Texas Instruments | How to Optimize Performance of AMC1304/05M25 in Voltage Sensing | Application notes | Texas Instruments How to Optimize Performance of AMC1304/05M25 in Voltage Sensing Application notes

Texas Instruments How to Optimize Performance of AMC1304/05M25 in Voltage Sensing Application notes
Application Report
SBAA214 – August 2015
How to Optimize Performance of AMC1304/05M25 in
Voltage Sensing
Polly Chung
ABSTRACT
Many applications such as motor drives and power inverters require measurements of both current and
voltage to obtain motor information, for example, speed, torque, and power, to control, monitor, and
protect the system. Meanwhile, these applications will be operated in harsh, noisy environments and high
voltage difference between power stage and control stage. Therefore, this is very important that the device
have precise performance and isolation functions simultaneously. In this case, the AMC1304/05M25 can
satisfy these criteria. AMC1304/05M25 is optimized for use in current-sensing applications using lowimpedance shunts. However, the device can also be used in isolated voltage sensing. In terms of that, this
application note will give you an idea of how large a shunt resistor can be used that will not influence
device performance and also how to optimize the system performance if you want to use large shunt
resistors. In the following content, we will focus on the AMC1305M25 device.
1
2
3
Contents
Design Consideration of AMC1305M25 .................................................................................. 2
Optimize the Performance of AMC1305M25 With Large Shunt Resistor ............................................ 5
Conclusion .................................................................................................................... 8
List of Figures
1
First Rough Solution to Perform Isolated Voltage Monitoring .......................................................... 2
2
Offset Error vs Different Shunt Resistor (AMC1305M25)
3
4
5
6
7
8
9
10
11
..............................................................
Gain Error vs Different Shunt Resistor (AMC1305M25) ................................................................
INL vs Different Shunt Resistor (AMC1305M25) ........................................................................
Simplified Circuit With Series Shunt Resistor ...........................................................................
Offset Error vs Different Shunt Resistor With Series Shunt Resistor (AMC1305M25) ............................
Gain Error vs Different Shunt Resistor With Series Shunt Resistor (AMC1305M25) ..............................
Rout Simulation of OPA376 ................................................................................................
Simplified Circuit With OPA376 ...........................................................................................
Offset Error vs Different Shunt Resistor With OPA376 (AMC1305M25) .............................................
Gain Error vs Different Shunt Resistor With OPA376 (AMC1305M25) ...............................................
3
4
4
5
5
6
7
7
8
8
All trademarks are the property of their respective owners.
SBAA214 – August 2015
Submit Documentation Feedback
How to Optimize Performance of AMC1304/05M25 in Voltage Sensing
Copyright © 2015, Texas Instruments Incorporated
1
Design Consideration of AMC1305M25
1
www.ti.com
Design Consideration of AMC1305M25
Consider the input impedance of the AMC1305M25 (RID: 25 kΩ) in designs with high-impedance shunt
resistors that can cause degradation of gain and offset specifications. However, the importance of this
effect depends on the desired system performance. Therefore, if AMC1305M25 is designed by such
applications, there are important details that need to be remembered when you choose the resistor
divider.
First, in order to efficiently use the available linear input range of AMC1305M25, the voltage across R2
must be within ±250 mV, because the linear input range of AMC1305M25 is ±250 mV.
The first inclination to carry out the voltage sensing is to implement the circuit which is shown in Figure 1.
5V
Vbus
AMC1305
AVDD
R1
AINP
R4
IIB
RID
R2
R5
û-Modulator
+
AINN
R4'
R5'
AGND
VCM = 1.88 V
GND
Figure 1. First Rough Solution to Perform Isolated Voltage Monitoring
The resistor divider formed by R1 and R2 in Figure 1 can be governed with Equation 1.
R2
VR2 = Vbus
R1 + R2
(1)
Where VR2 = 0.25 V for AMC1305M25.
For instance, if the system supply is 30 V and shunt resistor is 1 kΩ, then R1 can be calculated which is
119 kΩ for AMC1305M25.
Second, as mentioned before, the larger shunt resistor is used, the larger offset, gain error will be
obtained. Equation 2 helps easily estimate offset error.
VOS = R2 × Iib
(2)
Where Iib ≈ 30 µA when Vcm = 0 V for AMC1305M25.
The gain error is calculated with Equation 3:
Vbus
´ (R2 / / RIN ) - VIN
R1 + (R2 / / RIN )
EG (%) =
VIN
(3)
Where VIN = 0.25 V for AMC1305M25.
