AD824 Datasheet, PDF (13/16 Pages) Analog Devices – Single Supply, Rail-to-Rail Low Power, FET-Input Op Amp

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.
In Figure 33, 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 be-
tween 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 µF 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.
CHANNEL 1
1µF
MYLAR
95.3k
CHANNEL 2
1µF
MYLAR
+3V
95.3k
0.1µF
0.1µF
1/4
47.5k AD824
500µF
4.99k
L
10k
HEADPHONES
32Ω IMPEDANCE
10k
R
4.99k
47.5k
1/4
AD824
500µF
Figure 33. 3 Volt Single Supply Stereo Headphone Driver
Low Dropout Bipolar Bridge Driver
The AD824 can be used for driving a 350 ohm Wheatstone
bridge. Figure 34 shows one half of the AD824 being used to
buffer the AD589—a 1.235 V low power reference. The output
49.9k
+1.235V
AD589
10k
1%
10k
1%
+V_S
1/4
AD824
26.4k, 1%
350Ω
350Ω
10k
1%
1/4
AD824
R1
20Ω
TO A/D CONVERTER
REFERENCE INPUT
350Ω
350Ω ^RG
+V_S
3
7
AD620 ⁶
5
24
V_REF
–V_S
–4.5V
R2
20Ω
–V_S
+V_S
0.1µF
GND
0.1µF
–V_S
+5V
1µF
1µF
–5V
Figure 34. Low Dropout Bipolar Bridge Driver
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 R_G and determined by:
G = ^{49.4 kΩ} + 1
R_G
A 3.3 Volt/5 Volt 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 35, 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 appli-
cation requires the use of rail-to-rail, input/output operational
amplifiers. This design highlights the ability of the AD824 to oper-
ate 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 cur-
rents (3 pA, typical) and high input impedances (3 × 10¹³ Ω,
typical). The AD824 also exhibits very low supply currents so
the total supply current in this circuit is less than 2.5 mA.
3.3/5V
3.3/5V
R1 AD824A ^0.1µF
50k
3
4
1
2 ^A1
FALSE GROUND (FG)
R2
11
50k
R4
2kΩ
R5
2kΩ
SAMPLE/
HOLD
3.3/5V
13
15
14 ^ADG513
16
10
11
AD824B
5
7
2
6 ^A2
7
AD824D
12
FG
4
14
13 ^A4
9
3
1
6
8
5
FG
CH
500pF
10
8
9 ^A3
AD824C
C
500pF
FG
+
– ^VOUT
Figure 35. 3.3 V/5.5 V Precision Sample and Hold
In many single supply applications, the use of a false ground
generator is required. In this circuit, R1 and R2 divide the sup-
ply voltage symmetrically, creating the false ground voltage at
one-half the supply. Amplifier A1 then buffers this voltage cre-
ating a low impedance output drive. The S/H circuit is config-
ured in an inverting topology centered around this false ground
level.
REV. A
–13–