AD823
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.ꢀ V.
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.
1
V
= 2.15V p-p
IN
The other amplifier is configured as a gain of 2 to drive the
reference input from a 1.2ꢀ 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 +5VD
G = +1
FI = 490kHz
2
4
9
7
6
5
8
3
0.1µF
10µF
0.1µF
10µF
+5VA
8
28 19
0.1µF
+5VD
10µF
Figure 41. FFT of AD1672 Output Driven by AD823
2
3
20
21
22
0.1µF
OTR
REFOUT
AIN1
AIN2
1
15
3 V, Single-Supply Stereo Headphone Driver
49.9Ω
V
IN
13
14
12
11
10
9
BIT1 (MSB)
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.
AD823
AD1672
BIT2
BIT3
BIT4
BIT5
BIT6
5
V
7
REF
(1.25V)
23
24
25
26
REFIN
8
6
REFCOM
NCOMP2
NCOMP1
4
7
6
5
4
3
2
1
BIT7
BIT8
BIT9
BIT10
BIT11
1kΩ
1kΩ
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.ꢀ V). The gain is 1.ꢀ. Each half
of the AD823 can then be used to drive a headphone channel. A
ꢀ Hz high-pass filter is realized by the ꢀ00 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.
3V
27
16
ACOM
REF
BIT12 (LSB)
CLOCK
19
18
Figure 40. AD823 Driving Input and Reference of the
AD1672, a 12-Bit, 3-MSPS ADC
The circuit was tested with a ꢀ00 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).
+
0.1µF
0.1µF
95.3kΩ
95.3kΩ
47.5kΩ
8
3
CHANNEL 1
During the testing, it was observed that at ꢀ00 kHz, the output
of the AD823 cannot go below ~3ꢀ0 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.
+
1/2
1µF
MYLAR
1
2 AD823
500µF
L
4.99kΩ
95.3kΩ
10kΩ
10kΩ
HEADPHONES
32Ω IMPEDANCE
R
4.99kΩ
6
500µF
+
1/2
47.5kΩ
7
1µF
AD823
5
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 2.1ꢀ V p-p and the lower voltage excursion was
3ꢀ0 mV. The input frequency was 490 kHz, which was chosen
to spread the location of the harmonics.
4
CHANNEL 2
MYLAR
Figure 42. 3 V Single-Supply Stereo Headphone Driver
Rev. B | Page 16 of 20
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