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sames

sames

SA9903B

Figure 3: Analog input internal configuration

http://www.sames.co.za

FUNCTIONAL DESCRIPTION

The SA9903B is a CMOS mixed signal Analog/Digital

integrated circuit, which performs the measurement of active

power, reactive power, RMS voltage and mains frequency. The

integrated circuit includes all the required functions for single-

phase power and energy measurement such as two

oversampling A/D converters for the voltage and current sense

inputs, power calculation and energy integration.

The SA9903B integrates instantaneous active and reactive

power in 24 bit integrators. RMS voltage and frequency is

continuously measured and stored in respective registers. The

mains voltage zero crossover is available on the FMO output.

The SPI interface of the SA9903B has a tri-state output that

allows connection of more than one metering device on a

single SPI bus.

INPUT SIGNALS

Analog Input Configuration

The input circuitry of the current and voltage sensor inputs is

illustrated in figure 3. These inputs are protected against

electrostatic discharge through clamping diodes. The

feedback loops from the outputs of the amplifiers A and A

IV

generate virtual shorts on the signal inputs. Exact duplications

of the input currents are generated for the analog signal

processing circuitry. The current and voltage sense inputs are

identical. Both inputs are differential current driven up to ±25µA

peak. One of the voltage sense amplifier input terminals is

internally connected to GND. This is possible because the

voltage sense input is much less sensitive to externally

induced parasitic signals compared to the current sense

inputs.

Current Sense Input (IIP and IIN)

Figure 8 shows the typical connections for the current sensor

input. The resistor R6 and R7 define the current level into the

current sense inputs of the SA9903B. At rated current the

resistor values should be selected for input currents of

16µA

. Values for resistors R6 and R7 may be calculated as

RMS

follows:

R6 = R7 = (I / 16µA ) x RSH / 2

L

Where I = Max line current

L

RSH = Shunt resistor or termination resistor.

The voltage drop across RSH should not be less than 16mV at

rated currents. In case a current transformer is used for current

sensing the value of RSH should be less than the resistance of

the CT's secondary winding.

Voltage Sense Input (IVP)

The mains voltage is divided to 14V

.at nominal mains

RMS

voltage by means of resistors R1, R2, R3 and R4. The current

into the voltage sense input is set at 14µA

with resistor R5

RMS

from the voltage divider. The voltage sense input of the AD

converter saturates at an input current of ±25µA peak.

Reference Voltage (VREF)

The VREF pin is the reference for the bias resistor. With a bias

resistor of 24kW optimum conditions are set. It may be varied

within ±10% for calibration purposes.

Serial Clock (SCK)

The SCK pin is used to synchronize data interchange between

the micro controller and the SA9903B. The clock signal on this

pin is generated by the micro controller and determines the

data transfer rate of the DO and DI pins.

Serial Data In (DI)

The DI pin is the serial data input pin for the SA9903B. Data will

be input at a rate determined by the Serial Clock (SCK). Data

will be accepted only during an active chip select (CS).

Chip Select (CS)

The CS input is used to address the SA9603B. An active high

on this pin enables the SA9903B to initiate data exchange.

VOLTAGE

SENSOR

INPUT

IVP

DR-01288

SS

V

CURRENT

SENSOR

INPUTS

IIP

IIN

SS

V

VDD

SS

V

VDD

DD

V

GND