Texas Instruments | Current Sensing With Near-Zero Common Mode Voltage with INA19xA-Q1 | Application notes | Texas Instruments Current Sensing With Near-Zero Common Mode Voltage with INA19xA-Q1 Application notes

Texas Instruments Current Sensing With Near-Zero Common Mode Voltage with INA19xA-Q1 Application notes
Application Report
SLOA228 – October 2015
Current Sensing With Near-Zero Common Mode Voltage
With INA19xA-Q1
Mahmoud Harmouch ........................................................................................ MSA - Auto Signal Chain
ABSTRACT
This report provides a summary of using the INA19xA-Q1 device when the common mode voltage is close
to zero volts. In this low common-mode voltage region, the output voltage has a large error. The example
for this case is low-side current sensing with a differential input below 20 mV. The INA19xA-Q1 device is
not a good fit for situations where the common mode voltage is near zero. Instead TI recommends devices
such as the INA282-Q1 device.
1
2
3
Contents
Block Diagram Description ................................................................................................. 2
Setup for Low Common-Mode Voltage Sensing......................................................................... 3
Summary ...................................................................................................................... 8
List of Figures
1
Block Diagram ................................................................................................................ 2
2
Test Circuit.................................................................................................................... 3
3
VOUT Plots Across Temperature for 30 units at V+ = 5 V, [(VIN+) ― (VIN–)] Varies from 0 to 20 mV and
Common Mode Voltage Varies from –0.7 V to 19.6 V .................................................................. 5
4
VOUT Plots Across Temperature for 30 Units at V+ = 5 V, [(VIN+) ― (VIN–)] Varies from 0 to 20 mV
and Common Mode Voltage Varies from 8 V to 16 V .................................................................. 6
5
Linearity Plots Across Temperature for 30 Units at V+ = 5 V, [(VIN+) ― (VIN–)] Varies from 0 to 20 mV
and Common Mode Voltages at 8.4 V, 12.6 V and 16.1 V............................................................. 7
6
Output Error Plots Across Temperature for 30 Units at V+ = 5 V, [(VIN+) ― (VIN–)] Varies from 0 to 20
mV and Common Mode Voltage Varies from –0.7 V to 19.6 V........................................................ 8
List of Tables
.......................................................................................................
1
Characterization Plan
2
Characterization Pattern .................................................................................................... 4
3
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1
Block Diagram Description
1
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Block Diagram Description
The INA19xA-Q1 device family uses two operational amplifier front ends, A1 and A2, to achieve a wide
input common-mode voltage range. A1 amplifier is active when the common mode voltage is negative,
and A2 amplifier is active when the common mode voltage is positive. When the common mode is close to
zero, both amplifiers, A1 and A2, can be active resulting in high output error if the sense voltage is below
20 mV.
VIN+
VIN
R1(1)
5k
R1(1)
5k
V+
A1
A2
G = 20, RL = 100 k
G = 50, RL = 250 k
G = 100, RL = 500 k
INA193A-INA198A
OUT
R1(1)
GND
Figure 1. Block Diagram
2
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Setup for Low Common-Mode Voltage Sensing
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2
Setup for Low Common-Mode Voltage Sensing
The test setup was done according to circuit in Figure 2. Table 1 lists a summary of the characterization
plan.
5V
0.1 µF
GND
VIN+
V+
3
Voltage
Source,
VCM
5
INA195A-Q1
25
1
VOUT
2
4
VIN±
Current
Source,
IS
GND
GND
GND
The sense voltage, VSNS, = 25 Ω × IS
Figure 2. Test Circuit
Table 1. Characterization Plan
Parameter
Characterization Plan
Supply voltage, V+
5 V ±6%
Sense voltage, VSNS [(VIN+) – (VIN–)]
0 mV to 20 mV in 2-mV steps
Common mode voltage, VCM
–0.7 V to 18.9 V in 0.7-V steps
Temperatures
–40°C, 25°C, 105°C, 125°C
Number of units from 1 lot
30 pieces
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Setup for Low Common-Mode Voltage Sensing
2.1
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Characterization
Automated test equipment (ATE) based on the values in Table 2 was used to complete the
characterization.
