OPA992 datasheet(32/70 Pages) TI

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The load current (ILOAD) produces a voltage drop across the shunt resistor (RSHUNT). 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 2:

SHUNT

MAX

SHUNT

LOAD

MAX

100mV

100m

(2)

Using Equation 2, RSHUNT is calculated to be 100 mΩ. The voltage drop produced by ILOAD and RSHUNT is

amplified by the OPA992 to produce an output voltage of 0 V to 4.9 V. The gain needed by the OPA992 to

produce the necessary output voltage is calculated using Equation 3:

OUT

MAX

OUT

MIN

MAX

MIN

Gain

(3)

Using Equation 3, the required gain is calculated to be 49 V/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 OPA992 to 49 V/V.

Gain

(4)

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 8-2 shows the measured transfer function of the circuit shown in Figure 8-1.

8.2.1.3 Application Curve

ILOAD (A)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Figure 8-2. Low-Side, Current-Sense, Transfer Function

8.2.2 High Voltage Buffered Multiplexer

The OPAx992S shutdown devices can be configured to create a high voltage, buffered multiplexer. Outputs can

be connected together on a common bus and the shutdown pins can be used to select the desired channel to

pass through. Since the amplifier circuitry has been designed such that disable transitions occur significantly

faster than enable transitions, the amplifier naturally exhibits a "break before make" switch topology. Amplifier

outputs enter a high impedance state when placed in shutdown, so there is no risk of bus contention when

connecting multiple channel outputs together. Additionally, because outputs are isolated from inputs, there is no

concern about the impedance at the input of each channel interacting undesirably with the impedance at the

output, like an amplifier gain stage or ADC driver circuit. Also, because this topology uses amplifiers instead of

MOSFET switches, other common issues with multiplexers such as charge injection or signal error due to RON

effects are eliminated.

OPA992, OPA2992, OPA4992

SBOSA10D – JUNE 2021 – REVISED AUGUST 2022

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