2
How to Optimize Performance of AMC1304/05M25 in Voltage Sensing
Copyright © 2015, Texas Instruments Incorporated
SBAA214 – August 2015
Submit Documentation Feedback
Design Consideration of AMC1305M25
www.ti.com
Actual influence of offset error and gain error of different shunt resistors for AMC1305M25 is shown in
Figure 2 and Figure 3, Table 1 and Table 2, respectively. When the shunt resistor increases, the offset
and gain error will grow linearly. For example, if the shunt resistor is equal to 0.754 Ω, the offset error and
gain error is 14 µV and 0.02%. But when the shunt resistor rises to 2.4924 kΩ, the offset error and gain
error will become 65.62 mV and –10.65%. That is because input bias current caused by the internal
common-mode voltage at the output of the differential amplifier will flow out to shunt resistor, which will
cause additional offset error. In addition, with a large shunt resistor, load effect will cause extra gain error.
In this calculation only the effect from the differential amplifier is considered, whereas both input filter and
Delta-sigma modulator error are not. By contrast, the simulation considers the effect from differential
amplifier and input filter, but it does not include Delta-sigma modulator error. So these will have slight
differences with measurement. The simulation result is done with TI-TINA.
80
70
Offset error (mV)
60
50
40
30
20
10
Voltage divider_meas.
Voltage divider_sim.
0
0
500
1000
1500
Shunt Resistor (:)
2000
2500
D001
Figure 2. Offset Error vs Different Shunt Resistor (AMC1305M25)
Table 1. Offset Error vs Different Shunt Resistor (AMC1305M25)
Offset Error-Shunt Resistor
(Ω)
Voltage divider_cal. (mV)
Voltage divider_meas. (mV)
Voltage divider_sim. (mV)
0.754
0.022620
0.013876
0.022467
64.88
1.946400
1.892609
1.925176
486.1
14.58300
13.97162
14.14728
999
29.97000
28.05624
28.39243
2492.4
74.77200
65.61527
66.35166
SBAA214 – August 2015
Submit Documentation Feedback
How to Optimize Performance of AMC1304/05M25 in Voltage Sensing
Copyright © 2015, Texas Instruments Incorporated
3
Design Consideration of AMC1305M25
www.ti.com
0
Voltage divider_meas.
Voltage divider_sim.
Gain error (%)
-3%
-6%
-9%
-12%
-15%
0
500
1000
1500
Shunt Resistor (:)
2000
2500
D002
Figure 3. Gain Error vs Different Shunt Resistor (AMC1305M25)
Table 2. Gain Error vs Different Shunt Resistor (AMC1305M25)
Gain ErrorShunt resistor (Ω)
Voltage divider_cal. (%)
Voltage divider_meas. (%)
Voltage divider_sim. (%)
0.754
–0.003586%
0.02%
–0.003587%
64.88
–0.307846%
–0.52%
–0.466670%
486.1
–2.262164%
–2.44%
–2.414913%
999
–4.539821%
–4.68%
–4.685772%
2492.4
–10.608349%
–10.65%
–10.736764%
The INL will not be influenced by different shunt resistors. No matter how large shunt resisters will be, INL
will always be within specification (max: ±4 LSB). The measurement result is shown in Figure 4 for
AMC1305M25.
4
Voltage divider
3
INL (LSB)
2
1
0
-1
-2
-3
-4
0
500
1000
1500
Shunt Resistor (:)
2000
2500
D003
Figure 4. INL vs Different Shunt Resistor (AMC1305M25)
If the voltage sensing system is not allowed to add additional compensated circuits, but AMC1305M25's
performance is desired such as offset, gain error within datasheet specification, then the shunt resistor,
R2, must be lower than 0.75 Ω.
4
How to Optimize Performance of AMC1304/05M25 in Voltage Sensing
Copyright © 2015, Texas Instruments Incorporated
SBAA214 – August 2015
Submit Documentation Feedback
Optimize the Performance of AMC1305M25 With Large Shunt Resistor
www.ti.com
2
Optimize the Performance of AMC1305M25 With Large Shunt Resistor
If the system needs to use a large shunt resistor to realize voltage sensing, then there are two methods
that can help you minimize system error.
Method 1:
Series resistor at the negative input (AINN) of the AMC1305M25 with a value equal to the shunt resistor
R2 (that is R2' = R2 in Figure 5) to eliminate the effect of the bias current. This method can help minimize
offset error, but will cause extra gain error. The effect can be calculated using Equation 4 with R5 = R5' =
50 kΩ and R4 = R4' = 12.5 kΩ for AMC1305M25.
æ
ö
R4
EG (%) = ç 1 +
÷ ´ 100%
R4 ' + R2' ø
è
(4)
The simulation and measurement result of AMC1305M25 is shown in Figure 6 and Figure 7. The offset
error can be minimized from 13.97 / 65.62 mV to –0.123 / –0.069 mV when the shunt resistor is equal to
486.1 / 2.492 kΩ, but gain error will increase from –2.44 / –10.65% to –3.87 / –16.44%, respectively.
