71M6543F/H and 71M6543G/GH Data Sheet
The 71M6543 AFE can also be directly interfaced to Current Transformers (CTs), as seen in Figure 3. In
this case, all voltage and current channels are multiplexed into a single second-order sigma-delta ADC in
the 71M6543 and the 71M6xx3 remote isolated sensors are not used. The fourth CT and the
measurement of Neutral current via the IADC0-IADC1 current channel are optional.
See Figure 32 for the meter wiring configuration corresponding to Figure 3.
VREF
IA
IADC2
MUX
∆Σ ADC
CONVERTER
CT
VREF
VADC
VREF
FIR
IADC3
CE RAM
22
IB
IADC4
IADC5
IADC6
IADC7
IADC0
IADC1
CT
IC
CT
IN*
CT
VADC8 (VA)
VADC9 (VB)
VADC10 (VC)
71M6543
*IN = Neutral Current
9/17/2010
Figure 3. AFE Block Diagram (Four CTs)
2.2.1 Signal Input Pins
The 71M6543 features eleven ADC input pins.
IADC0 through IADC7 are intended for use as current sensor inputs. These eight current sensor inputs can
be configured as eight single-ended inputs, or can be paired to form four differential inputs. For best
performance, it is recommended to configure the current sensor inputs as differential inputs (i.e., IADC0-
IADC1, IADC2-IADC3, IADC4-IADC5 and IADC6-IADC7). The first differential input (IADC0-IADC1)
features a pre-amplifier with a selectable gain of 1 or 8, and is intended for direct connection to a shunt
resistor sensor, and can also be used with a Current Transformer (CT). The three remaining differential
pairs (i.e., IADC2-IADC3, IADC4-IADC5 and IADC6-IADC7) may be used with CTs, or may be enabled to
interface to a remote 71M6xx3 isolated current sensor providing isolation for a shunt resistor sensor using a
low cost pulse transformer.
The remaining three inputs VADC8 (VA), VADC9 (VB) and VADC10 (VC) are single-ended, and are
intended for sensing each of the phase voltages in a polyphase meter application. These three single-ended
inputs are referenced to the V3P3A pin.
All ADC input pins measure voltage. In the case of shunt current sensors, currents are sensed as a voltage
drop in the shunt resistor sensor. In the case of Current Transformers (CT), the current is measured as a
voltage across a burden resistor that is connected to the secondary of the CT. Meanwhile, line voltages are
sensed through resistive voltage dividers. The VADC8 (VA), VADC9 (VB) and VADC10 (VC) pins are
single-ended and their common return is the V3P3A pin. See Figure 27, Figure 28, Figure 29 and Figure
30 for detailed connections for each type of sensor. Also refer to the 71M6543 Demonstration Board
schematic and bill of materials for typical component values used in these and other circuits.
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