A1

A2

Single-Pole Filter

Capacitor

+2.7 V to +18 V

5 kW

5 kW

96 kW

R

L

PRE OUT

BUF IN

GND

Output

IN–

IN+

V+

Load

Supply

R

S

Second-Order, Sallen-Key Filter Connection

C

FILT

C

FILT

GND

R

S

a ) S i n g le - P o le F ilt e r

b ) S e c o n d - O r d e r, S a lle n - K e y F i lt e r

IN A 270

IN A 270

5 kW

5 kW

IN–

IN+

+2.7 V to +18 V

PRE OUT

BUF IN

V+

A1

A2

Output

96 kW

R

L

R

S

Load

Supply

INA270-Q1, INA271-Q1

www.ti.com

SBOS401B – JULY 2007 – REVISED FEBRUARY 2010

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 will never allow 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, since the internal gain resistors can vary up to ±30%, but the internal resistors

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.

Output Voltage Range

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 (see Figure 2a).

The INA270 and INA271 readily lend themselves to second-order Sallen-Key configurations (see Figure 2b).

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.

A.

The INA270 and INA271 can be easily connected for first-order or second-order filtering. Remember to use the

appropriate buffer gain (INA270 = 1.4, INA271 = 2) when designing Sallen-Key configurations.

Figure 2. First-Order or Second-Order Filtering

Copyright © 2007–2010, Texas Instruments Incorporated

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Product Folder Link(s): INA270-Q1 INA271-Q1