Texas Instruments | How to Optimize Performance of AMC1204 in Voltage Sensing | Application notes | Texas Instruments How to Optimize Performance of AMC1204 in Voltage Sensing Application notes

Texas Instruments How to Optimize Performance of AMC1204 in Voltage Sensing Application notes
Application Report
SBAA216 – August 2015
How to Optimize Performance of AMC1204 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
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, AMC1204 can satisfy these
criteria. The AMC1204 is optimized for use in current-sensing applications using low-impedance 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 shunt resistor that you could use will not influence device performance
and how to optimize the system performance if you want to use large shunt resistors.
1
2
3
Contents
Design Consideration of AMC1204........................................................................................ 2
Optimize the Performance of AMC1204 with Large Shunt Resistor .................................................. 5
Conclusion .................................................................................................................... 6
List of Figures
1
Equivalent Analog Input Circuit ............................................................................................ 2
2
First Rough Solution to Perform Isolated Voltage Monitoring .......................................................... 2
3
Offset Error vs Different Shunt Resistor (AMC1204)
4
Gain Error vs Different Shunt Resistor (AMC1204) ..................................................................... 3
5
INL vs Different Shunt Resistor (AMC1305M25) ........................................................................ 4
6
Rout Simulation of OPA376 ................................................................................................ 5
7
Offset Error vs Different Shunt Resistor With OPA376 (AMC1204)
8
Gain Error vs Different Shunt Resistor With OPA376 (AMC1204) .................................................... 6
...................................................................
..................................................
3
6
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1
Design Consideration of AMC1204
1
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Design Consideration of AMC1204
The differential analog input of the AMC1204 is implemented with a switched-capacitor circuit. Figure 1
shows the simplified schematic of the ADC input circuitry; the right side of Figure 1 illustrates the input
circuitry with the capacitors and switches replaced by an equivalent circuit.
Figure 1. Equivalent Analog Input Circuit
Consider the input impedance of the AMC1204 (RID: 12.5 kΩ) in designs with high-impedance shunt
resistors that can cause degradation of specifications. However, the bias current of the AMC1204 is very
small, so it just will cause gain error and not have a big impact in offset error as occurs in the AMC1305.
Therefore, if AMC1204 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 AMC1204, the voltage across R2 must
be within ±250 mV, because the linear input range of AMC1204 is ±250 mV.
The first inclination to carry out the voltage sensing is to implement the circuit shown in Figure 2.
5V
Vbus
AMC1204
AVDD
R1
AINP
û-Modulator
R2
AINN
AGND
2.5-V
Ref
Isolation Barrier
IIB
DATA
CLKIN
GND
Figure 2. First Rough Solution to Perform Isolated Voltage Monitoring
The resistor divider formed by R1 and R2 in Figure 2 can be governed with Equation 1:
R2
VR2 = Vbus
R1 + R2
(1)
Where VR2 = 0.25 V for AMC1204.
2
How to Optimize Performance of AMC1204 in Voltage Sensing
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Design Consideration of AMC1204
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For instance, if the system supply is 30 V and the shunt resistor is 1 kΩ, then R1 can be calculated as 119
kΩ for AMC1204.
Second, as mentioned before, the larger shunt resistor is used, the larger gain error will be obtained.
Equation 2 can help you estimate gain error easily.
Vbus
´ (R2 / / RIN ) - VIN
R1 + (R2 / / RIN )
EG (%) =
VIN
(2)
Where VIN = 0.25 V for AMC1204.
Actual influence of offset error and gain error of different shunt resistors for AMC1204 is shown in
Figure 3, Figure 4, and Table 1, respectively. When the shunt resistor increases, the offset will not be
influenced too much, but gain error will grow linearly. For example, if the shunt resistor is equal to 1.004
Ω, the offset error and gain error is 0.32 mV and –0.97%. But when the shunt resistor rises to 2.4924 kΩ,
the offset error is still almost the same, 0.40 mV, however gain error will increase to –17.4%. That is
because input bias current of the AMC1204 is small enough, but input impedance is not big enough, so it
causes load effect that degrades gain error.
