AN142: X3100 Current Measurment Accuracy

X3100 Current Measurement Accuracy
®
Application Note
Description
This test measures the accuracy of the current sense
amplifier circuit of the X3100. In the test, a known current is
applied through a sense resistor to generate a voltage
between the current sense inputs of the X3100. The analog
output voltage of the X3100 is measured. This value
(adjusted by the amplifier gain) is compared to the input
voltage. The collected data includes the values from five
devices from three different lots. The data also includes tests
at 0°C, 25°C and 50°C and at each gain setting.
May 5, 2005
AN142.0
For each applied voltage the output was recorded, first in
one direction, then in the other. This involves the changing of
the analog multiplexer setting in the X3100. While this
procedure cancels the majority of the offset, as the data
shows, a small offset remained even after this procedure. In
gathering the data with this test, the output was sampled 20
times for each setting. The high and low values were
discarded and the remaining samples were averaged. This
procedure was used to reduce the effect of external noise on
the test environment.
Results
Test Setup
This test applied a current across the sense resistor to
develop a voltage ranging from approximately 200uV to
100mV. The current source was forced through a 20.37Ω
resistor that connected across the VCS1 and VCS2 pins.
The 20.37Ω resistor was chosen to improve the accuracy of
the resistance measurement. In an actual battery pack, a
resistor in the range of 25mΩ to 50mΩ would be used. At
200uV, the minimum current measured would be 4 - 8mA. In
this current measurement test, the current was forced in
specific discrete increments, such as 10uA, 20uA, etc.
However, since the resistor was 20.37Ωs, the applied
voltage is represented in less recognizable increments.
I = 10uA, 20uA, 30uA...5mA
X3100
The data for this test is available in raw form in an Excel
spreadsheet. This spreadsheet includes data for each
device, data for each lot, and data for all devices. Each set of
data includes results for the three temperature settings and
the four gain settings. The data was copied to other files for
the purposes of generating graphs.
For the result summaries there are raw error measurements
and error measurements assuming both an offset correction
and a gain correction. The offset correction was derived by
taking the average offset over all devices, temperatures and
lots. This offset value was then added to each of the
measured outputs at each gain. After this correction, the
gain was computed for each input voltage. These gain
values were averaged over the devices, temperature and lot.
This average value was then factored into the results.
Vss
Vcs1
20.37Ω
Vcs2
I
FIGURE 1. CURRENT SENSE MEASUREMENT
1
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a registered trademark of Intersil Americas Inc.
Copyright Intersil Americas Inc. 2005. All Rights Reserved
All other trademarks mentioned are the property of their respective owners.
Application Note 142
On the following charts, errors are calculated as follows:
Comments
Raw error:
The AO pin on the X3100 is biased to a nominal 2.5V when
reading the voltage across the VCS1 and VCS2 pins.
However, there is an offset to this value that can make the
output (at 0V input) either higher or lower than this 2.5V
nominal value. The output with 0V current sense input
ranged from 1.8 to 2.9V for the devices under test (at a gain
of 160). The difference in voltage between the nominal
output and the actual output becomes more apparent at
higher gains. This is due to a small input offset in the current
sense amplifier that is magnified at the output by the gain of
the amplifier. This offset is mostly cancelled by reading the
voltage across the CSI pins in both directions and
subtracting the two values.
O
---- – I
G
Raw = -----------I
Average error:
avgO
--------------- – I
G
Avg = ----------------------I
Average error after offset and gain adjust:
avgO – avgoff
---------------------------------------- – I
avgG
AvgOGadj = -----------------------------------------------I
Worst case error at 0°C, after offset and gain adjust:
avgO ( 0 ) + 3 × std ( 0 )
---------------------------------------------------------- – I
avgG
StdOGadj ( 0 ) = ----------------------------------------------------------------I
Worst case error at 50°C, after offset and gain adjust:
avgO ( 50 ) – 3 × std ( 50 )
---------------------------------------------------------------- – I
avgG
StdOGadj ( 50 ) = -----------------------------------------------------------------------I
Where:O = Output
I = Input
G = Gain
avg = Average
std = Standard Deviation
off = offset
OG = Offset/Gain
(x) = at x degrees C
Overall Performance
The following set of graphs shows the performance of the
X3100 over temperature and across three lots of five
devices. The graphs will show the basic input/output curve,
raw error, and error after correction for offset and gain.
