INA170

6

SBOS193D

www.ti.com

OPERATION

Figure 1 shows the basic circuit diagram for the INA170.

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.

Without offset, the transfer function for the INA170 is:

IO = gm (VIN

+ – V

IN

– )(1)

where gm = 1000µA/V

(2)

In the circuit of Figure 1, the input voltage, (VIN

+ – V

IN

– ), is

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

IO • RL. The transconductance, gm, of the INA170 is

1000

µA/V. The complete transfer function for the current

measurement amplifier in this application is:

VOUT = (IS) (RS) (1000µA/V) (RL)(3)

Applying a positive reference voltage to pin 3 causes a

current to flow through ROS, forcing output current IO to be

offset from zero. The transfer function then becomes:

V

VR

R

IRR

k

OUT

REF

L

OS

SS

L

=









± 





•

••

1

Ω

(4)

The maximum differential input voltage for accurate mea-

surements is 0.5V, which produces a 500

µA output current.

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

damage. Differential measurements (pins 1 and 2) can be

bipolar with a more-positive voltage applied to pin 2. If a

more-negative voltage is applied to pin 1, output current IO

will decrease towards zero.

BASIC CONNECTION

Figure 1 shows the basic connection of the INA170. The

input pins, VIN

+ and V

IN

– , 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 6 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 INA170. However, applications with noisy or high

impedance power supplies may require de-coupling capaci-

tors to reject power-supply noise. Connect bypass capacitors

close to the device pins.

POWER SUPPLIES

The input circuitry of the INA170 can accurately measure

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

power supply can be 5V, while the load power-supply

voltage (INA170 input voltage) is up to +60V. However, the

output-voltage range of the OUT terminal (pin 6) is limited

by the supply.

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

FIGURE 1. Basic Circuit Connections.

Shunt

R

S

INA170

4

6

OUT

Q1

R

L

I

0

+

V

O

–

Load

R

G1

1k

Ω

R

G2

1k

Ω

V

IN

V

IN

2

1

I

S

V+

V

REF

3

8

5

V+ power can be

common or

independent of

load supply.

2.7

≤ (V+) ≤ 40V

V

P

Load Power Supply

+2.7 to 60V

+

–

Q2

R

OS

VOLTAGE GAIN

EXACT RL (Ω)

NEAREST 1% RL (Ω)

11k

1k

22k

2k

5

5k

4.99k

10

10k

10k

20

20k

20k

50

50k

49k

100

100k

100k