 Hz. To improve the accuracy,
either gain register can be programmed to correct
for the round-off error in PR. This value would be
calculated as
In the last example, suppose that the designer must
use a value for MCLK/K of 3.05856MHz. When
MCLK/K is not equal to 4.096MHz, the result for
‘PR’ that is calculated for the Pulse-Rate Register
must be scaled by a correction factor of:
4.096MHz / (MCLK/K). In this case we would
scale the result by 4.096/3.05856 to get a final PR
result of ~2.583 Hz.
PulseRate
IR
PR
V
Vnom
250mV
------------------
V
Inom
250mV
------------------
⋅
⋅
=
=
PR
IR
V
Vnom
250mV
------------------
V
Inom
250mV
------------------
×
---------------------------------------------
100Hz
132mV
250mV
------------------
112.5mV
250mV
-----------------------
×
-------------------------------------------------
=
=
PR
IR
V
Vnom
250mV
------------------
V
Inom
50mV
---------------
×
------------------------------------------
=
PR
500
pulses
kW
hr
⋅
---------------------
1hr
3600s
--------------
1kW
1000W
------------------
250mV
K
V
------------------
250mV
K
I
------------------
⋅
⋅
⋅
⋅
=
Ign or Vgn
PR
1.929
-------------
1.00441
≅
0x404830
=
=
Содержание CS5460A
Страница 63: ... Notes ...
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