AD823ARZ-R7 datasheet(16/20 Pages) AD

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AD823

Data Sheet

Rev. E | Page 16 of 20

Figure 40 shows a schematic of an AD823 being used to drive

both the input and reference input of an AD1672, a 12-bit,

3-MSPS, single-supply ADC. One amplifier is configured as a

unity-gain follower to drive the analog input of the AD1672,

which is configured to accept an input voltage that ranges from

0 V to 2.5 V.

The other amplifier is configured as a gain of 2 to drive the

reference input from a 1.25 V reference. Although the AD1672

has its own internal reference, there are systems that require

greater accuracy than the internal reference provides. On the other

hand, if the AD1672 internal reference is used, the second AD823

amplifier can be used to buffer the reference voltage for driving

other circuitry while minimally loading the reference source.

+5VA

10µF

0.1µF

49.9Ω

10µF

0.1µF

10µF

0.1µF

+5VA +5VD

+5VD

CLOCK

1kΩ

VIN

VREF

(1.25V)

BIT1 (MSB)

BIT2

BIT3

BIT4

BIT5

BIT6

BIT7

BIT8

BIT9

BIT10

BIT11

BIT12 (LSB)

OTR

REFOUT

AIN1

AIN2

REFIN

REFCOM

NCOMP2

NCOMP1

ACOM

REF

AD823

28 19

AD1672

Figure 40. AD823 Driving Input and Reference of the

AD1672, a 12-Bit, 3-MSPS ADC

The circuit was tested with a 500 kHz sine wave input that was

heavily low-pass filtered (60 dB) to minimize the harmonic content

at the input to the AD823. The digital output of the AD1672 was

analyzed by performing a fast Fourier transform (FFT).

During the testing, it was observed that at 500 kHz, the output

of the AD823 cannot go below ~350 mV (operating with

negative supply at ground) without seriously degrading the

second harmonic distortion. Another test was performed with a

200 Ω pull-down resistor to ground that allowed the output to

go as low as 200 mV without seriously affecting the second

harmonic distortion. There was, however, a slight increase in

the third harmonic term with the resistor added, but it was still

less than the second harmonic.

Figure 41 is an FFT plot of the results of driving the AD1672

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

amplitude 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 provides the necessary information

for its applicability for these systems.

VIN = 2.15V p-p

G = +1

FI = 490kHz

Figure 41. FFT of AD1672 Output Driven by AD823

3 V, Single-Supply Stereo Headphone Driver

The AD823 exhibits good current drive and total harmonic

distortion plus noise (THD+N) performance, even at 3 V

single supplies. At 20 kHz, 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 42, 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

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 that can be modeled as 32 Ω load resistors to

ground. This ensures that all signals in the audio frequency

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

MYLAR

1µF

1/2

AD823

HEADPHONES

32Ω IMPEDANCE

4.99kΩ

MYLAR

1µF

4.99kΩ

10kΩ

47.5kΩ

95.3kΩ

47.5kΩ

500µF

95.3kΩ

0.1µF

CHANNEL 1

CHANNEL 2

95.3kΩ

1/2

AD823

Figure 42. 3 V Single-Supply Stereo Headphone Driver

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