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Recommendations for Signal Processing
Bourns developed the typical current source module block
diagram, shown in Figure 2, to evaluate a shunt-based
current measurement system's accuracy. The module
consists of an Analog Front End (AFE) with a current
sense amplifier with analog buffer, 24-bit ADC and Serial
Peripheral Interface (SPI). There are several high voltage
bidirectional current sense amplifiers from ADI, such as
the model AD8210 or AD8211, which have gains of 20 and
common mode voltages of up to 65V.
To evaluate the shunt, Bourns tested its Model CSM2F-8518
(100µohm nominal resistance), as shown in Figure 2.
A single-board microcontroller kit is programmed to
communicate with the module over an SPI connection. The
Figure 3 Data in Degrees Celsius Recorded by the Module
Figure 4 Data in Millivolts Recorded by the Current Sense Module
current for the measurement was generated
using a precision current source.
The first measurement to be determined is the
resistance of the shunt. This is done using the
known current from the current source and
a precision 4-wire voltmeter. Once the actual
resistance of the shunt is measured, then the
voltage across the shunt using the current
sense module can be compared with the actual
resistance value.
The current sense amplifier has a common
mode voltage of 80V maximum, allowing for
the module to be placed at the high end in 48V
battery systems. The module also contains a
surface mount temperature sensor with a PWM
output proportional to the ambient temperature.
Figure 3 shows the temperature sensor
output at room temperature. The amplifier
power is supplied by a DC supply of +5V. This
experiment's power comes from a Low Drop
Out (LDO) regulator with the original supply
coming from the USB interface. For an isolated
5V supply, a low-power micro converter using the ADI
LTC8301 (flyback or push-pull) can provide the necessary
isolation with the required safety level.
Figure 4 shows the data points collected from the A/D
input terminals with an average of 22.44364mV and peak-
to-peak variation of 0.007mV over the sampling period.
Determining Measurement Accuracy
The accuracy of the measurements at room temperature
can be calculated using the following formula.
The accuracy:
(Expected Voltage-Actual Measurement)mV*100
Full Range of Op Amp
Figure 2 Block Diagram of Signal Processing Circuit and Photo of Experiment
