INA217

7

SBOS247A

www.ti.com

FIGURE 3. Offset Voltage Adjustment Circuit.

OFFSET VOLTAGE TRIM

A variable voltage applied to pin 5, as shown in Figure 3, can

be used to adjust the output offset voltage. A voltage applied

to pin 5 is summed with the output signal. An op amp

connected as a buffer is used to provide a low impedance at

pin 5 to assure good common-mode rejection.

INA217

V+

V–

V

O

V+

150

Ω

10k

Ω

150

Ω

100

µA

100

µA

2

1

8

R

G

3

5

7

4

6

V–

OPA237

MICROPHONE AMPLIFIER

Figure 4 shows a typical circuit for a professional microphone

tional 48V phantom power source for a remotely located

microphone. An optional switch allows phantom power to be

the INA217 input circuitry. Non-polarized capacitors should

additional input protection against ESD and hot-plugging,

four IN4148 diodes may be connected from the input to

supply lines.

Input offset current (typically 100nA) creates a DC differential

input voltage that will produce an output offset voltage. This

is generally the dominant source of output offset voltage in

this application. With a maximum gain of 1000 (60dB), the

output offset voltage can be several volts. This may be

entirely acceptable if the output is AC-coupled into the

subsequent stage. An alternate technique is shown in Figure 4.

An inexpensive FET-input op amp in a feedback loop drives

the DC output voltage to 0V. A2 is not in the audio signal path

and does not affect signal quality.

change (in dB) with rotation.

R

5

2.2k

Ω

R

6

(2)

8

Ω

R

7

(3)

1.6k

Ω

A1

INA217

+

47

µF

R

3

47k

Ω

R

2

6.8k

Ω

R

1

6.8k

Ω

Phantom Power

+48V

+

+

R

4

2.2k

Ω

C

1

(1)

47

µF

60V

C

2

(1)

47

µF

60V

A2

OPA137

0.1

µF

+15V

0.1

µF

0.1

µF

1M

Ω

V

O

–15V

Optional DC

output control loop.

6

7

5

4

1

2

3

Female XLR

Connector

NOTES: (1) Use non-polar capacitors if phantom power is to be

turned off. (2) R

(4) Optional IN4148 prevents damage due to ESD and hot-plugging.

+15V

–15V

+15V

–15V

FIGURE 4. Phantom-Powered Microphone Preamplifier.