Texas Instruments | Precision Designs with Non-Ratiometric Magnetic Current Sensors | Application notes | Texas Instruments Precision Designs with Non-Ratiometric Magnetic Current Sensors Application notes

Texas Instruments Precision Designs with Non-Ratiometric Magnetic Current Sensors Application notes
Enabling Precision Current Sensing Designs with NonRatiometric Magnetic Current Sensors
Steven Loveless - Current Sensing Products
6
Voltage Output (V)
Electronically controlled systems use local or remote
sensor elements to monitor operating parameters for
loop control, diagnostics, and system feedback. The
quality and accuracy of this information is a key limit to
system performance and control capability. In the past,
where many electronics had poor control of sensor
voltage supplies and references, ratiometry was used
to reduce the errors due to parameter fluctuations. In
modern systems, with tight control of references to
signal chain elements such as analog to digital
converters (ADCs), non-ratiometric sensors like the
TMCS1100 magnetic current sensor enable improved
noise immunity, precision, and design flexibility.
5.5
5
2
4.5
1
-30
-10
10
Input Current (A)
30
(1)
In a fully ratiometric device, both sensitivity and offset
vary with the supply, so that full-scale input current
always results in an output at either ground or supply,
as illustrated by Figure 1.
6
Voltage Output (V)
3
Figure 2. Non-Ratiometric Current Sensor
Response
VOUT = IIN × S + VOUT,0A
5
VS = 5.5V
VS = 5V
VS = 4.5V
3
4
0
The linear transfer function of a current sensor is
shown in Equation 1 with sensitivity (S) and zero
current output voltage as gain and offset.
4
VS = 5.5V
VS = 5V
VS = 4.5V
5
5.5
5
2
Ratiometry is effective in systems where a common
sensor supply and ADC reference is expected to vary
widely in operation, as shown in Figure 3. Ratiometry
mitigates some of the error caused by a varying, ADC,
full-scale reference by also scaling the sensor output
range. However, ratiometry scaling is never perfect,
and introduces some additional error to the system. It
must be fine-tuned over a limited supply range for high
accuracy, which reduces design flexibility, as sensor
output range must identically match ADC input range.
In addition, supply noise is directly injected into the
output signal, which causes poor power supply
rejection (PSR).
4.5
Analog Supply
1
RefHi
VS
0
-30
-10
10
Input Current (A)
30
Sensor
ADC
±10%
Figure 1. Fully Ratiometric Current Sensor
Response
For a non-ratiometric current sensor, the change in
voltage output for a given input current change has no
dependence upon the supply, and the zero current
output voltage is always a fixed voltage, as shown in
Figure 2.
SBOA340 – July 2019
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Figure 3. Ratiometric Sensor Architecture for
Poorly Regulated Supplies
For systems where a stable ADC reference is
available, either with a dedicated internal supply or an
external reference, ratiometry only introduces
additional error and noise. In these cases, such as in
architectures shown in Figure 4, a current sensor with
fixed sensitivity provides a superior solution. With a
fixed sensitivity, the device has significant PSR, and
Enabling Precision Current Sensing Designs with Non-Ratiometric Magnetic
Current Sensors Steven Loveless - Current Sensing Products
Copyright © 2019, Texas Instruments Incorporated
1
www.ti.com
can even have a different supply voltage than the ADC
full scale. This is commonly the case with integrated
microcontroller ADCs. This also allows for optimization
of fixed-sensitivity internal circuits, which provide
higher total accuracy and lower drift.
5-V Supply
VS
Precision Internal or
External Reference
RefHi
pseudo- or fully-differential sensing, rejecting noise
coupling into output signals. This results in lower
system-level noise and improved dynamic range, as
the differential measurement cancels any drift in the
zero current output voltage.
5-V Supply
VS
Sensor
Sensor
Int/Ext Reference
RefHi
ADC
VOUT
IN+
VREF
IN-
ADC
Precision
Reference
Figure 5. TMCS1100 Optimized Signal Chain
Figure 4. Non-Ratiometric Architecture for
Precision Signal Chains
The TMCS1100 and TMCS1101 are precision, isolated
magnetic current sensors with fixed sensitivity. The
TMCS1100 has an externally supplied reference pin
that sets the zero current output voltage, which allows
for both custom dynamic measurable ranges and a
fully differential signal chain all the way to the ADC, as
shown in Figure 5. This architecture, coupled with a
precision, fixed-sensitivity signal chain, enables an
industry-leading temperature stability with better than
1% accuracy from –40°C to 125°C.
Current sensors are often utilized in power systems
where the sensor is often located near the power
switching elements, far from the ADC and controller.
This results in switching noise and transient events
coupling directly into analog supplies and signals. A
fixed-sensitivity sensor with an external reference
allows the system to reject both of these noise paths.
The improved PSR rejects noise injection through the
analog supply and the external reference allows for
2
Design flexibility is greatly enhanced by this
architecture, as the zero current output can be tailored
to any use case condition. Bi-directional, unidirectional, and custom dynamic sensing ranges are
achieved by appropriately selecting the reference
voltage. Because there is no constraint between the
sensor supply, reference, and ADC reference, the
sensor output can cross voltage supply domains with
no scaling required.
The TMCS1101 has an internal resistor divider
providing the reference, with variants of either 50% or
10%, of the supply for bi-directional and uni-directional
current sensing respectively. It features a fixed
sensitivity as well, and provides better than 1.5%
accuracy across the full temperature range.
Table 1. Adjacent Tech Notes
Document Type
Title
Application Note
Low-Drift, Precision, In-Line Isolated
Magnetic Motor Current Measurements
(SBOA351)
Application Note
Integrating the Current Sensing Path
(SBOA167)
Enabling Precision Current Sensing Designs with Non-Ratiometric Magnetic
Current Sensors Steven Loveless - Current Sensing Products
Copyright © 2019, Texas Instruments Incorporated
SBOA340 – July 2019
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