SA2007P
TYPICAL APPLICATION
The analog (metering) interface described in this section is
designed for measuring 230V/60A with precision better than
Class 1.
The most important external components for the SA2007P
integrated circuit are the current sense resistors, the voltage
sense resistors and the bias setting resistor. The resistors
used in the metering section should be of the same type so
temperature effects are minimized.
Current Input IIN1, IIP1, IIN2, IIP2
Two current transformers are used to measure the current in
the live and neutral phases. The output of the current
transformer is terminated with a low impedance resistor. The
voltage drop across the termination resistor is converted to a
current that is fed to the differential current inputs of the
SA2007P.
CT Termination Resistor
The voltage drop across the CT termination resistor at rated
current should be at least 20mV. The CT have low phase shift
s
and a ratio of 1:2500. The CT is terminated with a 3.6W resistor
giving a voltage drop of 86.4mV across the termination resistor
at rated conditions (I
max
for the meter).
Current Sensor Input Resistors
The resistors R10, R11 and R12, R13 define the current level
into the current sense inputs of the SA2007P. The resistor
values are selected for an input current of 16µA on the current
inputs of the SA2007P at rated conditions. For a 60A meter at
2500:1 CT the resistor values are calculated as follows:
R10 = R11 = ( I
L
/ 16µA ) x R
SH
/ 2
= 60A / 2500 / 16µA x 3.6W / 2
= 2.7kW
I
L
= Line current
RSH = CT Termination resistor
2500 = CT ratio
The two current channels are identical so R10 = R11 = R12 =
R13.
Voltage Input IVP
The voltage input of the SA2007P (IVP) is driven with a current
of 14µA at nominal mains voltage. The voltage input saturates
sames
at approximately 17µA. At a nominal voltage current of 14µA
allows for 20% overdriving. The mains voltage is divided with a
voltage divider to 14V that is fed to the voltage input pins via a
1MW resistor.
Voltage Divider
The voltage divider is calculated for a voltage drop of 14V.
Equations for the voltage divider in figure 4 are:
RA = R1 + R2 + R3
RB = R7 || R5
Combining the two equations gives:
( RA + RB ) / 230V = RB / 14V
Values for resistors R5 = 24kW and R7 = 1MW is chosen.
Substituting the values result in:
RB = 23.437kW
RA = RB x ( 230V / 14V – 1 )
RA = 362kW.
Standard resistor values for R1, R2 and R3 are chosen to be
120kW each.
The capacitor C1 is used to compensate for phase shift
between the voltage sense inputs and the current sense inputs
of the device, in cases where CTs with phase errors are used.
The phase shift caused by the CT may be corrected by
inserting a capacitor in the voltage divider circuit. To
compensate for a phase shift of 0.18 degrees the capacitor
value is calculated as follows:
C = 1 / (2 x
p
x Mains frequency x R5 x tan (Phase shift angle))
C = 1 / ( 2 x
p
x 50 x 1MW tan (0.18 degrees ))
C = 1.013µF
Reference Voltage Bias resistor
R6 defines all on chip and reference currents. With R6 = 24kW
optimum conditions are set. Device calibration is done with
calibration data.
9/10
PRELIMINARY
SAMES [ SAMES ]
