V

RTI(Referred-To-Input)=

OS

V

OUT1

G

- 100mV

G=

V

V

-

OUT1

OUT2

100mV

20mV

-

13

INA270, INA271

www.ti.com

SBOS381E – FEBRUARY 2007 – REVISED JANUARY 2018

Product Folder Links: INA270 INA271

Submit Documentation Feedback

Copyright © 2007–2018, Texas Instruments Incorporated

Device Functional Modes (continued)

8.4.2 Accuracy Variations as a Result of VSENSE and Common-Mode Voltage

The accuracy of the INA270 and INA271 current shunt monitors is a function of two main variables: VSENSE

(VIN+ – VIN–) and common-mode voltage (VCM) relative to the supply voltage, VS. VCM is expressed as (VIN+ +

VIN–) / 2; however, in practice, VCM is used as the voltage at VIN+ because the voltage drop across VSENSE is

usually small.

This section addresses the accuracy of these specific operating regions:

Normal Case 1: VSENSE ≥ 20 mV, VCM ≥ VS

Normal Case 2: VSENSE ≥ 20 mV, VCM < VS

Low VSENSE Case 1:

VSENSE < 20 mV, –16 V ≤ VCM < 0

Low VSENSE Case 2:

VSENSE < 20 mV, 0 V ≤ VCM ≤ VS

Low VSENSE Case 3:

VSENSE < 20 mV, VS < VCM ≤ 80 V

8.4.2.1 Normal Case 1: VSENSE ≥ 20 mV, VCM ≥ VS

This region of operation provides the highest accuracy. Here, the input offset voltage is characterized and

measured using a two-step method. First, the gain is determined by Equation 1.

where

•

VOUT1 = Output voltage with VSENSE = 100 mV and

•

VOUT2 = Output voltage with VSENSE = 20 mV.

(1)

Then the offset voltage is measured at VSENSE = 100 mV and referred to the input (RTI) of the current shunt

monitor, as shown in Equation 2.

(2)

In the Typical Characteristics, the Output Error vs Common-Mode Voltage curve (Figure 6) shows the highest

accuracy for the this region of operation. In this plot, VS = 12 V; for VCM ≥ 12 V, the output error is at its

minimum. This case is also used to create the VSENSE ≥ 20 mV output specifications in the Electrical

Characteristics table.

8.4.2.2 Normal Case 2: VSENSE ≥ 20 mV, VCM < VS

This region of operation has slightly less accuracy than Normal Case 1 as a result of the common-mode

operating area in which the device functions, as illustrated in the Output Error vs Common-Mode Voltage curve

(Figure 6). As noted, for this graph VS = 12 V; for VCM < 12 V, the output error increases when VCM becomes less

than 12 V, with a typical maximum error of 0.005% at the most negative VCM = –16 V.

8.4.2.3 Low VSENSE Case 1: VSENSE < 20 mV, –16 V ≤ VCM < 0; and Low VSENSE Case 3: VSENSE < 20 mV,

VS < VCM ≤ 80 V

Although the INA270 family of devices are not designed for accurate operation in either of these regions, some

applications are exposed to these conditions. For example, when monitoring power supplies that are switched on

and off while VS is still applied to the INA270 or INA271, knowing what the behavior of the devices is in these

regions is important.