The below data shows that the error at the x10 gain is the
lowest. Using a 20mΩ current sense resistor, and
considering 3 standard deviations from the worst case
temperature (over 3 manufacturing lots), the X3100 output is
accurate to less than 1.4% at 300mA of load current and
above. At a gain of 25, the X3100 has less than 5% error at a
load current of 50mA. This error declines at higher currents
to about the same as the x10 gain. At all gains, the error
increases as the input current decreases.
2
While the process of reading the voltage in both directions and
subtracting the two values cancels most of the offset on the
X3100 current sense amplifier, it does not cancel it all. The
remaining output offset is approximately 668uV at a gain of 10,
1.7mV at a gain of 25, 5.7mV at a gain of 80 and 11mV at a
gain of 160. This offset becomes significant at the lowest input
voltages, since it can be nearly the same voltage as the
expected output. In some of the charts that follow, the output
has been compensated by subtracting this offset from the
recorded values. This cancels out the error for average
conditions, but leaves large errors at small input voltages due to
variations in this offset. With the test setup used it is not
possible to determine whether these offset error variations are
caused by the environment or the device.
The charts in this document that show errors after offset
compensation use an average offset value. This value is an
average over all devices across all temperatures. To improve
the accuracy, especially at lower input voltages, a calibration
procedure could be used. This calibration procedure would
apply 0V across the VCS1 and VCS2 pins. Then, at each gain
setting, the measured output (after reading in both directions
and subtracting) is saved as a common offset value.
The gain adjust is determined by dividing the output by the
input voltage. The gain is averaged over all inputs and all
devices at all temperatures. This average value is then used to
divide into the output reading to determine the compensated
error. Based on the data collected, the average gain for the 160
setting is 160.17, the average gain for the 80 setting is 79.24,
the average gain for the 25 setting is 24.53 and the average
gain for the 10 setting is 9.77. Using these average values will
reduce the error when computing the input current. However to
