Rainbow Electronics 71M6542G - Manual (Page 14)

14 © 2008–2011 Teridian Semiconductor Corporation v1.1
Figure 5 shows the 71M6542F multiplexer interface with one local and one remote resistive shunt sensor.
As seen in Figure 5 , when a remote isolated shunt sensor is connected via the 71M6x01, the samples
associated with this current channel are not routed to the multiplexer, and are instead transferred digitally
to the 71M6542F via the digital isolation interface and are directly stored in CE RAM.
See Figure 6 for the multiplexer timing sequence corresponding to Figure 5 . See Figure 38 for the meter
configurations corresponding to Figure 5 .
∆Σ ADC
CONVERTER
VREF
MUX
VREF
VREF
VADC
22
FIR
IBP
VADC9 (VB)
IAP
VADC10 (VA)
IAN
IBN
71M6542F
CE RAM
71M6x01
SP
SN
INP
INN
Remote
Shunt
IB
Digital
Isolation
Interface
Local
Shunt
IA
22
11/5/2010
Figure 5. 71M6542F AFE Block Diagram with 71M6x01
2.2.1 Signal Input Pins
The 71M6541D/F features five ADC inputs. The 71M6542F features six ADC inputs.
IAP-IAN and IBP-IBN are intended for use as current sensor inputs. These four current sensor inputs can be
configured as four single-ended inputs, or can be paired to form two differential inputs. For best
performance, it is recommended to configure the current sensor inputs as differential inputs (i.e., IAP-IAN
and IBP-IBN). The first differential input (IAP-IAN) 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 remaining differential pair (i.e., IBP-IBN) may be used with CTs, or may be enabled
to interface to a remote 71M6x01 isolated current sensor providing isolation for a shunt resistor sensor using
a low cost pulse transformer.
The remaining input in the 71M6541D/F (VA) is single-ended, and is intended for sensing the line voltage in
a single-phase meter application using Equation 0 or 1 (see
2.3.4 Meter Equations on page 25 ). The
71M6542F features an additional single-ended voltage sensing input (VB) to support bi-phase applications
using Equation 2. These single-ended inputs are referenced to the V3P3A pin.
All analog signal input pins measure voltage. In the case of shunt current sensors, currents are sensed as a
voltage drop in the shunt resistor sensor. Referring to
Figure 3 , shunt sensors can be connected directly to
the 71M654x (referred to as a ‘local’ shunt sensor) or connected via an isolated 71M6x01 (referred to as a
‘remote’ shunt 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 winding of the CT. Meanwhile, line voltages are
sensed through resistive voltage dividers. The VA and VB pins (VB is available in the 71M6542F only) are
single-ended and their common return is the V3P3A pin.
Pins IAP-IAN can be programmed individually to be differential or single-ended as determined by the
DIFFA_E (I/O RAM 0x210C[4]) control bit. However, for most applications, IAP-IAN are configured as a
differential input to work with a shunt or CT directly interfaced to the IAP-IAN differential input with the
appropriate external signal conditioning components (see
4.2 Direct Connection of Sensors on page 92 ).
The performance of the IAP-IAN pins can be enhanced by enabling a pre-amplifier with a fixed gain of 8,
using the I/O RAM control bit PRE_E (I/O RAM 0x2704[5]). When PRE_E = 1, IAP-IAN become the inputs
to the 8x pre-amplifier, and the output of this amplifier is supplied to the multiplexer. The 8x amplification