AD8644 Analog Devices, AD8644 Datasheet - Page 11

• Manufacturer Part Number: AD8644
• Description: 18 V Operational Amplifier (Quad)
• Manufacturer: Analog Devices

Specifications of AD8644

• -3db Bandwidth: 5.5MHz
• Slew Rate: 7.5V/µs
• Vos: 1mV
• Ib: 80nA
• # Opamps Per Pkg: 4
• Input Noise (nv/rthz): 12nV/rtHz
• Vcc-vee: 5V to 18V
• Isy Per Amplifier: 1.1mA
• Packages: SOIC,SOP

CAPACITIVE LOAD DRIVE
The AD8614/AD8644 exhibit excellent capacitive load driving
capabilities. Although the device is stable with large capacitive
loads, there is a decrease in amplifier bandwidth as the
capacitive load increases.
When driving heavy capacitive loads directly from the
AD8614/AD8644 output, a snubber network can be used to
improve the transient response. This network consists of a
series R-C connected from the amplifier’s output to ground,
placing it in parallel with the capacitive load. The configuration
is shown in Figure 28. Although this network does not increase
the bandwidth of the amplifier, it does significantly reduce the
amount of overshoot.
The optimum values for the snubber network should be
determined empirically based on the size of the capacitive load.
Table 4 shows a few sample snubber network values for a given
load capacitance.
Table 4. Snubber Networks for Large Capacitive Loads
Load Capacitance (C
0.47 nF
4.7 nF
47 nF
DIRECT ACCESS ARRANGEMENT
Figure 29 shows a schematic for a 5 V single-supply transmit/
receive telephone line interface for 600 Ω transmission systems. It
allows full duplex transmission of signals on a transformer-
coupled 600 Ω line. Amplifier A1 provides gain that can be
adjusted to meet the modem’s output drive requirements. Both
A1 and A2 are configured to apply the largest possible differential
signal to the transformer. The largest signal available on a single
5 V supply is approximately 4.0 V p-p into a 600 Ω transmission
system. Amplifier A3 is configured as a difference amplifier to
extract the receive information from the transmission line for
amplification by A4. A3 also prevents the transmit signal from
interfering with the receive signal. The gain of A4 can be adjusted
in the same manner as A1 to meet the modem input signal
requirements. Standard resistor values permit the use of single
in-line package (SIP) format resistor arrays. Couple this with
the AD8644 14-lead SOIC or TSSOP package and this circuit
can offer a compact solution.
Figure 28. Snubber Network Compensation for Capacitive Loads
V
IN
AD86x4
L
)
5V
Snubber Network (R
300 Ω, 0.1 μF
30 Ω, 1 μF
5 Ω, 10 μF
R
C
X
X
C
L
V
OUT
X
, C
X
)
Rev. B | Page 11 of 16
A ONE-CHIP HEADPHONE/MICROPHONE
PREAMPLIFIER SOLUTION
Because of its high output current performance, the AD8644
makes an excellent amplifier for driving an audio output jack in
a computer application. Figure 30 shows how the AD8644 can
be interfaced with an ac codec to drive headphones or speakers.
TO TELEPHONE
600Ω
LINE_OUT_R
A1, A2 = 1/2 AD8644
A3, A4 = 1/2 AD8644
LINE_OUT_L
Z
AD1881A
NOTES
1. ADDITIONAL PINS OMITTED FOR CLARITY.
O
(AC'97)
VREFOUT
MIDCOM
671-8005
LINE
Figure 29. A Single-Supply Direct Access Arrangement for Modems
1:1
AV
AV
T1
Figure 30. A PC-99-Compliant Headphone/Line Out Amplifier
DD1
SS1
25
28
35
36
26
6.2V
6.2V
10kΩ
R11
5V
10kΩ
10kΩ
360Ω
R9
R12
R3
2
3
7
8
5
6
U1-A
U1-B
2
3
5V
A3
10kΩ
10kΩ
10kΩ
R10
10
Tx GAIN
ADJUST
R5
R6
4
U1 = AD8644
P1
2kΩ
1
9
1
1
7
10kΩ
A1
A2
R13
9.09kΩ
R2
100µF
100µF
6
5
2
3
+
+
C1
C2
AD8614/AD8644
2kΩ
2kΩ
14.3kΩ
6
5
R1
R2
R14
A4
10kΩ
R1
Rx GAIN
ADJUST
10µF
2kΩ
20Ω
20Ω
R4
7
R3
0.1µF
P2
C1
5V DC
0.1µF
C2
R7
10kΩ
R8
10kΩ
TRANSMIT
RECEIVE
RxA
TxA