INA21x

V+

OUT

GND

IN-

IN+

PRODUCT

R and R

3

4

INA213

20 kW

C

BYPASS

NOTE: 1-MW paths from shunt inputs to reference and INA21x outputs.

Shutdown

Control

REF

Reference

Voltage

1 MW

R

3

R

2

R

4

Supply

Load

R

SHUNT

Output

INA213

10 kW

INA213-Q1

INA214-Q1

www.ti.com

SBOS475B – MARCH 2009 – REVISED JUNE 2010

SHUTTING DOWN

While the INA21x does not have a shutdown pin, its low power consumption allows powering from the output of a

logic gate or transistor switch that can turn on and turn off the INA21x power-supply quiescent current.

However, in current shunt monitoring applications. there is also a concern for how much current is drained from

the shunt circuit in shutdown conditions. Evaluating this current drain involves considering the simplified

schematic of the INA21x in shutdown mode shown in Figure 25.

Figure 25. Basic Circuit for Shutting Down INA21x With Grounded Reference

Note that there is typically slightly more than 1-M

Ω impedance (from the combination of 1-MΩ feedback and

5-k

Ω input resistors) from each input of the INA21x to the OUT pin and to the REF pin. The amount of current

flowing through these pins depends on the respective ultimate connection. For example, if the REF pin is

grounded, the calculation of the effect of the 1-M

Ω impedance from the shunt to ground is straightforward.

However, if the reference or op amp is powered while the INA21x is shut down, the calculation is direct; instead

of assuming 1 M

Ω to ground, however, assume 1 MΩ to the reference voltage. If the reference or op amp is also

shut down, some knowledge of the reference or op amp output impedance under shutdown conditions is

required. For instance, if the reference source behaves as an open circuit when it is unpowered, little or no

current flows through the 1-M

Ω path.

Regarding the 1-M

Ω path to the output pin, the output stage of a disabled INA21x does constitute a good path to

ground; consequently, this current is directly proportional to a shunt common-mode voltage impressed across a

1-M

Ω resistor.

As a final note, when the device is powered up, there is an additional, nearly constant, and well-matched 25 µA

that flows in each of the inputs as long as the shunt common-mode voltage is 3 V or higher. Below 2-V

common-mode, the only current effects are the result of the 1-M

Ω resistors.

REF INPUT IMPEDANCE EFFECTS

As with any difference amplifier, the INA21x common-mode rejection ratio is affected by any impedance present

at the REF input. This concern is not a problem when the REF pin is connected directly to most references or

power supplies. When using resistive dividers from the power supply or a reference voltage, the REF pin should

be buffered by an op amp.

In systems where the INA21x output can be sensed differentially, such as by a differential input analog-to-digital

converter (ADC) or by using two separate ADC inputs, the effects of external impedance on the REF input can

be cancelled. Figure 26 depicts a method of taking the output from the INA21x by using the REF pin as a

reference.

Copyright © 2009–2010, Texas Instruments Incorporated

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Product Folder Link(s): INA213-Q1 INA214-Q1