71M6521BE
Energy Meter IC
DATA SHEET
JANUARY 2008
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.
When CROSS is high, the hookup of the amplifier input devices is reversed. This preserves the overall polarity of that
amplifier gain, it inverts its input offset. By alternately reversing the connection, the amplifier’s offset is averaged to zero. This
removes the most significant long-term drift mechanism in the voltage reference. The CHOP_E bits control the behavior of
CROSS. The CROSS signal will reverse the amplifier connection in the voltage reference in order to negate the effects of its
offset. On the first CK32 rising edge after the last mux state of its sequence, the mux will wait one additional CK32 cycle
before beginning a new frame. At the beginning of this cycle, the value of CROSS will be updated according to the CHOP_E
bits. The extra CK32 cycle allows time for the chopped VREF to settle. During this cycle, MUXSYNC is held high. The
leading edge of muxsync initiates a pass through the CE program sequence. The beginning of the sequence is the serial
readout of the 4 RTM words.
CHOP_E has 3 states: positive, reverse, and chop. In the ‘positive’ state, CROSS is held low. In the ‘reverse’ state, CROSS is
held high. In the ‘chop’ state, CROSS is toggled near the end of each Mux Frame, as described above. It is desirable that
CROSS take on alternate values at the beginning of each Mux cycle. For this reason, if ‘chop’ state is selected, CROSS will
not toggle at the end of the last Mux cycle in a SUM cycle.
The internal bias voltage VBIAS (typically 1.6V) is used by the ADC when measuring the temperature and battery monitor
signals.
Temperature Sensor
The 71M6521BE 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”).
V1.0
© 2005-2008 TERIDIAN Semiconductor Corporation
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