ISL28022 Datasheet, PDF (22/28 Pages) Intersil Corporation – Precision Digital Power Monitor

ISL28022
FIGURE 30. BYTE TRANSACTION SEQUENCE FOR INITIATING DATA RATES ABOVE 400Kbs
Signal Integrity
The purity of the signal being measured by the ISL28022 is not
always ideal. Environmental noise or noise generated from a
regulator can degrade the measurement accuracy. The
ISL28022 maintains a high CMRR ratio from DC to
approximately 10kHz, as shown in Figure 31.
130
125
120
115
110
105
100
95
90
85
80
10
100
1k
10k
100k
1M
FREQUENCY (Hz)
FIGURE 31. CMRR vs FREQUENCY
The CMRR vs Frequency graph best represents the response of
the ISL28022 when an aberrant signal is applied to the circuit.
The normal state of the measured signal is DC and is the state at
which the ISL28022 internal calibration is performed.
The graph was generated by shorting the ISL28022 input without
any filtering and applying a 0 to 10V triangle wave to the shunt
inputs, Vinp and Vinm. The voltage shunt measurement was
recorded for each frequency applied to shunt input.
The CMRR of can be improved by designing an filter stage for the
of the ISL28022. The purpose of the filter stage is to attenuate
the amplitude of the unwanted signal to the noise level of the
ISL28022. Figure 32 is a simple filter example to attenuate
unwanted signals.
The filter circuit in Figure 32 attenuates unwanted signals. CSH
and RSH is single pole RC filter that differentially attenuates
unwanted signals to the ISL28022. Most power monitoring
applications require a shunt resistor to be low in value to
measure large currents. For small shunt resistors, a large value
capacitor is required to attenuate low frequency signals. Most
large value capacitors are not offered in space saving packages.
The corner frequency of the differential filter, CSH and RSH,
should be designed for higher value frequency filtering.
FROM
SOURCE
C1
R1
R1
C1
FIGURE 32. SIMPLIFIED FILTER DESIGN TO IMPROVE NOISE
PERFORMANCE TO THE ISL28022
R1 and C1 for both inputs are the single ended filter to the
ISL28022. The value of the series resistor to the ISL28022 can
be a larger value than the shunt resistor, RSH. A larger series
resistor to the input allows for a lower cutoff frequency filter
design to the ISL28022. The ISL28022 can source up to 20µA of
transient current in the measurement mode. The transient or
switching offset current can be a large as 10µA. The switching
offset current combined with the series resistance, R1, creates
an error offset voltage. A balance of the value of R1 and the
shunt measurement error should be achieved for this filter
design.
The common mode voltage of the shunt input stage ranges from
0V to 60V. The capacitor voltage rating for C1 and CSH should
comply with the nominal voltage being applied to the input.
Measurement Stability vs Acquisition Time
The BADC and SADC bits within the Configuration register
configures the conversion time and accuracy for the bus and
shunt inputs respectively. The faster the conversion time the less
accuracy and more noise introduced into the measurement.
Figure 33 is a graph that illustrates the shunt measurement
variability versus a set SADC mode. The standard deviation of
2048 shunt Vos measurements is used to quantify the
measurement variability of each mode.
22
FN8386.1
April 26, 2013