Voltage Output, High-Side or Low-Side Measurement,
Bidirectional Current Shunt Monitor
SGM8199
APPLICATION INFORMATION
voltage of power supply when input voltage is equal to
0mV.
Recommended Connection
The recommended connection of SGM8199 is shown in
Figure 1. Also, it is recommended that the shunt
resistor should be placed as close as possible to the
two input pins of the current shunt monitor, so the extra
resistance which is series with the measured shunt
resistor will be reduced.
Bidirectional Application
Bidirectional application indicates that the current shunt
monitor of SGM8199 can measure the current for two
different directions. For this special case, the REF pin
can be set anywhere between 0V to VCC for the bias of
output voltage. For simplification, it is recommended
that VREF = VCC/2 is a typical voltage point. However, if
the absolute values of positive and negative current are
not equal, the voltage of REF pin should be set other
than VCC/2.
The bypass capacitor is necessary for the current shunt
monitor to improve its stability. In most applications, the
power supply is noisy and it will affect the operation of
SGM8199, so the bypass capacitor is needed in this
case.
Power Supply
Input Filtering
The SGM8199 can accurately measure the current
when the common mode voltage exceeds the power
supply voltage presented at the VCC pin. For example,
the VCC power supply can be 5V and the load or
common mode power supply voltage is allowed to
reach up to 26V. The output voltage range is limited by
the level of the power supply.
It is not recommended to add a filter at the output of
SGM8199, as doing so increases impedance seen at
the output of the internal buffer. Filtering at the input
pins will be a good choice as long as the change of the
input impedance is taken in account. Figure 2 indicates
the application of input filtering.
For minimizing the error and enhancing the accuracy of
the result, the resistance of the external resistor RS
should be less than 10Ω.At the internal input structure
of SGM8199, there is a bias network which causes
mismatch in the bias current IB of two input pins, and
the external resistors result in the mismatch of voltage
drops (because of the mismatch of IB) across RS so that
the differential error will be created. Also, this differential
error will be reflected to the input pins of the current
shunt monitor and result in the accuracy. However, the
bias current difference can result in little effect on the
operation of the current shunt monitor so the users do
not need to care about that.
Selection of RSHUNT
For the typical range of the differential input, the current
shunt monitor of SGM8199 can work accurately with the
order of 10mV. The application of SGM8199 series will
determine the selection of the shunt resistor RSHUNT. Also,
the users should consider the trade-off between voltage
loss and the accuracy of small input signals. The effect
of offset can be minimized by using high values of
R
SHUNT, while the voltage loss can be minimized by
using low values of RSHUNT. For most applications, a
voltage drop of 60mV over RSHUNT is the appropriate
range for the selection of RSHUNT, and the corresponding
offset voltage is 350μV only.
Unidirectional Application
VCC
VCM
In unidirectional operation, the current measuring
direction of SGM8199 is fixed. Usually, the REF pin is
connected directly to GND pin to ensure that the output
is biased at 0V. Also, if the users desire to measure low
input voltage with high accuracy, biasing REF pin to
300mV is a good method to set the current shunt
monitor to its linear region.
RS < 10Ω
RINT
IN+
IN-
REF
OUT
+
RSHUNT
Bias
CF
_
Output
RS < 10Ω
RINT
Load
GND
For the least frequent case, REF pin can connect to the
power supply pin directly to measure the negative
current. In this case, the output voltage is equal to the
Figure 2. Filtering at Input
SG Micro Corp
www.sg-micro.com
MAY 2023
9