Table 2. Characterization Pattern
Parameter
Start
Stop
Delta
Step
5V
5V
0V
0V
1
Sense voltage, VSNS [(VIN+) – (VIN)]
0 mV
20 mV
20 mV
2 mV
11
Common mode, VCM
–0.7 V
18.9 V
19.6 V
0.7 V
29
Supply voltage, V+
Runs
Table 2 shows the number of test runs for each unit. The sense voltages from 0 to 20 mV in 2-mV steps
were tested at all common mode voltages from –0.7 to 18.9 V in 0.7-V steps. The table shows 21 runs for
sense voltages and 29 runs for common mode voltages
The ambient temperature was characterized at the following temperatures:
• –40°C
• 25°C
• 105°C
• 125°C
2.2
Output Voltage Level
Figure 3 shows the output voltage (VOUT) versus the common mode voltage for different temperatures and
sense voltages [(VIN+) ― (VIN–)] as the sense voltage varies from 0 to 20 mV and the common mode
voltage varies from –0.7 V to 18.9 V. The vertical axes show the output voltage on the left and the sense
voltage [(VIN+) ― (VIN–)] on the right. The horizontal axis shows the temperature variations on top and
the common mode voltage on the bottom. The horizontal dashed lines are set at ±20% error of ideal. As
shown in the graphs in Figure 3, the output voltage moves closer to ideal by increasing VSNS and VCM.
Figure 4 shows the same plots as Figure 3 but varies the common mode voltage from 8 V to 16 V. In this
higher range, only A2 amplifier is active and therefore the device performs as expected.
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Temperature (°C)
Output Voltage, VOUT (mV)
Sense Voltage [(VIN+) ± (VIN±)] (V)
Upper and
lower limits for
VOUT at ±20%
Common Mode Voltage, VCM (V)
Figure 3. VOUT Plots Across Temperature for 30 units at V+ = 5 V,
[(VIN+) ― (VIN–)] Varies from 0 to 20 mV and Common Mode Voltage Varies from –0.7 V to 19.6 V
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Setup for Low Common-Mode Voltage Sensing
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Temperature (°C)
Output Voltage, VOUT (mV)
Sense Voltage [(VIN+) ± (VIN±)] (V)
Common Mode Voltage, VCM (V)
The horizontal dashed lines are the upper and lower limits for VOUT at ±20%.
Figure 4. VOUT Plots Across Temperature for 30 Units at V+ = 5 V,
[(VIN+) ― (VIN–)] Varies from 0 to 20 mV and Common Mode Voltage Varies from 8 V to 16 V
6
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Setup for Low Common-Mode Voltage Sensing
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2.3
Linearity Plots
Figure 5 shows the output voltage (VOUT) versus the sense voltages [(VIN+) ― (VIN–)] for different
temperatures and common mode voltages as the sense voltage varies from 0 to 20 mV at 3 common
mode voltages 8.4 V, 12.6 V, and 16.1 V.
Temperature (°C)
Output Voltage, VOUT (V)
Common Mode Voltage, VCM (V)
Sense Voltage, VSNS [(VIN+) ± (VIN±)]
Figure 5. Linearity Plots Across Temperature for 30 Units at V+ = 5 V,
[(VIN+) ― (VIN–)] Varies from 0 to 20 mV and Common Mode Voltages at 8.4 V, 12.6 V and 16.1 V
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Setup for Low Common-Mode Voltage Sensing
2.4
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Output Error Visualization in 3 Dimension Plots
Figure 6 shows the sense voltage [(VIN+) ― (VIN–)] on the horizontal X axis, the common mode on the
horizontal Y axis, and the output error (%) on the vertical Z axis. As shown in this graph, the error
increase dramatically when the sense voltage and common mode voltage approach zero.
Figure 6. Output Error Plots Across Temperature for 30 Units at V+ = 5 V,
[(VIN+) ― (VIN–)] Varies from 0 to 20 mV and Common Mode Voltage Varies from –0.7 V to 19.6 V
3
Summary
While the INA19xA-Q1 device can be an excellent device in some applications, it should not be used in
cases where the sense voltage is below 20 mV and the common mode voltage around 0 V.
8
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