Datasheet ADS1015

V
SHUNT
LOAD
High-Voltage Bus
R
SHUNT
I
LOAD
R
3
R
2
R
1
R
5
VDD
AINP
AINN
ADS1015
I
2
C
OPA333
VDD
+
±
V
CM
V
OUT
R
6
C
CM1
C
DIFF
V
INX
4-Wire Kelvin
Connection
C
CM2
R
4
32
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SBAS473E –MAY 2009–REVISED JANUARY 2018 www.ti.com
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9.2 Typical Application
Shunt-based, current-measurement solutions are widely used to monitor load currents. Low-side, current-shunt
measurements are independent of the bus voltage because the shunt common-mode voltage is near ground.
Figure 29 shows an example circuit for a bidirectional, low-side, current-shunt measurement system. The load
current is determined by measuring the voltage across the shunt resistor that is amplified and level-shifted by a
low-drift operational amplifier, OPA333. The OPA333 output voltage is digitized with ADS1015 and sent to the
microcontroller using the I
2
C interface. This circuit is capable of measuring bidirectional currents flowing through
the shunt resistor with great accuracy and precision.
Figure 29. Low-Side Current Shunt Monitoring
9.2.1 Design Requirements
Table 8 shows the design parameters for this application.
(1) Does not account for inaccuracy of shunt resistor and the precision resistors used in the application.
Table 8. Design Parameters
DESIGN PARAMETER VALUE
Supply voltage (VDD) 5 V
Voltage across Shunt Resistor (V
SHUNT
) ±50 mV
Output Data Rate (DR) ≥200 readings per second
Typical measurement accuracy at T
A
= 25°C
(1)
±0.25%
9.2.2 Detailed Design Procedure
The first stage of the application circuit consists of an OPA333 in a noninverting summing amplifier configuration
and serves two purposes:
1. To level-shift the ground-referenced signal to allow bidirectional current measurements while running off a
unipolar supply. The voltage across the shunt resistor, V
SHUNT
, is level-shifted by a common-mode voltage,
V
CM
, as shown in Figure 29. The level-shifted voltage, V
INX
, at the noninverting input is given by Equation 3.
V
INX
= (V
CM
· R
3
+ V
SHUNT
· R
4
) / (R
3
+ R
4
) (3)
2. To amplify the level-shifted voltage (V
INX
). The OPA333 is configured in a noninverting gain configuration
with the output voltage, V
OUT
, given by Equation 4.
V
OUT
= V
INX
· (1 + R
2
/ R
1
) (4)
Using Equation 3 and Equation 4, V
OUT
is given as a function of V
SHUNT
and V
CM
by Equation 5.
V
OUT
= (V
CM
· R
3
+ V
SHUNT
· R
4
) / (R
3
+ R
4
) · (1 + R
2
/ R
1
) (5)
Using Equation 5 the ADC differential input voltage, before the first-order RC filter, is given by Equation 6.
V
OUT
– V
CM
= V
SHUNT
· (1 + R
2
/ R
1
) / (1 + R
4
/ R
3
) + V
CM
· (R
2
/ R
1
– R
3
/ R
4
) / (1 + R
3
/ R
4
) (6)
If R
1
= R
3
and R
2
= R
4
, Equation 6 is simplified to Equation 7.
V
OUT
– V
CM
= V
SHUNT
· (1 + R
2
/ R
1
) / (1 + R
4
/ R
3
) (7)