71M6511/71M6511H
Single-Phase Energy Meter IC
DATA SHEET
AUGUST 2007
The voltage reference is chopper stabilized, i.e. the polarity can be switched by the MPU using the I/O RAM register
CHOP_ENA (0x2002[5:4]). The two bits in the CHOP_ENA register enable the MPU to operate the chopper circuit in regular or
inverted operation, or in “toggling” mode. When the chopper circuit is toggled in between multiplexer cycles, DC offsets on the
measured signals will automatically be averaged out.
The general topology of a chopped amplifier is given in Figure 2.
A
B
A
B
A
Voutp
Vinp
Vinn
+
-
B
A
G
Voutn
B
CROSS
Figure 2: General Topology of a Chopped Amplifier
It is assumed that an offset voltage Voff appears at the positive amplifier input. With all switches, as controlled by CROSS in
the “A” position, the output voltage is:
Voutp – Voutn = G (Vinp + Voff – Vinn) = G (Vinp – Vinn) + G Voff
With all switches set to the “B” position by applying the inverted CROSS signal, the output voltage is:
Voutn – Voutp = G (Vinn – Vinp + Voff) = G (Vinn – Vinp) + G Voff, or
Voutp – Voutn = G (Vinp – Vinn) - G Voff
Thus, when CROSS is toggled, e.g. after each multiplexer cycle, the offset will alternately appear on the output as positive and
negative, which results in the offset effectively being eliminated, regardless of its polarity or magnitude.
The Functional Description Section contains a chapter with a detailed description on controlling the CHOP_ENA register.
Temperature Sensor
The 71M6511/6511H includes an on-chip temperature sensor implemented as a bandgap reference. It is used to determine
the die temperature The MPU may request an alternate multiplexer cycle containing the temperature sensor output by
asserting MUX_ALT.
The primary use of the temperature data is to determine the magnitude of compensation required to offset the thermal drift in
the system (see section titled “Temperature Compensation”).
The zero reference for the temperature sensor is VBIAS.
Functional Description
The AFE functions as a data acquisition system, controlled by the MPU. The main signals (IA, VA, IB) are sampled and the
ADC counts obtained are stored in CE RAM where they can be accessed by the CE and, if necessary, by the MPU. Alternate
multiplexer cycles are initiated less frequently by the MPU to gather access to the slow temperature signal.
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