AD8519/AD8529
Rev. D | Page 13 of 16
10× MICROPHONE PREAMP MEETS PC99
SPECIFICATIONS
This circuit, while lacking a unique topology, is anything but
featureless when an AD8519 is used as the op amp. This preamp
gives 20 dB gain over a frequency range of 20 Hz to 20 kHz and
is fully PC99 compliant in all parameters including THD + N,
dynamic range, frequency range, amplitude range, and crosstalk.
Not only does this preamp comply with the PC99 specifications,
it far surpasses them. In fact, when the input noise is 120 dB,
this preamp has a V
OUT
noise of around 100 dB, which is suit-
able for most professional 20-bit audio systems. At 120 dB THD
+ N in unity gain, the AD8519 is suitable for 24-bit professional
audio systems. In other words, the AD8519 will not be the
limiting performance factor in audio systems despite its small
size and low cost.
Slew rate related distortion is not present at the lower voltages
because the AD8519 is so fast at 2.1 V/μs. A general rule of
thumb for determining the necessary slew rate for an audio
system is to take the maximum output voltage range of the
device, given the design’s power rails, and divide by two. In
Figure 24, the power rails are 2.7 V and the output is rail-to-rail.
Enter these numbers into the equation: 2.7/2 = 1.35 V, and the
minimum ideal slew rate is 1.35 V/μs.
While this data sheet gives only one audio example, many audio
circuits are enhanced with the use of the AD8519. Examples
include: active audio filters such as bass, treble, and equalizers;
PWM filters at the output of audio DACs; buffers and summers
for mixing stations; and gain stages for volume control.
1kΩ
AD8519
2.7V
30.9kΩ
2.7V
3.09kΩ
46.4kΩ 93.1kΩ
2.7V
240p
48kΩ
CODEC LINE IN
OR MIC IN
10µF ELECT
MIC
IN
1nF
NPO
C1
1µF
01756-024
Figure 24. 10× Microphone Preamplifier
TWO-ELEMENT VARYING BRIDGE AMPLIFIER
There are a host of bridge configurations available to designers.
For a complete analysis, look at the ubiquitous bridge and its
different forms. Refer to the 1992
Amplifier Applications Guide
1
.
Figure 25 is a schematic of a two-element varying bridge. This
configuration is commonly found in pressure and flow transduc-
ers. With a two-element varying bridge, the signal is 2× as
compared to a single-element varying bridge. The advantages
of this type of bridge are gain setting range and single-supply
application. Negative characteristics are nonlinear operation
and required R matching. Given these sets of conditions,
requirements, and characteristics, the AD8519 can be successfully
used in this configuration because of its rail-to-rail output and
low offset. Perhaps the greatest benefits of the AD8519, when
used in the bridge configuration, are the advantages it can bring
when placed in a remote bridge sensor. For example, the tiny
SC70 and SOT-23 packages reduce the overall sensor size; low
power allows for remote powering via batteries or solar cells,
high output current drive to drive a long cable; and 2.7 V
operation for two-cell operation.
R
R R
R
AD8519
2.7
R
F
R
F
1756-025
Figure 25. Two-Element Varying Bridge Amplifier
1
Adolfo Garcia and James Wong, Chapter 2, 1992, Amplifier Applications Guide.
