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AND9750/D
Current Sense Amplifiers,Frequently Asked Questions(FAQ)
Summary

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The following questions are typical questions thatON Semiconductor’s application support team answers on the topic ofcurrent sense amplifiers. If you have a question not addressed here,
please do not hesitate to contact your local ON Semiconductor
representative, or visit our Technical Support site to call or email us, orvisit our Design Support site.
RSHUNTPower
SupplyAPPLICATION NOTE

Load

NCS21xR,
NCS199AxR

R1

R3IN−−

IN++

OutputOUT

R4ReferenceVoltage

R2VSGND

REF+2.2 V to +26 V
0.01 mF
to0.1 mF

Figure 1. Example Application Schematic of a Current Sense Amplifier© Semiconductor Components Industries, LLC, 2018

July, 2018 − Rev. 01Publication Order Number:AND9750/D

AND9750/D
Power
SupplyRSHUNT+ Load −NCS21xR,
NCS199AxR

R1

R3
IN−−

IN++

Output
OUT

R4

R2

VS

GND

REF

+2.2 V to +26 V
0.01 mF
to0.1 mF

Figure 2. Example Application Schematic of a Current Sense Amplifier Configured for High-side Current Sensing

FREQUENTLY ASKED QUESTIONSWhat are the Common Current Sensing Circuit
Configurations?

What is a Current Sense Amplifier?

The common current sensing circuit configurations are:

Current sense amplifiers such as the NCS21xR andNCS199AxR family of devices, (also called current shuntmonitors or current shunt amplifiers) are amplifiers thatfeature internal precision resistors to create a full differential
input. Current sense amplifiers also feature zero-driftarchitecture or low offset voltage and low offset drift overtemperature. These devices measure small voltage drops
across a shunt resistor that is connected to the inputs. Thesedevices accurately amplify the 1 mV to 10 mV signal fromthe shunt, at various gains depending on the chosen device,
up to a gain of 500. The output voltage is related to input
current across the shunt resistor by:• Low-side sensing,• High-side sensing,• Unidirectional sensing, and• Bidirectional sensing.

The NCS21xR and NCS199AxR family of devices makes
possible all of these configurations.
What is Low-side Current Sensing?

Low-side current sensing places the low side of the loadat one end of the shunt or sense resistor, the other side of thesense resistor is connected to ground. The common mode
voltage for low-side sensing will be close to 0 V. Low-side
has the advantage of being simple to implement andinexpensive. However, it has the disadvantage of not being
able to detect load high-side shorts, and the ground path isdisturbed since the low side of the load is connected toground through the sense resistor. See Figure 3 and take noteof the position of the load.

V OUT
I SENSE + Gain
R SENSE(eq. 1)

where ISENSE is the current flowing through the shunt
resistor, VOUT is the output voltage, Gain is the fixedinternal gain of the amplifier, and RSENSE is the sense (alsocalled shunt) resistor.

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2

AND9750/D
Power
SupplyRSHUNT+ Load −NCS21xR,
NCS199AxR

R1

R3−

IN−

OUT+

IN+
R4Output

R2VS

GND

REF
+2.2 V to +26 V
0.01 mF
to0.1 mF

Figure 3. Low-side Current Sensing Example
What is High-side Current Sensing?

load shorts and not having resistance in the ground path tothe load. In the past, high-side sensing was more difficultand expensive to implement. However, current sense
amplifiers have provided an economical and easy to use
solution. See Figure 4 and take note of the position of theload.

High-side current sensing places the high-side of the loadat one end of the shunt or sense resistor and the low-side ofthe load will be connected directly to ground. Common
mode voltages are typically quite high: 20 V, 40 V, or even
80 V. High-side has the advantage of being able to detect
RSHUNT+ Load −NCS21xR,
NCS199AxRR1R3IN−−IN++OUT

R4R2GNDREF
VSPower
Supply+2.2 V to +26 V
0.01 mF
to0.1 mF

Figure 4. High-side Current Sensing Example

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3Output

AND9750/D
What is Unidirectional Current Sensing?

Use unidirectional sensing only when current flowing inone direction (into a load) needs to be sensed. The output ofthe amplifier will only swing in one direction. Notice inPower
SupplyFigure 5 that the REF pin is connected to ground, this groundconnection configures the device for unidirectional currentsensing.

