Datasheet
Protection Network
3-4
3.5 Protection Network
Resistors R
7
and R
8
, along with Schottky diodes D6 through D9, provide input
protection for the PGA2500 preamplifier when using phantom power, or when
the input voltage exceeds the VA+ or VA− power supplies by more than 350mV
(the approximate turn-on voltage of the Schottky diodes).
A common fault condition is for either the hot (+) or cold (−) input of the pream-
plifier to be shorted to ground. With phantom voltage applied, this will cause
the blocking capacitors to discharge, with a large surge current presented at
the PGA2500 input pins. Without the protection network, the PGA2500 would
be permanently damaged by the surge current, which can reach several am-
peres in peak magnitude. The Schottky diodes are forced into conduction dur-
ing this fault condition, steering most of the charge away from the PGA2500
device and towards the power supplies. The series resistors can be set to a
value that will help limit the input current, although care must taken to avoid
adding too much resistance, since the added noise can degrade the overall
performance of the preamplifier.
The Schottky diodes add a nonlinear capacitance to the input circuit, which
can result in additional distortion. However, with the relatively small input volt-
age swing present when the preamplifier is set to gains between 10dB and
65dB, the effect on the THD+N of the PGA2500 is small or negligible. For uni-
ty-gain applications, where the voltage swing may become large enough in
magnitude to transition over a greater portion of the diodes nonlinear capaci-
tance, the THD+N ratio may degrade by as much as 3dB from the published
typical performance specifications.
3.6 Configurable Input Circuitry
The configurable portion of the input circuit includes R
5
, R
6
, R
10
, R
11
, R
14
, R
15
,
C
C
1, C
C
2, and jumper JMP2. These components support prototyping of addi-
tional circuitry, such as pads and filters. During assembly at the factory, resis-
tors R
5
and R
6
are not installed, while R
10
, R
11
, R
14
, R
15
, C
C
1, and C
C
2 are
replaced by wire shunts.
3.7 Configurable Output Circuitry and Preamp Output Connector
The configurable portion of the output circuit includes R
1
, R
2
, R
3
, R
12
, R
13
,
C
23
, C
24
, and C
25
. These components support prototyping of additional cir-
cuitry, such as pads and filters, as well as emulation of various loading condi-
tions. During assembly at the factory, resistors R
12
and R
13
are replaced by
wire shunts. Resistors R
1
through R
3
, as well as capacitors C
23
through C
25
,
are not installed.
The differential preamplifier output is provided at connector J3, which is a 3-pin
male XLR connector. Figure 3−3 illustrates the pin connections for connector
J3.
Figure 3−3. Preamp Output Connector Configuration
1
2
3
Hot (+), to V
OUT
+
Cold (
−
), to V
OUT
−
Shield, to GND