There is typically slightly more than 1-MΩ impedance (from the combination of 1-MΩ feedback and 5-kΩ input

resistors) from each input of the INA199 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-MΩ impedance from the shunt to ground is straightforward. However, if the

reference or operational amplifier is powered when the INA199 is shut down, the calculation is direct; instead

of assuming 1-MΩ to ground, however, assume 1-MΩ to the reference voltage. If the reference or operational

amplifier is also shut down, some knowledge of the reference or operational amplifier output impedance under

shutdown conditions is required. For instance, if the reference source functions as an open circuit when not

powered, little or no current flows through the 1-MΩ path.

Regarding the 1-MΩ path to the output pin, the output stage of a disabled INA199 does constitute a good path to

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

1-MΩ resistor.

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-MΩ resistors.

7.4.3 REF Input Impedance Effects

As with any difference amplifier, the INA199 series 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 must be buffered by an operational amplifier.

In systems where the INA199 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 7-4 depicts a method of taking the output from the INA199 by using the REF pin as a

reference.

Output

Load

Supply

ADC

V+

OUT

GND

IN-

IN+

C

BYPASS

0.01 F

to

0.1 F

m

m

2.7 V to 26 V

REF

R

1

R

3

R

2

R

4

R

SHUNT

Figure 7-4. Sensing the INA199 to Cancel Effects of Impedance on the REF Input

7.4.4 Using the INA199 With Common-Mode Transients Above 26 V

With a small amount of additional circuitry, the INA199 series can be used in circuits subject to transients higher

than 26 V, such as automotive applications. Use only Zener diode or Zener-type transient absorbers (sometimes

referred to as transzorbs); any other type of transient absorber has an unacceptable time delay. Start by adding

a pair of resistors (see Figure 7-5) as a working impedance for the Zener. Keeping these resistors as small

as possible is preferable, most often approximately 10 Ω. Larger values can be used with an effect on gain as

discussed in the Section 7.4.1 section. Because this circuit limits only short-term transients, many applications

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INA199

SBOS469H – APRIL 2009 – REVISED OCTOBER 2023

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