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OPA388-Q1 Fiches technique(PDF) 19 Page - Texas Instruments

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No de pièce OPA388-Q1
Description  OPA388-Q1 Automotive, Precision, Zero-Drift, Zero-Crossover, True Rail-to-Rail, Input/Output Operational Amplifier
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Fabricant  TI [Texas Instruments]
Site Internet  http://www.ti.com
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8.2.1 Design Requirements
This solution has the following requirements:
• Supply voltage: 3.3 V
• Input: –1 A to 1 A
• Output: 1.65 V ±1.54 V (110 mV to 3.19 V)
8.2.2 Detailed Design Procedure
The load current, ILOAD, flows through the shunt resistor (RSHUNT) to develop the shunt voltage, VSHUNT. The
shunt voltage is then amplified by the difference amplifier consisting of U1A and R1 through R4. The gain of the
difference amplifier is set by the ratio of R4 to R3. To minimize errors, set R2 = R4 and R1 = R3. The reference
voltage, VREF, is supplied by buffering a resistor divider using U1B. The transfer function is given by Equation 1.
V
= V
Gain
+ V
OUT
SHUNT
Diff_Amp
REF
´
(1)
where
V
= I
R
SHUNT
LOAD
´
SHUNT
Gain
=
Diff_Amp
R
R
4
3
V
= V
REF
CC
´
R
R
R
6
5
6
+
There are two types of errors in this design: offset and gain. Gain errors are introduced by the tolerance of the
shunt resistor and the ratios of R4 to R3 and, similarly, R2 to R1. Offset errors are introduced by the voltage
divider (R5 and R6) and how closely the ratio of R4 / R3 matches R2 / R1. The latter value affects the CMRR of
the difference amplifier, ultimately translating to an offset error.
The value of VSHUNT is the ground potential for the system load because VSHUNT is a low-side measurement.
Therefore, a maximum value must be placed on VSHUNT. In this design, the maximum value for VSHUNT is set to
100 mV. Equation 2 calculates the maximum value of the shunt resistor given a maximum shunt voltage of
100 mV and maximum load current of 1 A.
R
=
SHUNT(Max)
=
= 100 mW
V
I
SHUNT(Max)
LOAD(Max)
100 mV
1 A
(2)
The tolerance of RSHUNT is directly proportional to cost. For this design, a shunt resistor with a tolerance of 0.5%
was selected. If greater accuracy is required, select a 0.1% resistor or better.
The load current is bidirectional; therefore, the shunt voltage range is –100 mV to 100 mV. This voltage is
divided down by R1 and R2 before reaching the operational amplifier, U1A. Take care to ensure that the voltage
present at the noninverting node of U1A is within the common-mode range of the device. Therefore, use an
operational amplifier, such as the OPA388-Q1, that has a common-mode range that extends below the negative
supply voltage. Finally, to minimize offset error, note that the OPA388-Q1 has a typical offset voltage of merely
±0.25 µV (±5 µV maximum).
Given a symmetric load current of –1 A to +1 A, the voltage divider resistors (R5 and R6) must be equal. To be
consistent with the shunt resistor, a tolerance of 0.5% was selected. To minimize power consumption, 10-kΩ
resistors were used.
www.ti.com
OPA388-Q1
SBOS989 – AUGUST 2020
Copyright © 2020 Texas Instruments Incorporated
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