OPA191 Datasheet, PDF (31/45 Pages) Texas Instruments – Low Offset Voltage, Low Input Bias Current Op Amp

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OPA191, OPA2191, OPA4191
SBOS701 – DECEMEBER 2015
Typical Applications (continued)
9.2.1.2 Detailed Design Procedure
The transfer function of the circuit in Figure 60 is given in Equation 2
V_OUT I_LOAD uR_SHUNT u Gain
(2)
The load current (I_LOAD) produces a voltage drop across the shunt resistor (R_SHUNT). The load current is set from
0 A to 1 A. To keep the shunt voltage below 100 mV at maximum load current, the largest shunt resistor is
defined using Equation 3.
R_SHUNT
V_{SHUNT _ MAX}
I_{LOAD _ MAX}
100mV _100m:
1A
(3)
Using Equation 3, R_SHUNT is calculated to be 100 mΩ. The voltage drop produced by I_LOAD and R_SHUNT is
amplified by the OPA191 to produce an output voltage of 0 V to 4.9 V. The gain needed by the OPA191 to
produce the necessary output voltage is calculated using Equation 4:
Gain ^{VOUT _ MAX VOUT _ MIN}
V_{IN _ MAX} V_{IN _ MIN}
(4)
Using Equation 4, the required gain is calculated to be 49 V/V, which is set with resistors R_F and R_G. Equation 5
is used to size the resistors, R_F and R_G, to set the gain of the OPA191 to 49 V/V.
Gain 1 ^RF
R_G
(5)
Choosing R_F as 360 kΩ, R_G is calculated to be 7.5 kΩ. R_F and R_G were chosen as 360 kΩ and 7.5 kΩ because
they are standard value resistors that create a 49:1 ratio. Other resistors that create a 49:1 ratio can also be
used. Figure 2 shows the measured transfer function of the circuit shown in Figure 60.
9.2.1.3 Application Curves
5
0.1
4
0.08
3
0.06
2
0.04
1
0.02
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
I_LOAD (A)
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
I_LOAD (A)
Figure 61. Low-Side, Current-Sense, Transfer Function
Figure 62. Low-Side, Current-Sense, Full-Scale Error
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