REV. 0

–14–

AD823

MYLAR

1µF

1/2

AD823

L

R

HEADPHONES

32

Ω IMPEDANCE

4.99k

MYLAR

1µF

4.99k

10k

10k

47.5k

95.3k

47.5k

500µF

500µF

+3V

95.3k

0.1µF

0.1µF

CHANNEL 1

CHANNEL 2

95.3k

Ω

+

+

7

4

5

6

1/2

AD823

3

8

2

1

1

Figure 40. 3 Volt Single Supply Stereo Headphone Driver

Second Order Low-Pass Filter

Figure 41 depicts the AD823 configured as a second order

Butterworth low-pass filter. With the values as shown, the cor-

ner frequency will be 200 kHz. The equations for component

selection are shown below:

R1 = R2 = user selected (typical values: 10 k

Ω to 100 kΩ).

C1( farads )

=

1.414

2

πf

; C2

=

0.707

2

πf

1/2

AD823

C3

0.1µF

+5V

C4

0.1µF

C1

28pF

–5V

C2

56pF

R1

20k

R2

20k

50pF

Figure 41. Second Order Low-Pass Filter

A plot of the filter is shown below; better than 50 dB of high fre-

quency rejection is provided.

Figure 39 is an FFT plot of the results of driving the AD1672

with the AD823 with no pull-down resistor. The input ampli-

tude was 2.15 V p-p and the lower voltage excursion was

350 mV. The input frequency was 490 kHz, which was chosen

to spread the location of the harmonics.

The distortion analysis is important for systems requiring good

frequency domain performance. Other systems may require

good time domain performance. The noise and settling time

performance of the AD823 will provide the necessary informa-

tion for its applicability for these systems.

5

6

4

9

7

2

3

8

1

G = +1

Figure 39. FFT of AD1672 Output Driven by AD823

3 Volt, Single Supply Stereo Headphone Driver

The AD823 exhibits good current drive and THD+N perfor-

mance, even at 3 V single supplies. At 20 kHz, total harmonic

distortion plus noise (THD+N) equals –62 dB (0.079%) for a

300 mV p-p output signal. This is comparable to other single

supply op amps which consume more power and cannot run on

3 V power supplies.

In Figure 40, each channel’s input signal is coupled via a 1

µF

Mylar capacitor. Resistor dividers set the dc voltage at the non-

inverting inputs so that the output voltage is midway between

the power supplies (+1.5 V). The gain is 1.5. Each half of the

AD823 can then be used to drive a headphone channel. A 5 Hz

high-pass filter is realized by the 500

µF capacitors and the

headphones, which can be modeled as 32 ohm load resistors to

ground. This ensures that all signals in the audio frequency

range (20 Hz–20 kHz) are delivered to the headphones.