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Transcript
____________________________________________________
Integrating the Current Sensing Resistor
Scott Hill, Current Sensing Products
Current is one of the most common signals used for
evaluating and diagnosing the operational
effectiveness of an electronic system. However,
measuring this signal directly is very challenging.
Instead, many types of sensors are used to measure
the proportional effects that occur due to current
flowing throughout the system.
The most common sensing element used for detecting
current flowing in a system is a resistor. Placing a
resistor, called a shunt, in series with the current path
develops a differential voltage across the resistor as
current passes through it.
One common signal chain configuration for monitoring
a current signal involves an analog front-end (AFE), an
analog to digital converter (ADC), and a system
controller as shown in Figure 1. An AFE, such as an
operational amplifier or dedicated current sense
amplifier, converts the small differetial voltage
developed across the shunt resistor to a larger output
voltage that the ADC can digitize before sending the
information to a controller. The system controller uses
the current information to optimize the system's
operational performance or reduce functionality in the
event of an out-of-range condition to prevent potential
damaging conditions from occurring.
+
ADC
CONTROLLER
Figure 1. Current Sensing Signal Chain
The proper resistance value selection is critical in
optimizing the signal chain path. The resistance value
and corresponding voltage developed across the shunt
results in a system power loss. To limit the power
loss, it is preferred to minimize the shunt resistance.
The resistor value is directly proportional to the signal
developed and sent to the current sensing amplifier.
SBOA170A – October 2016 – Revised December 2016
Submit Documentation Feedback
Amplifiers have fixed inherent errors associated with
them, input offset voltage for example, that impact the
measurement accuracy. As the input signal increases,
the influence of these internal errors on the total
measurement accuracy decreases. When the input
signal decreases the corresponding measurement
error is a higher . This relationship between the signal
level and the acceptable measurement accuracy
provides general lower limits for the current sensing
resistor selection. The upper limit value for the current
sensing resistor should be limited based on an
application’s acceptable power loss for this
component.
One benefit of using resistors for current measurement
is the availability of accurate components that provide
both high precision and temperature stable
measurements. Precision current sensing amplifiers
are available featuring measurement capabilities
optimized for interfacing with very small signals to
accommodate small value resistors and low power
losses.
There are two trends for resistors as the ohmic value
decreases into the single digit milliohm level and
below. One trend for this segment of resistors is the
reduced package availability and resistor value
combinations. The other trend is the increased cost for
precision and low temperature coefficient components.
Pairing a low ohmic, low temperature coefficient
current sensing resistors with precision tolerance
levels (~0.1%) result in solution costs in the several
dollar range without including the cost associated with
the precision amplifier.
A component such as the INA250, shown in Figure 2,
helps reduce the challenges of selecting these
increased accuracy, higher cost resistors for
applications needing precise and temperature stable
measurements. This device pairs a precision, zerodrift, voltage output current sense amplifier with a 2mΩ
integrated current sensing resistor with a 0.1%
maximum tolerance and a temperature drift of
15ppm/ºC over the device's entire tempeature range of
-40ºC to +125ºC. This device can accommodate
continuous currents flowing through the on-board
resistor of up to 15A.
Integrating the Current Sensing Resistor Scott Hill, Current Sensing Products
Copyright © 2016, Texas Instruments Incorporated
1
www.ti.com
0V to 36V Supply
Supply
Voltage
0V to 36V
Supply
VS
IN+
INA250
IN-
IN+
VBUS
Load
INA260
SDA
V
SCL
ADC
I2C
Interface Alert
-
+
A0
A1
I
ADC
Reference
Voltage
IN-
REF
OUT
ADC
GPIO
Controller
Load
GND
Controller
Figure 2. Integrating The Current Sensing Resistor
In addition to the integrated precision resistor inside
this device, the INA250 also addresses one of the
most common issues associated with implementing a
current sensing solution. A low-ohmic shunt resistor is
used to reduce the current sensing power dissipation.
A challenge in accommodating this low resistance
value is the potential impact of parasitic resistance on
the PCB. Parasitic resistance in series with the shunt
resistor can cause additional measurement errors as
current flows through the resistance to create the
shunt voltage. The most common source for these
measurement errors is poor layout techniques. A
Kelvin connection, also known as a four terminal
connection or a force-sense, is required to ensure that
minimal additional resistance is present to alter the
differential voltage developed between the amplifier's
input pins. There are PCB layout techniques to reduce
the effect of parasitic resistance, however, this
concern is removed with the INA250.
As previously described, the typical current sensing
signal chain path includes the current sensing resistor,
the analog front-end, ADC and system controller. The
INA250 combines the shunt resistor and the current
sensing amplifier. The INA260 combines the current
sensing resistor, measurement front-end and the ADC
into one single device.
Figure 3 shows the INA260 featuring the same
precision, integrated sensing resistor, pairing it with a
16-bit, precision ADC optimized for current sensing
applications. This combination provides an even higher
performance measurement capability than the INA250
resulting in a maximum measurement gain error of
0.5% over the entire temperature range and a
maximum input offset current of 5mA.
2
GND
SDA
SCL
Integrating the Current Sensing Resistor Scott Hill, Current Sensing Products
Figure 3. Integrated Signal Path
Pairing the precision, low-drift current sensing with
these precision current sensing devices provides
measurement solutions that are challenging to
accomplish using discrete amplifier and resistor
combinations. There are few catalog current sensing
resistors available that are capable of enabling the
combination of precision and temperature stable
measurements but achieving this level of accuracy in a
solution size comparable to TSSOP-16 packaged
integrated solutions doesn't exist.
Alternate Device Recommendations
For additional design flexibility, many stand-alone
current sensing amplifiers and digital power monitors
are also available. For lower performance applications
with higher current requirements than the integrated
solutions support, use the INA210 stand-alone current
sensing amplifier. For applications requring a standalone digital power monitor, use the INA226. For
applications implementing over-current detection, the
INA301 features an integrated comparator for on-chip
over-current detection as fast as 1μs.
Table 1. Alternative Device Recommendations
Device
Optimized Parameter
Performance TradeOff
INA210
35μV VOS, Package: SC706, QFN-10
No on-boad current
sensing resistor
INA226
10μV VOS, Package:
MSOP-10
No on-boad current
sensing resistor
INA301
Signal Bandwidth, OnBoard Comparator
No on-boad current
sensing resistor
Table 2. Related TI TechNotes
SBOA162
Measuring Current To Detect Out-of-Range
Conditions
SBOA165
Precision Current Measurements on HighVoltage Power Supply Rails
SBOA167
Integrating the Current Sensing Signal Path
SBOA169
Precision, Low-Side Current Measurement
SBOA170A – October 2016 – Revised December 2016
Submit Documentation Feedback
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