AD824

REV. A

–13–

3 Volt, Single Supply Stereo Headphone Driver

The AD824 exhibits good current drive and THD+N perfor-

mance, even at 3 V single supplies. At 1 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 that consume more power and cannot run on 3

V power supplies.

In Figure 33, each channel’s input signal is coupled via a 1

µF

Mylar capacitor. Resistor dividers set the dc voltage at the

noninverting inputs so that the output voltage is midway be-

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

the AD824 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.

MYLAR

1µF

1/4

AD824

L

R

HEADPHONES

32

Ω IMPEDANCE

4.99k

MYLAR

1µF

4.99k

1/4

AD824

10k

10k

47.5k

95.3k

47.5k

500µF

500µF

+3V

0.1µF

0.1µF

CHANNEL 1

CHANNEL 2

95.3k

Figure 33. 3 Volt Single Supply Stereo Headphone Driver

Low Dropout Bipolar Bridge Driver

The AD824 can be used for driving a 350 ohm Wheatstone

bridge. Figure 34 shows one half of the AD824 being used to

buffer the AD589—a 1.235 V low power reference. The output

350

Ω

350

Ω

350

Ω

350

Ω

AD620

R2

20

Ω

–4.5V

10k

10k

10k

26.4k, 1%

R1

20

Ω

TO A/D CONVERTER

REFERENCE INPUT

AD589

49.9k

+1.235V

+5V

1µF

GND

1%

1%

1%

1/4

AD824

1/4

AD824

0.1µF

–5V

1µF

0.1µF

7

6

5

4

3

2

Figure 34. Low Dropout Bipolar Bridge Driver

of +4.5 V can be used to drive an A/D converter front end. The

other half of the AD824 is configured as a unity-gain inverter

and generates the other bridge input of –4.5 V. Resistors R1 and

R2 provide a constant current for bridge excitation. The AD620

low power instrumentation amplifier is used to condition the

differential output voltage of the bridge. The gain of the AD620

G

=

49.4 k

Ω

+ 1

A 3.3 Volt/5 Volt Precision Sample-and-Hold Amplifier

In battery-powered applications, low supply voltage operational

amplifiers are required for low power consumption. Also, low

supply voltage applications limit the signal range in precision

analog circuitry. Circuits like the sample-and-hold circuit

shown in Figure 35, illustrate techniques for designing precision

analog circuitry in low supply voltage applications. To maintain

high signal-to-noise ratios (SNRs) in a low supply voltage appli-

cation requires the use of rail-to-rail, input/output operational

amplifiers. This design highlights the ability of the AD824 to oper-

ate rail-to-rail from a single +3 V/+5 V supply, with the advantages

of high input impedance. The AD824, a quad JFET-input op

amp, is well suited to S/H circuits due to its low input bias cur-

rents (3 pA, typical) and high input impedances (3

typical). The AD824 also exhibits very low supply currents so

the total supply current in this circuit is less than 2.5 mA.

3.3/5V

3.3/5V

R1

50k

R2

50k

A1

3

2

4

1

11

0.1µF

FALSE GROUND (FG)

A4

12

13

14

SAMPLE/

HOLD

A3

10

9

8

A2

5

6

7

15

14

16

10

9

11

AD824B

3.3/5V

ADG513

R5

2k

Ω

AD824C

+

–

CH

C

500pF

FG

4

5

8

6

7

23

1

AD824A

AD824D

R4

2k

Ω

FG

13

500pF

FG

Figure 35. 3.3 V/5.5 V Precision Sample and Hold

In many single supply applications, the use of a false ground

generator is required. In this circuit, R1 and R2 divide the sup-

ply voltage symmetrically, creating the false ground voltage at

one-half the supply. Amplifier A1 then buffers this voltage cre-

ating a low impedance output drive. The S/H circuit is config-

ured in an inverting topology centered around this false ground

level.