5V
Vbus
AMC1305
AVDD
R1
AINP
R4
IIB
-
RID
R2
R5
û-Modulator
+
AINN
R2'
R4'
R5'
AGND
VCM = 1.88 V
GND
Figure 5. Simplified Circuit With Series Shunt Resistor
80
Voltage divider_meas.
Voltage divider_sim.
Series Rshunt_meas.
Series Rshunt_sim.
70
Offset error (mV)
60
50
40
30
20
10
0
-10
0
500
1000
1500
Shunt Resistor (:)
2000
2500
D004
Figure 6. Offset Error vs Different Shunt Resistor With Series Shunt Resistor (AMC1305M25)
SBAA214 – August 2015
Submit Documentation Feedback
How to Optimize Performance of AMC1304/05M25 in Voltage Sensing
Copyright © 2015, Texas Instruments Incorporated
5
Optimize the Performance of AMC1305M25 With Large Shunt Resistor
www.ti.com
0
Gain error (%)
-3%
-6%
-9%
-12%
Voltage divider_meas.
Voltage divider_sim.
Series Rshunt_meas.
Series Rshunt_sim.
-15%
-18%
0
500
1000
1500
Shunt Resistor (:)
2000
2500
D005
Figure 7. Gain Error vs Different Shunt Resistor With Series Shunt Resistor (AMC1305M25)
Method 2:
Adding an op amp between the voltage divider and the AMC1305M25 can optimize overall system
performance. The characteristic of an op amp can help minimize both offset and gain error from bias
current and load effect.
2.1
Choose Suitable Op Amp
In order to ignore errors which come from the op amp, some specifications must be considered. First of
all, the offset error of the op amp must be much lower than ±150 µV. Secondly, the bandwidth of the op
amp must be higher than 1 MHz. Third, the input bias current must be smaller to avoid offset error. Last,
closed-loop output impedance must be smaller than 0.75 Ω. Some op amp datasheets just provide openloop output impedance, use Equation 5 to translate or use TI-TINA to simulate.
RO
Rout =
1 + Aolb
(5)
Based on this criterion, OPA376 is used.
Vos(max): 25 µV, Vos(typ.): 5 µV
GBW: 5.5 MHz
Iib(max): 10 pA
Rout at DC: 163.5 µΩ
6
How to Optimize Performance of AMC1304/05M25 in Voltage Sensing
Copyright © 2015, Texas Instruments Incorporated
SBAA214 – August 2015
Submit Documentation Feedback
Optimize the Performance of AMC1305M25 With Large Shunt Resistor
www.ti.com
Figure 8. Rout Simulation of OPA376
The simplified circuit is presented in Figure 9. The offset and gain error can be minimized when you add
OPA376 between the voltage divider and AMC1305M25. Simulation and measurement results are shown
in Figure 10 and Figure 11. When the shunt resistor is equal to 486.1 / 2.492 kΩ, the offset error can be
reduced from 13.97 / 65.62 mV to 0.009 / –0.007 mV and gain error will be also reduced from –2.44 /
–10.65% to –0.04 / –0.04%, respectively.
5V
Vbus
AMC1305
AVDD
R1
+
AINP
R4
IIB
-
RID
OPA376
R5
û-Modulator
+
R2
AINN
R4'
GND
R5'
AGND
VCM = 1.88 V
GND
Figure 9. Simplified Circuit With OPA376
SBAA214 – August 2015
Submit Documentation Feedback
How to Optimize Performance of AMC1304/05M25 in Voltage Sensing
Copyright © 2015, Texas Instruments Incorporated
7
Conclusion
www.ti.com
80
Voltage divider_meas.
Voltage divider_sim.
Series Rshunt_meas.
Series_Rshunt_sim.
OPA376_meas.
OPA376_sim.
70
Offset error (mV)
60
50
40
30
20
10
0
-10
0
500
1000
1500
Shunt Resistor (:)
2000
2500
D006
Figure 10. Offset Error vs Different Shunt Resistor With OPA376 (AMC1305M25)
3%
0
Gain error (%)
-3%
-6%
-9%
-12%
Voltage divider_meas.
Voltage divider_sim.
Series Rshunt_meas.
Series Rshunt_sim.
OPA376_meas.
OPA376_sim.
-15%
-18%
0
500
1000
1500
Shunt Resistor (:)
2000
2500
D007
Figure 11. Gain Error vs Different Shunt Resistor With OPA376 (AMC1305M25)
The designer must be aware that if their system does not have suitable power for the op amp, you might
need to design other power paths. Therefore, there is a trade-off between performance and cost.
3
Conclusion
This application report provides straightforward equations to evaluate initial performance when you add a
large shunt resistor in voltage sensing, and also presents two methods to optimize performance of the
AMC1304/05M25. Hence, as long as you add a suitable compensated circuit in these modulators, it can
achieve good performance although TI’s isolated delta-sigma modulator is optimized by current sensing.