improve the accuracy further, each device can be calibrated for
the particular gain. To do this, apply a known current across the
current sense resistor. Use a current that is relatively high, such
as 500mA. At each gain read the output and divide this by the
known input. Use this gain value in subsequent calculations.
AN142.0
May 5, 2005
Application Note 142
HP6626
Power Supply
X3100
+
HP6626
VCC
VCELL1
-
Power Supply
+
VRGP
VCELL2
VRGC
+
VCELL3
VRGO
+
Keithley 2002
VCELL4
DMM
VSS
AO
Keithley 2000
DMM
SPI Serial
Analog Select
VCS1
VCS2
20.37Ω
Keithley 236
Source Measurement Unit
PC
FIGURE 2. CHARACTERIZATION TEST SETUP
3
AN142.0
May 5, 2005
Application Note 142
10 Gain
Lot All - 10 Gain
Lot All - Gain 10
1.2
@50C
97.8E-3
91.7E-3
85.6E-3
79.4E-3
73.3E-3
67.2E-3
61.1E-3
55.0E-3
48.9E-3
42.8E-3
0C -3 Stdev
0C Avg
0C +3 Stdev
25C -3 Stdev
25C Avg
25C +3 Stdev
50C -3 Stdev
50C Avg
50C +3 Stdev
FIGURE 3. INPUT TO OUTPUT VOLTAGE
FIGURE 4. INPUT TO OUTPUT ERROR. THIS IS THE RAW
ERROR WITH NO CORRECTION FOR OFFSET OR
GAIN. IT INCLUDES ERROR AT EACH
TEMPERATURE WITH THREE STANDARD
DEVIATIONS
Lot All - Gain 10
Lot All - Gain 10 (with Offset & Gain Adjust)
4.0%
3.5%
3.0%
2.5%
2.0%
1.5%
1.0%
0.5%
0.0%
0.8%
% Error
0.6%
0.4%
0.2%
0.0%
95.7E-3
0C Avg
0C +3 Stdev
25C -3 Stdev
25C Avg
25C +3 Stdev
50C -3 Stdev
50C Avg
50C +3 Stdev
97.8E-3
91.7E-3
85.6E-3
79.4E-3
73.3E-3
67.2E-3
61.1E-3
55.0E-3
48.9E-3
42.8E-3
36.7E-3
30.6E-3
24.4E-3
18.3E-3
6.1E-3
Vcs Set (V)
0C -3 Stdev
12.2E-3
-0.4%
000.0E+0
89.6E-3
83.5E-3
77.4E-3
71.3E-3
65.2E-3
59.1E-3
53.0E-3
46.9E-3
40.7E-3
34.6E-3
28.5E-3
22.4E-3
16.3E-3
-0.2%
10.2E-3
% Error
@25C
36.7E-3
Vcs Set (V)
Input Voltage
@0C
30.6E-3
97.8E-3
91.7E-3
85.6E-3
79.4E-3
73.3E-3
67.2E-3
61.1E-3
55.0E-3
48.9E-3
42.8E-3
36.7E-3
30.6E-3
24.4E-3
18.3E-3
6.1E-3
000.0E+0
12.2E-3
0.0
24.4E-3
000.0E+0
0.2
18.3E-3
0.4
6.1E-3
0.6
4.0E-3
3.5E-3
3.0E-3
2.5E-3
2.0E-3
1.5E-3
1.0E-3
500.0E-6
000.0E+0
-500.0E-6
12.2E-3
0.8
Error (V)
Output Voltage
1.0
Vcs Set (V)
@0C
@25C
@50C
FIGURE 6. AVERAGE INPUT TO OUTPUT ERROR WITH
CORRECTION FOR OFFSET AND GAIN FOR EACH
INPUT VOLTAGE AT 0°C, 25°C AND 50°C
FIGURE 5. INPUT TO OUTPUT % ERROR (EXPANDED - STARTS
AT 10mV INPUT). THIS IS THE RAW ERROR WITH
NO CORRECTION FOR OFFSET OR GAIN. IT
INCLUDES ERROR AT EACH TEMPERATURE WITH
THREE STANDARD DEVIATIONS
Current Measurement Accuracy
After Offset & Gain Adjust (Gain=10)
