ISL28022_15 Datasheet, PDF (23/32 Pages) Intersil Corporation – Precision Digital Power Monitor

ISL28022
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 33.
130
125
120
115
110
105
100
95
90
85
8010
100
1k
10k
100k
1M
FREQUENCY (Hz)
FIGURE 33. 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 graph was generated by shorting the ISL28022 input without
any filtering and applying a 0V to 10V triangle wave to the Shunt
inputs, VINP and VINM. The voltage shunt measurement was
recorded for each frequency applied to the shunt input.
The CMRR can be improved by designing a filter stage before 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 34 is a simple filter example to attenuate
unwanted signals.
CSH and RSH are single pole RC filters that differentially
attenuate 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.
C1
FROM
SOURCE
R1
RSH
CSH
R1
C1
FIGURE 34. SIMPLIFIED FILTER DESIGN TO IMPROVE NOISE
PERFORMANCE TO THE ISL28022
R₁ and C₁ for both inputs are single ended low pass filters. The
value of the series resistor to the ISL28022 can be a larger value
than the shunt resistor, R_SH. 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 as 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 35 is a graph that illustrates the shunt measurement
variability versus a set SADC mode. The standard deviation of
2048 shunt V_OS measurements is used to quantify the
measurement variability of each mode.
0.20
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
SADC MODE
FIGURE 35. MEASUREMENT STABILITY vs SADC MODE
Fast Transients
A small isolation resistor placed between ISL28022 inputs and
the source is recommended. In hot swap or other fast transient
events, the amplitude of a signal can exceed the recommended
operating voltage of the part due to the line inductance. The
isolation resistor creates a low pass filter between the device and
the source. The value of the isolation resistor should not be too
large. A large value isolation resistor can effect the
measurement accuracy. The offset current for shunt input can be
as large as 10µA. The value of the isolation resistor combined
with the offset current creates an error offset voltage at the
shunt input. The input of the Bus channel is connected to the top
of a precision resistor divider. The accuracy of the resistor divider
determines the gain error of the Bus channel. The input
resistance of the Bus channel is 600kΩ. Placing an isolation
resistor of the 10Ω will change the gain error of the Bus channel
by 0.0016%.
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FN8386.7
October 2, 2015