a) Single-Pole Filter

b) Second-Order, Sallen-Key Filter

A1

A2

Single-Pole Filter

Capacitor

+2.7V to +18V

5kW

5kW

96kW

R

L

PRE OUT

BUF IN

GND

Output

IN-

IN+

V+

Load

Supply

R

S

INA270

A1

A2

Second-Order, Sallen-Key Filter Connection

+2.7V to +18V

C

FILT

C

FILT

5kW

5kW

96kW

R

L

PRE OUT

BUF IN

GND

Output

IN-

IN+

V+

Load

Supply

R

S

R

S

INA270

12

INA270, INA271

SBOS381E – FEBRUARY 2007 – REVISED JANUARY 2018

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Product Folder Links: INA270 INA271

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Copyright © 2007–2018, Texas Instruments Incorporated

Feature Description (continued)

Use of MOVs or VDRs is not recommended except when they are used in addition to a semiconductor transient

absorber. Select the transient absorber such that it never allows the INA270 and INA271 to be exposed to

transients greater than 80 V (that is, allow for transient absorber tolerance, as well as additional voltage because

of transient absorber dynamic impedance). Despite the use of internal zener-type ESD protection, the INA270

and INA271 are not suited to using external resistors in series with the inputs because the internal gain resistors

can vary up to ±30%, but are tightly matched (if gain accuracy is not important, then resistors can be added in

series with the INA270 and INA271 inputs with two equal resistors on each input).

8.4 Device Functional Modes

8.4.1 First- or Second-Order Filtering

The output of the INA270 and INA271 is accurate within the output voltage swing range set by the power-supply

pin, V+.

The INA270 and INA271 readily enable the inclusion of filtering between the preamp output and buffer input.

Single-pole filtering can be accomplished with a single capacitor because of the 96-k

Ω output impedance at PRE

OUT on pin 3, as shown in Figure 16a.

The INA270 and INA271 readily lend themselves to second-order Sallen-Key configurations, as shown in

Figure 16b. When designing these configurations consider that the PRE OUT 96-k

Ω output impedance exhibits

an initial variation of ±30% with the addition of a –2200-ppm/°C temperature coefficient.

NOTE: Remember to use the appropriate buffer gain (INA270 = 1.4, INA271 = 2) when designing Sallen-Key configurations.

Figure 16. The INA270–INA271 can be Easily Connected for First- or Second-Order Filtering