4.0%
Output Error
3.0%
2.0%
1.0%
0.0%
-1.0%
-2.0%
97.8E-3
91.7E-3
85.6E-3
79.4E-3
73.3E-3
67.2E-3
61.1E-3
55.0E-3
48.9E-3
42.8E-3
36.7E-3
30.6E-3
24.4E-3
18.3E-3
12.2E-3
6.1E-3
000.0E+0
-3.0%
Input Voltage
Average
0deg+3std
50deg-3std
FIGURE 7. WORST CASE INPUT TO OUTPUT ERROR WITH CORRECTION FOR OFFSET AND GAIN. THE TERM +3 STDEV IS WORST
CASE VALUE FOR 0°C. THE TERM -3 STDEV IS THE WORST CASE VALUE FOR 50°C
4
AN142.0
May 5, 2005
Application Note 142
25 Gain
Lot All - 25 Gain
L o t All - Gain 25
1.4
48.9E-3
45.8E-3
42.8E-3
39.7E-3
36.7E-3
33.6E-3
30.6E-3
27.5E-3
24.4E-3
0C -3 Stdev
0C Avg
0C + 3 S tdev
25C -3 S tdev
25C Avg
25C + 3 S tdev
50C -3 Stdev
50C Avg
50C + 3 S tdev
@50C
FIGURE 8. INPUT TO OUTPUT VOLTAGE
FIGURE 9. INPUT TO OUTPUT ERROR. THIS IS THE RAW
ERROR WITH NO CORRECTION FOR OFFSET OR
GAIN. IT INCLUDES ERROR AT EACH
TEMPERATURE WITH THREE STANDARD
DEVIATIONS
Lot All - Gain 25
Lot All - Gain 25 (with Offset & Gain Adjust)
3.5%
3.0%
2.5%
2.0%
1.5%
1.0%
0.5%
0.0%
-0.5%
-1.0%
-1.5%
2.0%
1.5%
% Error
1.0%
0.5%
0.0%
0C Avg
0C +3 Stdev
25C -3 Stdev
25C Avg
25C +3 Stdev
50C -3 Stdev
50C Avg
50C +3 Stdev
48.9E-3
45.8E-3
42.8E-3
39.7E-3
36.7E-3
33.6E-3
30.6E-3
27.5E-3
24.4E-3
21.4E-3
18.3E-3
15.3E-3
12.2E-3
9.2E-3
6.1E-3
000.0E+0
Vcs Se t (V)
0C -3 Stdev
3.1E-3
-1.0%
47.9E-3
44.8E-3
41.8E-3
38.7E-3
35.6E-3
32.6E-3
29.5E-3
26.5E-3
23.4E-3
20.4E-3
17.3E-3
14.3E-3
11.2E-3
8.1E-3
-0.5%
5.1E-3
% Error
@25C
21.4E-3
Vcs S e t (V)
Input Voltage
@0C
18.3E-3
48.9E-3
45.8E-3
42.8E-3
39.7E-3
36.7E-3
33.6E-3
30.6E-3
27.5E-3
24.4E-3
21.4E-3
18.3E-3
15.3E-3
9.2E-3
12.2E-3
6.1E-3
000.0E+0
3.1E-3
0.0
15.3E-3
000.0E+0
0.2
9.2E-3
0.4
12.2E-3
0.6
6.1E-3
0.8
1.6E-3
1.4E-3
1.2E-3
1.0E-3
800.0E-6
600.0E-6
400.0E-6
200.0E-6
000.0E + 0
-200.0E-6
3.1E-3
1.0
Error (V)
Output Voltage
1.2
Vcs Set (V)
@0C
@25C
@50C
FIGURE 11. AVERAGE INPUT TO OUTPUT ERROR WITH
CORRECTION FOR OFFSET AND GAIN FOR EACH
INPUT VOLTAGE AT 0°C, 25°C AND 50°C
FIGURE 10. INPUT TO OUTPUT % ERROR (EXPANDED - STARTS
AT 5mV INPUT). THIS IS THE RAW ERROR WITH NO
CORRECTION FOR OFFSET OR GAIN. IT INCLUDES
ERROR AT EACH TEMPERATURE WITH THREE
STANDARD DEVIATIONS
Current Measurement Accuracy
48.9E-3
45.8E-3
42.8E-3
39.7E-3
36.7E-3
33.6E-3
30.6E-3
27.5E-3
24.4E-3
21.4E-3
18.3E-3
15.3E-3
12.2E-3
9.2E-3
6.1E-3
3.1E-3
6%
4%
2%
0%
-2%
-4%
-6%
000.0E+0
Output Error
After Offset & Gain Adjust (Gain=25)
