Rainbow Electronics 71M6542G - Manual (Page 20)

20 © 2008–2011 Teridian Semiconductor Corporation v1.1
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θ, resulting in the residual phase error between the current and its corresponding voltage of θ – Ф.
The residual phase error is negligible, and is typically less than ±1.5 milli-degrees at 100Hz, thus it does
not contribute to errors in the energy measurements.
When using remote sensors, the CE performs the same delay compensation described above to align
each voltage sample with its corresponding current sample. Even though the remote current samples do
not pass through the 71M654x multiplexer, their timing relationship to their corresponding voltages is
fixed and precisely known, provided that the MUXn_SEL[3:0] slot assignment fields are programmed as
shown in
Table 1 and Table 2 .
2.2.4 ADC Pre-Amplifier
The ADC pre-amplifier is a low-noise differential amplifier with a fixed gain of 8 available only on the IAP-
IAN sensor input pins. A gain of 8 is enabled by setting PRE_E = 1 (I/O RAM 0x2704[5]). When disabled,
the supply current of the pre-amplifier is <10 nA and the gain is unity. With proper settings of the PRE_E
and DIFFA_E (I/O RAM 0x210C[4]) bits, the pre-amplifier can be used whether differential mode is
selected or not. For best performance, the differential mode is recommended. In order to save power, the
bias current of the pre-amplifier and ADC is adjusted according to the ADC_DIV control bit (I/O RAM
0x2200[5]).
2.2.5 A/D Converter (ADC)
A single 2
nd
order delta-sigma A/D converter digitizes the voltage and current inputs to the device. The
resolution of the ADC, including the sign bit, is 21 bits (FIR_LEN[1:0] = 1, I/O RAM 0x210C[2:1]), or 22 bits
(FIR_LEN[1:0] = 2). The ADC is clocked by CKADC.
Initiation of each ADC conversion is controlled by MUX_CTRL internal circuit as described above. At the
end of each ADC conversion, the FIR filter output data is stored into the CE RAM location determined by
the multiplexer selection. FIR data is stored LSB justified, but shifted left 9 bits.
2.2.6 FIR Filter
The finite impulse response filter is an integral part of the ADC and it is optimized for use with the multiplexer.
The purpose of the FIR filter is to decimate the ADC output to the desired resolution. At the end of each
ADC conversion, the output data is stored into the fixed CE RAM location determined by the multiplexer
selection as shown in
Table 1 and Table 2 .
2.2.7 Voltage References
A bandgap circuit provides the reference voltage to the ADC. The amplifier within the reference is chopper
stabilized, i.e., the chopper circuit can be enabled or disabled by the MPU using the I/O RAM control field
CHOP_E[1:0] (I/O RAM 0x2106[3:2]). The two bits in the CHOP_E[1:0] field enable the MPU to operate the
chopper circuit in regular or inverted operation, or in toggling modes (recommended). When the
chopper circuit is toggled in between multiplexer cycles, dc offsets on VREF are automatically be
averaged out, therefore the chopper circuit should always be configured for one of the toggling modes.
Since the VREF band-gap amplifier is chopper-stabilized, the dc offset voltage, which is the most
significant long-term drift mechanism in the voltage references (VREF), is automatically removed by the
chopper circuit. Both the 71M654x and the 71M6x01 feature chopper circuits for their respective VREF
voltage reference.
The general topology of a chopped amplifier is shown in Figure 8 . The CROSS signal is an internal on-
chip signal and is not accessible on any pin or register.