8
How to Optimize Performance of AMC1304/05M25 in Voltage Sensing
Copyright © 2015, Texas Instruments Incorporated
SBAA214 – August 2015
Submit Documentation Feedback
IMPORTANT NOTICE
Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and other
changes to its semiconductor products and services per JESD46, latest issue, and to discontinue any product or service per JESD48, latest
issue. Buyers should obtain the latest relevant information before placing orders and should verify that such information is current and
complete. All semiconductor products (also referred to herein as “components”) are sold subject to TI’s terms and conditions of sale
supplied at the time of order acknowledgment.
TI warrants performance of its components to the specifications applicable at the time of sale, in accordance with the warranty in TI’s terms
and conditions of sale of semiconductor products. Testing and other quality control techniques are used to the extent TI deems necessary
to support this warranty. Except where mandated by applicable law, testing of all parameters of each component is not necessarily
performed.
TI assumes no liability for applications assistance or the design of Buyers’ products. Buyers are responsible for their products and
applications using TI components. To minimize the risks associated with Buyers’ products and applications, Buyers should provide
adequate design and operating safeguards.
TI does not warrant or represent that any license, either express or implied, is granted under any patent right, copyright, mask work right, or
other intellectual property right relating to any combination, machine, or process in which TI components or services are used. Information
published by TI regarding third-party products or services does not constitute a license to use such products or services or a warranty or
endorsement thereof. Use of such information may require a license from a third party under the patents or other intellectual property of the
third party, or a license from TI under the patents or other intellectual property of TI.
Reproduction of significant portions of TI information in TI data books or data sheets is permissible only if reproduction is without alteration
and is accompanied by all associated warranties, conditions, limitations, and notices. TI is not responsible or liable for such altered
documentation. Information of third parties may be subject to additional restrictions.
Resale of TI components or services with statements different from or beyond the parameters stated by TI for that component or service
voids all express and any implied warranties for the associated TI component or service and is an unfair and deceptive business practice.
TI is not responsible or liable for any such statements.
Buyer acknowledges and agrees that it is solely responsible for compliance with all legal, regulatory and safety-related requirements
concerning its products, and any use of TI components in its applications, notwithstanding any applications-related information or support
that may be provided by TI. Buyer represents and agrees that it has all the necessary expertise to create and implement safeguards which
anticipate dangerous consequences of failures, monitor failures and their consequences, lessen the likelihood of failures that might cause
harm and take appropriate remedial actions. Buyer will fully indemnify TI and its representatives against any damages arising out of the use
of any TI components in safety-critical applications.
In some cases, TI components may be promoted specifically to facilitate safety-related applications. With such components, TI’s goal is to
help enable customers to design and create their own end-product solutions that meet applicable functional safety standards and
requirements. Nonetheless, such components are subject to these terms.
No TI components are authorized for use in FDA Class III (or similar life-critical medical equipment) unless authorized officers of the parties
have executed a special agreement specifically governing such use.
Only those TI components which TI has specifically designated as military grade or “enhanced plastic” are designed and intended for use in
military/aerospace applications or environments. Buyer acknowledges and agrees that any military or aerospace use of TI components
which have not been so designated is solely at the Buyer's risk, and that Buyer is solely responsible for compliance with all legal and
regulatory requirements in connection with such use.
TI has specifically designated certain components as meeting ISO/TS16949 requirements, mainly for automotive use. In any case of use of
non-designated products, TI will not be responsible for any failure to meet ISO/TS16949.
Products
Applications
Audio
www.ti.com/audio
Automotive and Transportation
www.ti.com/automotive
Amplifiers
amplifier.ti.com
Communications and Telecom
www.ti.com/communications
Data Converters
dataconverter.ti.com
Computers and Peripherals
www.ti.com/computers
DLP® Products
www.dlp.com
Consumer Electronics
www.ti.com/consumer-apps
DSP
dsp.ti.com
Energy and Lighting
www.ti.com/energy
Clocks and Timers
www.ti.com/clocks
Industrial
www.ti.com/industrial
Interface
interface.ti.com
Medical
www.ti.com/medical
Logic
logic.ti.com
Security
www.ti.com/security
Power Mgmt
power.ti.com
Space, Avionics and Defense
www.ti.com/space-avionics-defense
Microcontrollers
microcontroller.ti.com
Video and Imaging
www.ti.com/video
RFID
www.ti-rfid.com
OMAP Applications Processors
www.ti.com/omap
TI E2E Community
e2e.ti.com
Wireless Connectivity
www.ti.com/wirelessconnectivity
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2015, Texas Instruments Incorporated
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Related manuals

Download PDF

advertising