AD8275

Rev. A | Page 15 of 16

INCREASING INPUT IMPEDANCE

In applications where a high input impedance is needed, low

input bias current op amps can be used to buffer the AD8275.

In Figure 41, an AD8620 is used to provide high input imped-

ance. Input bias current is limited to 10 pA.

AD8275

7

4

5

6

8

1

2

1

3

2

7

6

5

4

8

50k

Ω

0.1µF

50k

Ω

20k

Ω

20k

Ω

10k

Ω

3

–IN

1/2

2/2

+IN

REF2

REF1

–13V

0.1µF

0.1µF

+5V

+13V

OUT

SENSE

INVERTING

INPUT

NON-

INVERTING

INPUT

AD8620

AD8620

Figure 41. Adding Op Amp Buffers for High Input Impedance

AC COUPLING

An integrator can be tied to the AD8275 in feedback to create a

high-pass filter as shown in Figure 42. This circuit can be used

to reject dc voltages and offsets. At low frequencies, the impedance

of the capacitor, C, is high. Thus, the gain of the integrator is

high. DC voltage at the output of the AD8275 is inverted and

gained by the integrator. The inverted signal is injected back

into the REFx pins, nulling the output. In contrast, at high fre-

quencies, the integrator has low gain because the impedance of

C is low. Voltage changes at high frequencies are inverted but at

a low gain. The signal is injected into the REFx pins but it is not

enough to null the output. High frequency signals are, therefore,

allowed to pass.

conditioned input signal.

AD8275

7

4

5

6

8

2

50k

Ω

0.1µF

0.1µF

50k

Ω

20k

Ω

20k

Ω

R

C

10k

Ω

3

+IN

–IN

REF2

REF1

+5V

+5V

OUT

SENSE

1

1

2

πRC

OP

AMP

Figure 42. AC-Coupled Level Translator

USING THE AD8275 AS A LEVEL TRANSLATOR IN

A DATA ACQUISITION SYSTEM

Signal size varies dramatically in some data acquisition applica-

tions. Instrumentation amplifiers, such as the AD8253, AD8228,

or AD8221, are often used at the inputs to provide CMRR and

high input impedance. However, the instrumentation amplifiers

output ±10 V signals and the ADC full scale is 5 V or 4.096 V.

In Figure 43, the AD8275 serves as a level translator between

the in-amp and the ADC. The AD8275, along with the AD8228

and the AD8253, have very low gain drift because all gain setting

resistors are internal and laser-trimmed.

VREF

AD8275

7

4

5

6

8

2

50k

Ω

0.1µF

50k

Ω

20k

Ω

20k

Ω

33

Ω

10k

Ω

3

+IN

–IN

REF2

REF1

+5V

OUT

SENSE

0.1µF

2.7nF

10µF

1

ADC

VCC

GND

REF

+IN

–IN

0.1µF

+15V

–15V

0.1µF

IN-AMP

Figure 43. Level Translation in a Data Acquisition System