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71M6521FE Datasheet(PDF) 11 Page - Teridian Semiconductor Corporation |
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71M6521FE Datasheet(HTML) 11 Page - Teridian Semiconductor Corporation |
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11 / 101 page  71M6521DE/71M6521FE Energy Meter IC DATASHEET JANUARY 2008 v1.0 © 2005-2008 TERIDIAN Semiconductor Corporation Page: 11 of 101 G - + V inp V outp V outn V inn CROSS A B A B A B A B 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 71M6521DE/FE 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”). |
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