JAJSQK5B June 2023 – March 2024 OPA2994 , OPA994
PRODMIX
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The transfer function of the circuit in Figure 7-1 is given in Equation 1:
The load current (ILOAD) produces a voltage drop across the shunt resistor (RSHUNT). The load current is set from 0A to 1 A. To keep the shunt voltage below 100 mV at maximum load current, the largest shunt resistor is defined using Equation 2:
Using Equation 2, RSHUNT is calculated to be 100 mΩ. The voltage drop produced by ILOAD and RSHUNT is amplified by the OPA994 to produce an output voltage of 0V to 4.9V. The gain needed by the OPA994 to produce the necessary output voltage is calculated using Equation 3:
Using Equation 3, the required gain is calculated to be 49V/V, which is set with resistors RF and RG. Equation 4 is used to size the resistors, RF and RG, to set the gain of the OPA994 to 49V/V.
Choosing RF as 5.76 kΩ, RG is calculated to be 120 Ω. RF and RG were chosen as 5.76 kΩ and 120 Ω because they are standard value resistors that create a 49:1 ratio. Other resistors that create a 49:1 ratio can also be used. However, excessively large resistors will generate thermal noise that exceeds the intrinsic noise of the op amp. Figure 7-2 shows the measured transfer function of the circuit shown in Figure 7-1.