AD824
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.
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
is programmed using an external resistor RG and determined by:
In Figure 6, each channel’s input signal is coupled via a 1 mF
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
AD824 can then be used to drive a headphone channel. A 5 Hz
high-pass filter is realized by the 500 mF 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.
49.4 kW
RG
G =
+ 1
A 3.3 V/5 V 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 8, 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 application
requires the use of rail-to-rail, input/output operational amplifi-
ers. This design highlights the ability of the AD824 to operate
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 currents
(3 pA, typical) and high input impedances (3 ¥ 1013 W, typical).
The AD824 also exhibits very low supply currents so the total
supply current in this circuit is less than 2.5 mA.
3V
0.1ꢀF
0.1ꢀF
95.3kꢃ
1ꢀF
CHANNEL 1
1/4
MYLAR
AD824
47.5kꢃ AD824
500ꢀF
95.3kꢃ
L
4.99kꢃ
10kꢃ
10kꢃ
HEADPHONES
32ꢃ IMPEDANCE
3.3/5V
R1
3.3/5V
R
0.1ꢀF
4.99kꢃ
AD824A
50kꢃ
3
4
1
1/4
AD824
47.5kꢃ
1ꢀF
A1
A1
FALSE GROUND (FG)
AD824
2
R2
50kꢃ
500ꢀF
CHANNEL 2
11
R4
2kꢃ
MYLAR
3.3/5V
13
Figure 6. 3 Volt Single Supply Stereo Headphone Driver
ADG513
FG
15
14
Low Dropout Bipolar Bridge Driver
R5
16
9
The AD824 can be used for driving a 350 ohm Wheatstone
bridge. Figure 7 shows one half of the AD824 being used to
buffer the AD589—a 1.235 V low power reference. The output
2kꢃ
11
10
AD824B
CH
500pF
5
7
2
3
A2
A2
+V
S
6
1
8
10
9
\
49.9kꢃ
+1.235V
8
R1
A3
A3
+
–
20ꢃ
V
OUT
7
4
6
5
1/4
AD824
1/4
TO A/D CONVERTER
REFERENCE INPUT
AD824C
AD824
AD589
C
AD824D
10kꢃ
26.4kꢃ, 1%
350ꢃ
FG
14
500pF
+V
S
12
1%
A4
A4
SAMPLE/
HOLD
FG
350ꢃ
3
2
13
7
6
AD824
AD620
R
G
350ꢃ
350ꢃ
5
Figure 8. 3.3 V/5.5 V Precision Sample and Hold
4
10kꢃ
V
In many single supply applications, the use of a false ground
generator is required. In this circuit, R1 and R2 divide the
supply voltage symmetrically, creating the false ground voltage
at one-half the supply. Amplifier A1 then buffers this voltage
creating a low impedance output drive. The S/H circuit is con-
figured in an inverting topology centered around this false
ground level.
REF
–V
S
1%
10kꢃ
1/4
1/4
1%
+V
+5V
–5V
S
–4.5V
AD824
AD824
1ꢀF
1ꢀF
0.1ꢀF
GND
0.1ꢀF
–V
R2
20ꢃ
–V
S
S
Figure 7. Low Dropout Bipolar Bridge Driver
REV. C
–13–
ADI [ ADI ]