RSHUNT+ Load −R1

NCS21xR,NCS199AxR
R3Output−IN−

OUT+IN+

R4

R2VSGND

REF+2.2 V to +26 V
0.01 mF
to0.1 mF

Figure 5. Unidirectional Current Sensing Example
What is Bidirectional Current Sensing?

power is disconnected would use bidirectional current
sensing. Notice in Figure 6 that the REF pin is connected tosome voltage potential above ground. This voltage potential
allows the output of the amplifier to swing in both directions,
positive and negative. Thus, current can be sensed in bothdirections.

Use bidirectional sensing when sensing current flowing inboth directions. For example, an application that monitors
the current flowing in one direction as the system charges
a battery and then monitors the current flowing the otherdirection as the battery discharges once the battery charger
RSHUNT+ Battery −NCS21xR,
NCS199AxR

R1

R3−

IN−

OUT+IN+

OutputR4R2GNDREF
VSPower
Supply+2.2 V to +26 V
0.01 mF
to0.1 mF

Figure 6. Bidirectional Current Sensing Example

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4Reference+ Voltage−

AND9750/D
Why is Low Input Offset Voltage (VOS) Important?

Why is Low Gain Error vs Temperature, or Gain Error
Drift Important?

It is desirable to keep the shunt voltage drop as low as
possible, and input offset voltage is the key parameter thatsets the low limit of what can be sensed. Low offset and theresulting low voltage drop makes possible smaller and more
economical shunts and improves system efficiency. For
example, measuring a 10 mV shunt drop on an amplifier
with a 1 mV offset voltage would result in a 10% error due
to the 1 mV offset voltage. By comparison, the NCS213R
has a 100 mV max offset voltage spec, which means only
a 1% error with the same 10 mV shunt drop. The NCS210R
has a max offset voltage of just 35 mV. The error witha 10 mV shunt drop in this case would be only 0.35%.

The Gain Error can be calibrated out of a system, but gain
error drift over temperature cannot be calibrated out and so
it is important that the spec for this parameter be as low as
possible. The NCS21xR and NCS199AxR offer low
10 ppm/°C max gain error drift over the full temperature
range of −40°C to +125°C.
How Important is the Connection to the Current Shunt
Resistor?

It is important to always follow the shunt resistor
manufacturer’s recommendations for connecting to theshunt resistor so that the current sense amplifier will
accurately sense the current.

Why is Low Input Offset Voltage vs Temperature,or Offset Drift Important?

Where Can I Find More Information on Current Sense
Amplifiers (CSA)?

The Input Offset Voltage can be calibrated out of a system,
but offset drift over temperature cannot be calibrated out, so
it is important that the spec for this parameter be as low as
possible. The NCS21xR and NCS199AxR series of current
sense amplifiers offer low 0.5 mV/°C max offset drift overthe full temperature range of −40°C to +125°C.

You can start here: ON Semiconductor’s Current Sense
Amplifiers.

Why is Low Gain Error Important?

Gain error defines how accurate the measurement will beover the entire dynamic range. Inaccuracy in current
measurements can reduce system efficiency through lostpower.

ON Semiconductor andare trademarks of Semiconductor Components Industries, LLC dba ON Semiconductor or its subsidiaries in the United States and/or other countries.
ON Semiconductor owns the rights to a number of patents, trademarks, copyrights, trade secrets, and other intellectual property. A listing of ON Semiconductor’s product/patent coverage
may be accessed at www.onsemi.com/site/pdf/Patent−Marking.pdf. ON Semiconductor reserves the right to make changes without further notice to any products herein.
ON Semiconductor makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does ON Semiconductor assume any liability
arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages. Buyer
is responsible for its products and applications using ON Semiconductor products, including compliance with all laws, regulations and safety requirements or standards, regardless ofany support or applications information provided by ON Semiconductor. “Typical” parameters which may be provided in ON Semiconductor data sheets and/or specifications can anddo vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for each customer application by customer’s
technical experts. ON Semiconductor does not convey any license under its patent rights nor the rights of others. ON Semiconductor products are not designed, intended, or authorized
for use as a critical component in life support systems or any FDA Class 3 medical devices or medical devices with a same or similar classification in a foreign jurisdiction or any devices
intended for implantation in the human body. Should Buyer purchase or use ON Semiconductor products for any such unintended or unauthorized application, Buyer shall indemnify andhold ON Semiconductor and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees
arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that ON Semiconductor wasnegligent regarding the design or manufacture of the part. ON Semiconductor is an Equal Opportunity/Affirmative Action Employer. This literature is subject to all applicable copyrightlaws and is not for resale in any manner.

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