AD823A Analog Devices, AD823A Datasheet - Page 14

• Manufacturer Part Number: AD823A
• Description: Dual, 16 Mhz, Rail-to-rail Fet Input Amplifier
• Manufacturer: Analog Devices

AD823
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.
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 F
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 F 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.
Figure 39. FFT of AD1672 Output Driven by AD823
5
6
1
4
9
V
G = +1
F
I
IN
= 490kHz
7
= 2.15Vp-p
2
3
8
–14–
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 to 100 k ).
A plot of the filter is shown below; better than 50 dB of high fre-
quency rejection is provided.
Figure 40. 3 Volt Single Supply Stereo Headphone Driver
CHANNEL 1
CHANNEL 2
V
IN
C1( farads )
Figure 41. Second Order Low-Pass Filter
20k
R1
MYLAR
MYLAR
1µF
95.3k
1µF
95.3k
28pF
20k
R2
C1
2 f
56pF
C2
1.414
95.3k
47.5k
cutoff
47.5k
+3V
AD823
10k
1/2
10k
–5V
3
2
+5V
6
R1
5
AD823
AD823
1/2
1/2
0.1µF
0.1µF
; C2
C3
C4
8
4
4.99k
4.99k
7
1 1
32 IMPEDANCE
2 f
HEADPHONES
500µF
500µF
+
+
0.1µF
0.707
cutoff
50pF
R1
0.1µF
V
OUT
L
R
REV. 0