Input Voltage
Average
0deg+3std
50deg-3std
FIGURE 12. WORST CASE INPUT TO OUTPUT ERROR WITH CORRECTION FOR OFFSET AND GAIN. THE TERM +3 STDEV IS WORST
CASE VALUE FOR 0°C. THE TERM -3 STDEV IS THE WORST CASE VALUE FOR 50°C
5
AN142.0
May 5, 2005
Application Note 142
80 Gain
Lot All - 80 Gain
Lot All - Gain 80
1.6
1.4
800.0E-6
1.2
1.0
400.0E-6
Error (V)
600.0E-6
0.8
0.6
200.0E-6
000.0E+0
-200.0E-6
0.4
0.2
0.0
@25C
19.6E-3
18.3E-3
17.1E-3
15.9E-3
14.7E-3
13.4E-3
12.2E-3
9.8E-3
11.0E-3
8.6E-3
7.3E-3
6.1E-3
0C -3 Stdev
25C -3 Stdev
50C -3 Stdev
0C Avg
25C Avg
50C Avg
0C +3 Stdev
25C +3 Stdev
50C +3 Stdev
@50C
FIGURE 14. INPUT TO OUTPUT ERROR. THIS IS THE RAW
ERROR WITH NO CORRECTION FOR OFFSET OR
GAIN. IT INCLUDES ERROR AT EACH
TEMPERATURE WITH THREE STANDARD
DEVIATIONS
FIGURE 13. INPUT TO OUTPUT VOLTAGE
Lot All - Gain 80
Lot All - Gain 80 (with Offset & Gain Adjust)
5%
4%
3%
2%
1%
0%
-1%
-2%
-3%
-4%
-5%
-6%
-7%
4%
% Error
3%
2%
1%
0%
-1%
19.6E-3
18.3E-3
17.1E-3
15.9E-3
14.7E-3
13.4E-3
12.2E-3
11.0E-3
9.8E-3
8.6E-3
7.3E-3
6.1E-3
4.9E-3
3.7E-3
2.4E-3
1.2E-3
000.0E+0
19.1E-3
17.9E-3
16.7E-3
15.5E-3
14.3E-3
13.0E-3
11.8E-3
10.6E-3
9.4E-3
8.1E-3
6.9E-3
5.7E-3
4.5E-3
3.3E-3
-2%
2.0E-3
% Error
4.9E-3
Vcs Set (V)
Input Voltage
@0C
3.7E-3
2.4E-3
1.2E-3
000.0E+0
19.6E-3
18.3E-3
17.1E-3
15.9E-3
14.7E-3
13.4E-3
12.2E-3
9.8E-3
11.0E-3
8.6E-3
7.3E-3
6.1E-3
4.9E-3
3.7E-3
2.4E-3
1.2E-3
-400.0E-6
000.0E+0
Output Voltage
1.8
Vcs Set (V)
Vcs Set (V)
0C -3 Stdev
0C Avg
0C +3 Stdev
25C -3 Stdev
25C +3 Stdev
50C -3 Stdev
50C Avg
50C +3 Stdev
25C Avg
@0C
@25C
@50C
FIGURE 16. AVERAGE INPUT TO OUTPUT ERROR WITH
CORRECTION FOR OFFSET AND GAIN FOR EACH
INPUT VOLTAGE AT 0°C, 25°C AND 50°C
FIGURE 15. INPUT TO OUTPUT % ERROR (EXPANDED - STARTS
AT 2mV INPUT). THIS IS THE RAW ERROR WITH NO
CORRECTION FOR OFFSET OR GAIN. IT INCLUDES
ERROR AT EACH TEMPERATURE WITH THREE
STANDARD DEVIATIONS
Current Measurement Accuracy
After Offset & Gain Adjust (Gain=80)
Output Error
15%
10%
5%
0%
-5%
19.6E-3
18.3E-3
17.1E-3
15.9E-3
14.7E-3
13.4E-3
12.2E-3
9.8E-3
11.0E-3
8.6E-3
7.3E-3
6.1E-3
4.9E-3
3.7E-3
2.4E-3
1.2E-3
000.0E+0
-10%
Input Voltage
Average
0deg+3std
50deg-3std
FIGURE 17. WORST CASE INPUT TO OUTPUT ERROR WITH CORRECTION FOR OFFSET AND GAIN. THE TERM +3 STDEV IS WORST
CASE VALUE FOR 0°C. THE TERM -3 STDEV IS THE WORST CASE VALUE FOR 50°C
6
AN142.0
May 5, 2005