Besides, in the calculation, the load effect is considered, while Delta-sigma modulator error is not. Thus,
the result of calculation will be slightly different from that of measurement.
0.5
Offset Error (%)
0.4
0.3
0.2
0.1
Voltage divider_meas.
0
0
500
1000
1500
Shunt Resistor (:)
2000
2500
D001
Figure 3. Offset Error vs Different Shunt Resistor (AMC1204)
0.00
Gain Error (%)
-4.00%
-8.00%
-12.00%
-16.00%
Voltage divider_meas.
-20.00%
0
500
1000
1500
Shunt Resistor (:)
2000
2500
D002
Figure 4. Gain Error vs Different Shunt Resistor (AMC1204)
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Design Consideration of AMC1204
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Table 1. Gain Error vs Different Shunt Resistor (AMC1204)
Gain Error-Shunt Resistor (Ω)
Voltage divider_cal. (%)
Voltage divider_meas. (%)
1.004
–0.008%
–0.97%
129.57
–1.02%
–2.00%
486.1
–3.71%
–4.65%
999
–7.34%
–8.26%
2492.4
–16.51%
–17.40%
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: ±8 LSB). The measurement result for AMC1204 is shown in
Figure 5.
4.00
Voltage divider
3.00
2.00
INL (LSB)
1.00
0.00
-1.00
-2.00
-3.00
-4.00
0
500
1000
1500
Shunt Resistor (:)
2000
2500
D003
Figure 5. INL vs Different Shunt Resistor (AMC1305M25)
If voltage sensing system is not allowed to add additional compensated circuits, but AMC1204’s
performance is desired such as offset, gain error within datasheet specification, then shunt resistor, R2,
must be lower than 1 Ω.
4
How to Optimize Performance of AMC1204 in Voltage Sensing
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Optimize the Performance of AMC1204 with Large Shunt Resistor
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2
Optimize the Performance of AMC1204 with Large Shunt Resistor
In order to ignore errors which come from the op amp, you need to consider some specifications. First of
all, the offset error of the op amp must be much lower than ±1 mV. Secondly, 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 1 Ω. Some op amp datasheets just provide open-loop
output impedance, you could use Equation 3 to translate or use TI-TINA to simulate.
RO
Rout =
1 + Aolb
(3)
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
Figure 6. Rout Simulation of OPA376
Simulation and measurement results are shown in Figure 7 and Figure 8. The offset error is almost the
same when the shunt resistor is equal to 486.1 / 2.492 kΩ, but the gain error will be reduced from –4.65 /
–17.4% to –0.93 / –0.90%, respectively.
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Conclusion
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0.80
0.70
Offset Error (mV)
0.60
0.50
0.40
0.30
0.20
0.10
OPA376_meas.
Voltage divider_meas.
0.00
0
500
1000
1500
Shunt Resistor (:)
2000
2500
D004
Figure 7. Offset Error vs Different Shunt Resistor With OPA376 (AMC1204)
0.00
Gain Error (%)
-4.00%
-8.00%
-12.00%
-16.00%
OPA376_meas.
Voltage divider_meas.
-20.00%
0
500
1000
1500
Shunt Resistor (:)
2000
2500
D005
Figure 8. Gain Error vs Different Shunt Resistor With OPA376 (AMC1204)
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, this is a trade-off between performance and cost.
3
Conclusion
The application report provides straightforward equations to evaluate initial performance when you add
large shunt resistor in voltage sensing, and also presents a method to optimize performance of the
AMC1204. Hence, as long as you add suitable compensated circuit in these modulators, it can achieve
good performance although TI’s isolated delta-sigma modulator is optimized by current sensing.
6
How to Optimize Performance of AMC1204 in Voltage Sensing
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