Texas Instruments | Increase Measurement Accuracy With HS Amps for Low-Side Shunt Current Monitoring (Rev. A) | Application notes | Texas Instruments Increase Measurement Accuracy With HS Amps for Low-Side Shunt Current Monitoring (Rev. A) Application notes

Texas Instruments Increase Measurement Accuracy With HS Amps for Low-Side Shunt Current Monitoring (Rev. A) Application notes
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Increase Measurement Accuracy Using High-Speed
Amplifiers for Low-Side Shunt Current Monitoring
Author: Leaphar Castro
The need to accurately and quickly detect the load
current through a low-side shunt resistor is a critical
application in systems requiring overcurrent, feedback
control loops, battery monitoring, and power-supply
monitoring. Load current is often measured using lowside current sensing, which is when the voltage is
measured across a shunt-resistor that is placed
between the load and ground. One common way to
discretely implement low-side current monitoring is to
use a current-sense amplifier in a difference
configuration, as shown in Figure 1.
VCC
VEE
+
VCC
+
5V
0V
GND
GND
5k
GND
VCC
I_IN
100
R_Sense
15 m
OPA354
Out
100
GND
VEE
5k
Figure 1. Low-Side Current Sensing Test Circuit
Using the OPA354
Traditionally, this low-side current measurement is
done with a dedicated current sense amplifier or a
lower-speed amplifier connected to an external shuntresistor. However, in applications that are required to
detect a small, high-speed transient pulse, these
amplifiers tend to lack the adequate bandwidth needed
SBOA298A – May 2018 – Revised August 2019
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to replicate the pulse accurately in a single gain stage.
A possible solution is to use multiple gain stages with
a lower bandwidth device, thus increasing the amount
of components and potentially increasing the shunt
resistance in order to use a smaller gain. By having a
large shunt-resistor, you introduce noise to your signal,
increase the power dissipation, and cause ground
disturbances. Instead, an alternative solution is to use
a single high-speed amplifier. By using the high-speed
amplifier. you have more gain-bandwidth, which allows
you to use a single high-gain stage with a small shuntresistor. For current sensing applications, you want to
choose an amplifier with low offset and noise so that it
does not degrade the accuracy of low-voltage
measurements. Consider a widely used op-amp such
as the OPA365. This device has a maximum input
offset voltage as of 200 µV and an input voltage noise
of 4.5 nV/√Hz at 100 kHz. Using an amplifier such as
OPA365 allows you to implement the circuit in a single
high-gain stage, save board space, keep the shuntresistor low, and enable driving an analog-to-digital
converter (ADC) with a single device. The OPA365 is
available in an AEC-Q100 version (OPA365-Q1) that
supports automotive applications.
Choosing the correct amplifier simplifies detection of
high-current spikes that may cause damage to the
system or reduce motor and servo efficiency, all while
maximizing system efficiency. There are several
benefits to using a high-speed amplifier in currentsensing circuit. For example, in applications such as
power-supply monitoring, the duration of the pulse
may be as low as 1 µs. Without being able to detect
these transients, short-duration pulses may go
unnoticed, and thus cause glitches or potential
damage to the rest of the system. As shown in Figure
2, with a short-duration 1-µs pulse input in a gain of
50, the OPA354 is able to reach 3-V output and is able
to replicate the original input signal much closer than a
400-kHz or a 20-MHz bandwidth op amp. As Figure 3
shows, 100-nA input pulse in a gain of 50 is
introduced, and again the output response of the
OPA354 is much closer than the other two devices.
Increase Measurement Accuracy Using High-Speed Amplifiers for Low-Side
Shunt Current Monitoring Author: Leaphar Castro
Copyright © 2018–2019, Texas Instruments Incorporated
1
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microseconds. Use a high-speed amplifier for motorcontrol applications to provide a fast and precise
current measurement for the best dynamic motor
control, minimum torque ripple, and minimal audible
noise.
Figure 2. Output Response With a 3-A, 1-µs Input
Into 15 mΩ With a Gain of 50 vs an Integrated and
Lower-Bandwidth Device
For maximum system efficiency when using an op
amp to measure a small differential voltage signal from
the shunt-resistor, make sure the op amp has enough
bandwidth to make a precise and accurate
measurement without introducing error to the signal.
Measuring short duration pulses are a challenge, but
by using a high-speed amplifier, high slew rates and
plenty of bandwidth are available to track the input
signal. This article used the OPA354 as an example,
but there are many other available amplifiers offered
by Texas Instruments. For automotive applications that
require AEC-Q100 devices, and applications that
require higher supply ranges or higher bandwidth, see
Table 1. The amplifiers listed inTable 1 give alternative
recommendations with optimized parameter benefits,
and what the performance trade-offs are of using each
of the devices.
Table 1. Alternative Device Recommendations
Figure 3. 100-nA Pulse Input Into 15 mΩ With a
Gain of 50 vs an Integrated and Lower-Bandwidth
Device
In another example, a three-phase inverter shuntresistor is sensing large negative phase voltages.
These PWM duty cycles tend to be very small (around
2 µs). The current sense amplifier must be able to
settle to < 1% in this time frame, and in many cases
drive an ADC. In applications such as three-phase
inverters, maintain low distortion at the maximum rate
at which the output changes with respect to time. In
general, high-speed amplifiers offer slew rates > 25
V/µs and fast settling times of < 0.5 µs. These features
make high-speed amplifiers a great choice when a
high rate of change exists in the output voltage that is
caused by a step change on the input in the form of
short current pulses. High slew rate, larger bandwidth,
and fast settling high-speed amplifiers contribute to
keeping the detection time down to a few
2
Performance
Trade-Offs
Device
Optimized Parameters
OPA836
Higher bandwidth,
lower power consumption
IN to –V rail,
slightly less output
current
OPA2836-Q1
Dual channel,
higher bandwidth,
lower power consumption,
automotive qualification
IN to –V rail,
slightly less output
current
OPA354
OPA354-Q1
Higher bandwidth,
higher slew rate,
higher output current,
automotive qualification
(Q1)
Slightly higher
offset and power
consumption
LMH6618
Higher supply (maximum),
higher bandwidth,
lower IQ
Slightly higher
noise and less
output current
LMH6611
Higher supply (maximum),
higher bandwidth,
higher slew rate
IN to –V rail,
slightly higher
power consumption
Single, dual, quad channels,
higher supply (maximum),
IN to –V rail,
LMH6642
lower IQ ,
slightly higher
LMH6642Q-Q1
automotive qualification
noise
(Q1)
Increase Measurement Accuracy Using High-Speed Amplifiers for Low-Side
Shunt Current Monitoring Author: Leaphar Castro
SBOA298A – May 2018 – Revised August 2019
Submit Documentation Feedback
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