Datasheet

INPUT CIRCUIT CONSIDERATIONS
(3)
6.81kW
0.25W
(4) (4)
(4) (4)
(3)
10 F 47m m- F
63WV
+
+
10 F 47m - mF
63WV
1
MicInput
2
3
1000pF
1000pF
1000pF
6.81kW
0.25W
Phantom
Power
Switch
+48V
(1)
(2)
(2)
VA+
VA-
V +
IN
V -
IN
NOTES:
PGA2505
www.ti.com ......................................................................................................................................................... SBOS396B – MARCH 2009 – REVISED JUNE 2009
The blocking capacitors, along with the PGA2505
input resistance, form a high-pass filter circuit. With
For proper operation, the input circuit for the
the typical input resistance of the PGA2505 specified
PGA2505 must include several items that are
in the Electrical Characteristics table, the value of the
common to most microphone preamplifiers. Figure 14
capacitor can be chosen to meet the desired low
shows a typical input circuit configuration. Other
frequency response for the end application. At the
functions, such as input attenuation (pads), filters,
same time, the value should be no greater than
and polarity reversal switches are commonly found in
required, because larger capacitors store more
preamplifier circuits, but are not shown here in order
charge and increase the surge current seen at the
to focus on the basic input circuit requirements.
preamplifier when a short circuit occurs on the
microphone input connector.
The microphone input is typically taken from a
balanced XLR or TRS input connection (XLR shown).
To protect the PGA2505 from large surge currents,
Three 1000pF capacitors provide simple EMI filtering
power Schottky diodes are placed on the input pins to
for the circuit. Additional filtering for low- or
both the VA+ and VA – power supplies. Schottky
high-frequency noise may be added, depending on
diodes are used because of the lower turn-on voltage
the end application environment. A bridging resistor is
compared to standard rectifier diodes. Power devices
shown and may be selected to provide the desired
are required because the surge currents from a large
overall input impedance required for a given
valued blocking capacitor (47 µ F) can exceed 4.5A for
microphone. This resistance is in parallel with the
a very short duration of time. It is recommended that
phantom power bias resistors and the PGA2505 input
the Schottky diode chosen for this application be
resistance to set the actual impedance seen by the
specified for at least a 10A surge current.
microphone.
The use of a series current-limiting resistor before the
Connections for +48V phantom power, required for
protection diodes aids in handling surge currents,
condenser microphones, are shown in Figure 14 . The
although the resistor adds noise to the circuit. Select
phantom power requires an On/Off switch, because
a current-liming resistor value that is as high as
dynamic microphones do not require phantom power
tolerable for the desired noise performance of the
and may be damaged if power is applied.
preamplifier circuit.
DC-blocking capacitors are required between the
phantom power connections and the PGA2505
inputs. The blocking capacitors are selected to have
a high working voltage rating, with 50V being the
minimum and 63V recommended for long-term
reliability.
(1) Bridging resistor; used to set the impedance seen by the microphone.
(2) The blocking capacitor value is selected based upon the desired low frequency response.
(3) Current-limiting resistor. Select the highest value tolerable based upon input noise requirements.
(4) Schottky diode; selected for fast turn-on and rated for a minimum of a 10A surge current. Recommended device is the
MBRA120LT3 from ON Semiconductor.
Figure 14. Typical Input Circuit for the PGA2505
Copyright © 2009, Texas Instruments Incorporated 13
Product Folder Link(s): PGA2505