Datasheet
AD823
Rev. A | 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.
13
14
12
11
10
9
8
7
6
5
4
3
2
1
19 18
+5VA
10µF
0.1µF
2
3
5
6
4
7
1
8
49.9Ω
10µF0.1µF0.1µF10µF
0.1µF
+5VA +5VD +5VD
20
21
22
23
24
25
26
27
16
CLOCK
1kΩ
1kΩ
V
IN
V
REF
(1.25V)
BIT1 (MSB)
BIT2
BIT3
BIT4
BIT5
BIT6
BIT7
BIT8
BIT9
BIT10
BIT11
BIT12 (LSB)
15
OTR
REFOUT
AIN1
AIN2
REFIN
REFCOM
NCOMP2
NCOMP1
ACOM
COM
REF
DCOM
AD823
+V
CC
+V
DD
28 19
AD1672
00901-A-039
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 FFT.
During the testing, it was observed that at 500 kHz, the output
of the AD823 cannot go below about 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 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
2
3
1
V
IN
= 2.15V p-p
G = +1
F
I
= 490kHz
15dB/DIV
00901-A-040
7
8
Figure 41. FFT of AD1672 Output Driven by AD823
3 V, Single-Supply Stereo Headphone Driver
The AD823 exhibits good current drive and THD+N perform-
ance, 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 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 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 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
L
R
HEADPHONES
32Ω IMPEDANCE
4.99kΩ
MYLAR
1µF
4.99kΩ
10kΩ
10kΩ
47.5kΩ
95.3kΩ
47.5kΩ
500µF
500µF
3V
95.3kΩ
0.1µF0.1µF
CHANNEL 1
CHANNEL 2
95.3kΩ
+
+
7
4
5
6
1/2
AD823
3
8
2
1
1
00901-A-041
Figure 42. 3 V Single-Supply Stereo Headphone Driver