Application Note 142
160 Gain
Lot All - Gain 160
300.0E-6
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Error (V)
200.0E-6
100.0E-6
000.0E+0
-100.0E-6
-200.0E-6
-300.0E-6
@25C
@50C
FIGURE 18. INPUT TO OUTPUT VOLTAGE
9.8E-3
9.2E-3
8.6E-3
7.9E-3
7.3E-3
6.7E-3
6.1E-3
5.5E-3
4.9E-3
4.3E-3
3.7E-3
3.1E-3
2.4E-3
+3 Stdev (0C)
Avg (25C)
+3 Stdev (25C)
-3 Stdev (50C)
Avg (50C)
+3 Stdev (50C)
0C Avg
25C Avg
50C Avg
9.8E-3
9.2E-3
8.6E-3
7.9E-3
7.3E-3
6.7E-3
6.1E-3
5.5E-3
4.9E-3
4.3E-3
3.7E-3
3.1E-3
2.4E-3
1.8E-3
000.0E+0
Input Voltage (V)
1.2E-3
6%
5%
4%
3%
2%
1%
0%
-1%
-2%
611.1E-6
% Error
9.6E-3
9.0E-3
8.4E-3
7.7E-3
7.1E-3
6.5E-3
5.9E-3
5.3E-3
4.7E-3
4.1E-3
3.5E-3
2.9E-3
2.2E-3
1.6E-3
1.0E-3
Avg (0C)
-3 Stdev (25C)
Lot All - Gain 160 (with Offset & Gain Adjust)
Lot All - Gain 160
% Error
-3 Stdev (0C)
FIGURE 19. INPUT TO OUTPUT ERROR. THIS IS THE RAW ERROR
WITH NO CORRECTION FOR OFFSET OR GAIN. IT
INCLUDES ERROR AT EACH TEMPERATURE WITH
THREE STANDARD DEVIATIONS
4%
2%
0%
-2%
-4%
-6%
-8%
-10%
-12%
-14%
0C -3 Stdev
25C -3 Stdev
50C -3 Stdev
1.8E-3
Input Voltage (V)
Input Voltage
@0C
1.2E-3
611.1E-6
000.0E+0
9.8E-3
9.2E-3
8.6E-3
7.9E-3
7.3E-3
6.7E-3
6.1E-3
5.5E-3
4.9E-3
4.3E-3
3.7E-3
3.1E-3
2.4E-3
1.8E-3
1.2E-3
611.1E-6
-400.0E-6
000.0E+0
Output Voltage
Lot All - 160 Gain
Vcs Set (V)
0C +3 Stdev
25C +3 Stdev
50C +3 Stdev
@0C
FIGURE 20. INPUT TO OUTPUT % ERROR (EXPANDED - STARTS
AT 1mV INPUT). THIS IS THE RAW ERROR WITH NO
CORRECTION FOR OFFSET OR GAIN. IT INCLUDES
ERROR AT EACH TEMPERATURE WITH THREE
STANDARD DEVIATIONS
@25C
@50C
FIGURE 21. AVERAGE INPUT TO OUTPUT ERROR WITH
CORRECTION FOR OFFSET AND GAIN FOR EACH
INPUT VOLTAGE AT 0°C, 25°C AND 50°C
Current Measurement Accuracy
After Offset & Gain Adjust (Gain=160)
20%
10%
0%
9.8E-3
9.2E-3
8.6E-3
7.9E-3
7.3E-3
6.7E-3
6.1E-3
5.5E-3
4.9E-3
4.3E-3
3.7E-3
3.1E-3
2.4E-3
1.8E-3
1.2E-3
611.1E-6
-10%
-20%
000.0E+0
Output Error
30%
Input Voltage
Average
0deg+3std
50deg-3std
FIGURE 22. WORST CASE INPUT TO OUTPUT ERROR WITH CORRECTION FOR OFFSET AND GAIN. THE TERM +3 STDEV IS WORST
CASE VALUE FOR 0°C. THE TERM -3 STDEV IS THE WORST CASE VALUE FOR 50°C
Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to
verify that the Application Note or Technical Brief is current before proceeding.
For information regarding Intersil Corporation and its products, see www.intersil.com
7
AN142.0
May 5, 2005
Similar pages