INA138 datasheet(5/15 Pages) TI1

5 / 15 page

INA138, INA168

SBOS122C

www.ti.com

TYPICAL CHARACTERISTICS (Cont.)

At TA = +25°C, V+ = 5V, VIN+ = 12V, and RL = 125kΩ, unless otherwise noted.

STEP RESPONSE

µs/div

200mV

G = 1

100mV

G = 1

0mV

50mV/div

STEP RESPONSE

µs/div

G = 25

G = 10

1V/div

500mV/div

OPERATION

Figure 1 illustrates the basic circuit diagram for both the

INA138 and INA168. Load current IS is drawn from supply VS

through shunt resistor RS. The voltage drop in shunt resistor

VS is forced across RG1 by the internal op amp, causing

current to flow into the collector of Q1. External resistor RL

converts the output current to a voltage, VOUT, at the OUT pin.

The transfer function for the INA138 is:

IO = gm (VIN+ – VIN– )(1)

where gm = 200µA/V.

In the circuit of Figure 1, the input voltage, (VIN+ – VIN– ), is

equal to IS • RS and the output voltage, VOUT, is equal to

IO • RL. The transconductance, gm, of the INA138 is

200

µA/V. The complete transfer function for the current

measurement amplifier in this application is:

VOUT = (IS) (RS) (200µA/V) (RL)(2)

The maximum differential input voltage for accurate mea-

surements is 0.5V, which produces a 100

µA output current.

A differential input voltage of up to 2V will not cause damage.

Differential measurements (pins 3 and 4) must be unipolar

with a more-positive voltage applied to pin 3. If a more-

negative voltage is applied to pin 3, the output current, IO, will

be zero, but it will not cause damage.

BASIC CONNECTION

Figure 1 shows the basic connection of the INA138. The

input pins, VIN+ and VIN– , should be connected as closely as

possible to the shunt resistor to minimize any resistance in

series with the shunt resistance. The output resistor, RL, is

shown connected between pin 1 and ground. Best accuracy

is achieved with the output voltage measured directly across

RL. This is especially important in high-current systems

where load current could flow in the ground connections,

affecting the measurement accuracy.

No power-supply bypass capacitors are required for stability

of the INA138. However, applications with noisy or high-

impedance power supplies may require decoupling capaci-

tors to reject power-supply noise. Connect bypass capacitors

close to the device pins.

POWER SUPPLIES

The input circuitry of the INA138 can accurately measure

beyond its power-supply voltage, V+. For example, the V+

power supply can be 5V, whereas the load power supply

voltage is up to +36V (or +60V with the INA168). The output

voltage range of the OUT terminal, however, is limited by the

lesser of the two voltages (see “Output Voltage Range” section).

SELECTING RS AND RL

The value chosen for the shunt resistor, RS, depends on the

application and is a compromise between small-signal accu-

racy and maximum permissible voltage loss in the measure-

ment line. High values of RS provide better accuracy at lower

currents by minimizing the effects of offset, while low values of

RS minimize voltage loss in the supply line. For most applica-

tions, best performance is attained with an RS value that

provides a full-scale shunt voltage range of 50mV to 100mV.

Maximum input voltage for accurate measurements is 500mV.

RL is chosen to provide the desired full-scale output voltage.

The output impedance of the INA138 OUT terminal is very

high which permits using values of RL up to 500kΩ with

excellent accuracy. The input impedance of any additional

circuitry at the output should be much higher than the value

of RL to avoid degrading accuracy.

INA138 Datasheet(PDF) 5 Page - Texas Instruments

INA138 Datasheet(HTML) 5 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Datasheet Download

Link URL