AD8517/AD8527
–11–REV. B
Table III. Snubber Network Values for Large Capacitive Loads
C
LOAD
Rx Cx
680 pF 300 Ω3 nF
1 nF 100 Ω10 nF
10 nF 400 Ω30
nF
TOTAL HARMONIC DISTORTION + NOISE
The AD85x7 family offers a low total harmonic distortion, which
makes this amplifier ideal for audio applications. Figure 26 shows a
graph of THD + N, for a V
S
> 3 V the THD + N is about 0.001%
and 0.03% for V
S
≥ 1.8 V in a noninverting configuration with a
gain of 1. In an inverting configuration, the noise is 0.003% for
all V
SY
.
FREQUENCY – Hz
0.01
0.0001
10 20k
THD + N – %
0.001
0.1
1
100 1k 10k
VS > 3V TO 5V
VS = 1.8V
AV = +2
Figure 26. THD + N vs. Frequency Graph
A MICROPOWER REFERENCE VOLTAGE GENERATOR
Many single supply circuits are configured with the circuit-biased
to one-half of the supply voltage. In these cases, a false-ground
reference can be created by using a voltage divider buffered by an
amplifier. Figure 27 shows the schematic for such a circuit.
The two 1 MΩ resistors generate the reference voltages while
drawing only 0.9 µA of current from a 1.8 V supply. A capacitor
connected from the inverting terminal to the output of the op
amp provides compensation to allow for a bypass capacitor to be
connected at the reference output. This bypass capacitor helps
establish an ac ground for the reference output.
AD8517
10k⍀
0.022F
VREF
0.9V TO 2.5V
1F
1F1M⍀
1.8V TO 5V
100⍀
1M⍀
Figure 27. A Micropower Reference Voltage Generator
MICROPHONE PREAMPLIFIER
The AD8517 is ideal to use as a microphone preamplifier.
Figure 28 shows this implementation.
R3
220k⍀
V
CC
V
REF
C1
0.1F
V
CC
R1
2.2k⍀
ELECTRET
MIC
V
OUT
AD8517
V
IN
R2
22k⍀
|
A
V
|
= R3
R2
Figure 28. A Microphone Preamplifier
R1 is used to bias an electret microphone and C1 blocks dc
voltage from the amplifier. The magnitude of the gain of the
amplifier is approximately R3/R2 when R2 ≥ 10 × R1. V
REF
should be equal to 1/2 1.8 V for maximum voltage swing.
Direct Access Arrangement for Telephone Line Interface
Figure 28 illustrates a 1.8 V transmit/receive telephone line
interface for 600 Ω transmission 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 ad-
justed 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. Amplifier A3 is configured as a
difference amplifier for two reasons: (1) It prevents the transmit
signal from interfering with the receive signal and (2) it extracts
the receive signal from the transmission line for amplification by
A4. A4’s gain can be adjusted in the same manner as A1’s to
meet the modem’s input signal requirements. Standard resistor
values permit the use of SIP (Single In-line Package) format
resistor arrays. Couple this with the AD8517/AD8527’s 5-lead
SOT-23, 8-lead MSOP, and 8-lead SOIC footprint and this
circuit offers a compact solution.
6.2V
6.2V
TRANSMIT
TxA
RECEIVE
RxA
C1
0.1F
R1
10k⍀
R2
9.09k⍀
2k⍀
P1
Tx GAIN
ADJUST
A1
R3
360⍀
1:1
T1
TO
TELEPHONE
LINE 1
2
3
7
6
5
2
31
6
5
7
10F
R7
10k⍀
R8
10k⍀
R5
10k⍀
R6
10k⍀
R9
10k⍀
R14
14.3k⍀
R10
10k⍀
R11
10k⍀
R12
10k⍀
R13
10k⍀
C2
0.1F
P2
Rx GAIN
ADJUST
2k⍀
ZO
600⍀
+1.8V DC
MIDCOM
671-8005 A2
A3
A4
1/2
AD8517
1/2
AD8517
1/2
AD8527
1/2
AD8527
Figure 29. A Single-Supply Direct Access Arrangement
for Modems