Datasheet

AD8614/AD8644
Rev. B | Page 11 of 16
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.
AD86x4
V
OUT
5V
R
X
C
X
C
L
V
IN
06485-028
Figure 28. Snubber Network Compensation for Capacitive Loads
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
L
) Snubber Network (R
X
, C
X
)
0.47 nF 300 Ω, 0.1 μF
4.7 nF 30 Ω, 1 μF
47 nF 5 Ω, 10 μF
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.
6.2V
6.2V
2kΩ
1:1
1
2
3
7
6
5
2
3
1
6
5
7
5V DC
A1, A2 = 1/2 AD8644
A3, A4 = 1/2 AD8644
R12
10kΩ
R11
10kΩ
R10
10kΩ
R13
10kΩ
R14
14.3kΩ
C2
0.1µF
RECEIVE
RxA
2kΩ
10µF
P2
Rx GAIN
ADJUST
R7
10kΩ
R8
10kΩ
R9
10kΩ
R6
10kΩ
R5
10kΩ
TRANSMIT
TxA
C1
0.1µF
R1
10kΩ
R2
9.09kΩ
P1
Tx GAIN
ADJUST
R3
360Ω
TO TELEPHONE
LINE
Z
O
600Ω
T1
MIDCOM
671-8005
06485-029
A1
A2
A3
A4
Figure 29. A Single-Supply Direct Access Arrangement for Modems
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.
U1-A
4
5V
1
10
2
3
5
5
V
AV
DD1
VREFOUT
LINE_OUT_L
LINE_OUT_R
AV
SS1
7
8
6
9
NOTES
1. ADDITIONAL PINS OMITTED FOR CLARITY.
U1-B
U1 = AD8644
28
35
36
26
25
+
AD1881A
(AC'97)
R3
20Ω
+
C1
100µF
R1
2kΩ
R4
20Ω
C2
100µF
R2
2kΩ
06485-030
Figure 30. A PC-99-Compliant Headphone/Line Out Amplifier