AD8591/AD8592/AD8594
The U1-A amplifier is configured as a unity-gain buffer driving
a 1 nF capacitor. The input signal is connected to the noninverting
input, and the sample clock controls the shutdown for that
amplifier. When the sample clock is high, the U1-A amplifier is
active and the output follows V
IN
. When the sample clock goes
low, U1-A shuts down with the output of the amplifier going to
a high impedance state, holding the voltage on the C1 capacitor.
The U1-B amplifier is used as a unity-gain buffer to prevent
loading on C1. Because of the low input bias current of the U1-B
CMOS input stage and the high impedance state of the U1-A
output in shutdown, there is little voltage droop from C1 during
the hold period. This circuit can be used with sample frequencies as
high as 500 kHz and as low as 1 Hz. By increasing the C1 value,
lower voltage droop is achieved for very low sample rates.
SINGLE-SUPPLY DIFFERENTIAL LINE DRIVER
that can drive a 600 Ω load with less than 0.7% distortion from
20 Hz to 15 kHz with an input signal of 4 V p-p and a single 5 V
supply. The design uses an AD8594 to mimic the performance
of a fully balanced transformer-based solution. However, this
design occupies much less board space, while maintaining low
distortion, and can operate down to dc. Like the transformer-based
design, either output can be shorted to ground for unbalanced
line driver applications without changing the circuit gain of 1.
R3
10kΩ
2
3
R2
10kΩ
+5V
C1
22µF
V
IN
2
3
A1
4
R1
10kΩ
R10
10kΩ
A1, A2 = 1/2 AD8592
GAIN = R3
R2
SET: R7, R10, R11 = R2
SET: R6, R12, R13 = R3
10
1
9
A2
1
R6
10kΩ
R5
50Ω
C3
47µF
V
O1
DIRECT ACCESS ARRANGEMENT FOR PCMCIA
MODEMS (TELEPHONE LINE INTERFACE)
interface for 600 Ω systems. It allows full duplex transmission
of signals on a transformer-coupled 600 Ω line in a differential
manner. Amplifier A1 provides gain that can be adjusted to
meet the modem output drive requirements. Both A1 and A2
are configured to apply the largest possible signal on a single
supply to the transformer. Because of the high output current
drive and low dropout voltages of the AD8594, the largest signal
available on a single 5 V supply is approximately 4.5 V p-p into
a 600 Ω transmission system. Amplifier A3 is configured as a
difference amplifier for two reasons. It prevents the transmit
signal from interfering with the receive signal, and it extracts
the receive signal from the transmission line for amplification
by A4. The gain of A4 can be adjusted in the same manner as
the gain of A1 to meet the input signal requirements of the
modem. Standard resistor values permit the use of single
inline package (SIP) format resistor arrays. Couple this with
the 16-lead TSSOP or SOIC footprint of the AD8594, and this
circuit offers a compact, cost-effective solution.
P1
Tx GAIN
ADJUST
R2
9.09kΩ
2
A1
9
3
SHUTDOWN
+5V
R6
10kΩ
9
7
A2
6
5
10µF
R9
10kΩ
11
R11
10kΩ
R12
10kΩ
12
R10
10kΩ
9
A3
10
R14
R13
10kΩ 14.3kΩ
15
14
9
A4
R7
10kΩ
R8
10kΩ
R7
10kΩ
+5V
10
A1
4
8
7
+5V
R8
100kΩ
C2
1µF
R
L
600Ω
R11
10kΩ
8
7
A2
R9
100kΩ
R12
10kΩ
9
R13
10kΩ
R14
50Ω
C4
47µF
V
O2
Figure 43. Low Noise, Single-Supply Differential Line Driver
R8 and R9 set up the common-mode output voltage equal to
half of the supply voltage. C1 is used to couple the input signal
and can be omitted if the dc voltage of the input is equal to half
of the supply voltage.
The circuit can also be configured to provide additional gain, if
desired. The gain of the circuit is
A
V
=
V
OUT
R
3
=
V
IN
R
2
(7)
TO TELEPHONE
LINE
1:1
Z
O
600Ω
T1
MIDCOM
671-8005
6.2V
6.2V
R3
360Ω
R5
10kΩ
2kΩ
1
C1
R1
10kΩ 0.1µF
TRANSMIT
TxA
where:
V
OUT
= V
O1
− V
O2
R2
=
R7
=
R10
=
R11
R3
=
R6
=
R12
=
R1
P2
Rx GAIN
ADJUST
RECEIVE
RxA
2kΩ
16
A1, A2 = 1/4 AD8594
A3, A4 = 1/4 AD8594
Figure 42. Single-Supply Direct Access Arrangement for PCMCIA Modems
Rev. B | Page 14 of 16
01106-043
C2
0.1µF
01106-044
AD [ ANALOG DEVICES ]
