71M6533 Datasheet, PDF (12/124 Pages) Teridian Semiconductor Corporation – Energy Meter IC

71M6533/71M6534 Data Sheet
Table 3: ADC RAM Locations
Signal Number
0
1
2
3
4
5
6
0x0A
0x0B
Address (HEX)
0x00
0x01
0x02
0x03
0x04
0x05
0x06
0x0A
0x0B
Name
IA
VA
IB
VB
IC
VC
ID
TEMP
VBAT
FDS_6533_6534_004
1.2.5 Voltage References
The device includes an on-chip precision bandgap voltage reference that incorporates auto-zero tech-
niques. The reference of the 71M6533H/71M6534H is trimmed in production to minimize errors caused
by component mismatch and drift. The result is a voltage output with a predictable temperature coeffi-
cient.
The amplifier within the reference is chopper stabilized, i.e. the polarity can be switched by the MPU us-
ing the I/O RAM register CHOP_E (0x2002[5:4]). The two bits in the CHOP_E 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 aver-
aged out.
The general topology of a chopped amplifier is shown in Figure 2.
V_inp
A
B
V_inn
A
B
CROSS
+
A
G
B
-
A
B
V_outp
V_outn
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 con-
trolled 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 connection of the amplifier input devices is reversed. This preserves the over-
all polarity of that amplifier gain; it inverts its input offset. By alternately reversing the connection, the am-
plifier’s offset is averaged to zero. This removes the most significant long-term drift mechanism in the
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