Texas Instruments | Precision, Low-Side Current Measurement (Rev. B) | Application notes | Texas Instruments Precision, Low-Side Current Measurement (Rev. B) Application notes

Texas Instruments Precision, Low-Side Current Measurement (Rev. B) Application notes
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Precision, low-side current measurement
Dennis Hudgins, Current Sensing Products
For most applications, current measurements are
made by sensing the voltage drop across a resistor.
There are two locations in a circuit that resistors are
commonly placed for current measurements. The first
location is between the power supply and load. This
measurement method is referred to as high-side
sensing. The second location a sense resistor is
commonly placed is between the load and ground.
This method for sensing the current is referred to as
low-side current sensing. Figure 1 shows the two
methods to sense current in a load.
Power
Supply
Current Flow
Direction
circuits are referenced to the supply ground. To
minimize this issue, reference all circuits that have
interactions to the same ground. Reducing the value of
the current sense resistor helps minimize any ground
shifts.
Low-side current sensing is the easiest method to use
when designing circuits or choosing devices to do
current measurements. Due to the low common mode
voltage at the inputs, a difference amplifier typology
can be used. Figure 2 shows the classical difference
amplifier typology using an operational amplifier (opamp).
Power
Supply
+
High-Side
Sensing
RSENSE
-
VOUT
LOAD
Common-mode voltage
(VCM) is supply dependent
R2
R1
LOAD
Current Flow
Direction
RSENSE
V1
+
+
Low-Side
Sensing
-
+
VOUT
RSENSE
V2
Common-mode voltage
is always near ground and is
isolated from supply spikes
Figure 1. Current Sensing Methods
R1
R2
Figure 2. Operational Amplifier Configuration for
Low-Side Sensing
There are advantages and disadvantages of doing
either measurement. One of the advantages of lowside current measurements is the common-mode
voltage, or the average voltage at the measurement
inputs is near zero. This makes it easier to design
application circuits or select devices for this
measurement. Since the voltages seen by the current
sensing circuit are near the ground, this is the
preferred method of measuring currents when dealing
with very high voltages, or in applications where the
supply voltage may be prone to spikes or surges. The
immunity to high voltage spikes and ability to monitor
currents in high-voltage systems make low-side
current sensing popular in many automotive, industrial,
and telecommunication applications. The major
disadvantage of low-side current sensing is that the
voltage drop across the sense resistor appears as a
difference between the supply ground and the
load/system ground. This can be an issue if other
SBOA169B – October 2016 – Revised March 2019
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R2
V1 V2
R1
When using an op-amp for current sense
measurements, there are several performance
requirements that need to be met to ensure correct
operation. First, the operation amplifier needs to
support common-mode input voltages to ground when
operated from a signal supply. Since the difference
amplifier typically gains the differential input signal, the
swing to rail specification of the op-amp is important in
order to ensure the signal is correctly passed to the
output. For these reasons, rail-to-rail input and output
operation amplifiers are generally preferred for current
sense measurements. Since operational amplifiers are
not specified in the difference amplifier configuration, it
is difficult to tell what the performance can be in the
real application. Parameters such as slew rate,
bandwidth, input current, common mode rejection, and
drift are all degraded when resistors around the opamp are added to create the current sense circuit. The
parametric degradation depends on the closed loop
Precision, low-side current measurement Dennis Hudgins, Current Sensing Products
Copyright © 2016–2019, Texas Instruments Incorporated
1
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gain of the amplifier and values of the gain setting
resistors. Figure 2 shows the matching and tolerance
of R1 and R2. They need to be considered when
implementing a discrete solution since variations in
these components directly affect the circuit gain error.
area, and simplifies the layout. Integration of these
resistors does not necessarily mean an increase in
package size. The INA199 is available both in the 2mm x 1.25-mm SC70-6 leaded package and the 1.8mm x 1.4-mm UQFN package.
Another factor to consider when implementing a
discrete current sense amplifier is the PCB layout. R1
and R2 need placed as closely as possible to the
operational amplifier and current sense resistor. By
placing these components close the op-amp the
likelihood of noise pickup on the operational amplifier
positive input is reduced. Since many current sense
amplifiers are used with DC/DC convertors, the
placement of the entire current sense circuit needs to
be carefully considered to avoid radiated noise by the
DC/DC power supplies. Figure 2 shows how to
calculate the difference amplifier gain. However, any
increase or decrease in the gain affects the solution
stability and bandwidth. The stability of the op-amp
requires special consideration in applications where a
capacitive load is present to avoid oscillations or
excessive output ringing.
The current measurement accuracy of the INA199 is
better than what is achievable with cost effective
discrete op-amp designs. The device features a
maximum gain error of 1.5% over the temperature
range of -40°C to 105°C. The offset of the INA199 is
less than 150 μV and drifts less than 0.5 μV/°C.
Figure 3 shows a current sense amplifier, which is an
effective way to address the weaknesses of the
discrete implementation.
Power
Supply
LOAD
VS = 2.7 V to 26 V
IN+
Reference
Voltage
REF
+
The INA199 also features a REF pin. The voltage
applied at the REF pin adds to the voltage seen at the
output. This is useful if down-stream devices need to
have the current signal level-shifted.
Alternate Device Recommendations
For a smaller current sense solution with improved
accuracy, the INA185 provides 0.2% gain error in the
very small SOT-563 package. For applications
requiring higher performance, the INA210-215 series
of devices provide low offset (35 μV maximum) and
gain error (1% Max). If a high accuracy current monitor
with a digital interface is needed, the INA226 features
a maximum offset of 10 μV and a gain error of 0.1%. If
a small digital based current monitor is needed, the
INA231 is offered in a tiny 1.68-mm x 1.43-mm
package and is well-suited for portable or other space
constrained applications. If a voltage output current
monitor is needed with pin strappable gain settings,
use the INA225.
Table 1. Alternative Device Recommendations
OUT
RSENSE
DEVICE
OPTIMIZED PARAMETERS
PERFORMAN
CE TRADEOFF
INA185
Solution size, Accuracy
Slightly higher
cost
Accuracy
Slightly higher
cost
INGND
INA210
Figure 3. Low-Side Current Sensing with INA199
Current Sense Amplifier
INA215
INA231
INA226
A current sense amplifier integrates the gain setting
resistors, reducing many of the layout concerns that
exist with discrete implementations. The internal
resistors are designed to reduce mismatch, which
optimizes the gain error specification. Current sense
amplifiers come preconfigured to address many
different gain requirements. For example, the INA199
is available with gains of 50, 100, and 200 V/V. The
bandwidth and capacitor load stability is optimized for
each gain setting with maximum capacitive loads
specified in the datasheet. Integration of the gain
setting resistors reduces noise susceptibility, PCB
2
Precision, low-side current measurement Dennis Hudgins, Current Sensing Products
Digital interface, small size
Cost
Digital interface, high accuracy
Package size,
cost
Table 2. Related Documentation
SBOA161
Low-Drift, Low-Side Current Measurements
for Three Phase Systems
SBOA167
Integrating The Current Sensing Signal Path
SBOA165
Precision Current Measurements on HighVoltage Power Supply Rails
SBOA169B – October 2016 – Revised March 2019
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