4.2 Microphone Input
Dynamic and condenser microphones or audio test equipment are connected to the PGA2500EVMV2 input
through combo connector J3. The combo connector combines both a 3-pin female XLR and a 1/4-inch TRS jack
for connecting to microphones and test signal sources. Both the XLR and TRS jacks are wired for a balanced
input.
illustrates the combo connector pin configuration for the PGA2500EVMV2.
Figure 4-2. Microphone Input Connector Configuration
4.3 Phantom Power Connections
The PGA2500EVMV2 supports connection of a phantom power source across the inputs of the preamplifier
using terminal block J1. The voltage source is connected to the hot (+) and cold (−) sides of the preamplifier
input through 6.81-kΩ resistors. Phantom power may be operated at voltages up to +50 V DC.
Phantom power is required for condenser microphones. When using a condenser microphone requiring a
phantom power source, short terminals 1 and 2 of jumper JMP1, and leave terminals 3 and 4 open. It is
important to note that when using a phantom power source, connect the microphone and cable before turning on
phantom power. In addition, phantom power should be turned off before disconnecting the microphone or cable.
Making or breaking the input connection with +48 V may cause large transient spikes which could damage the
PGA2500 or anything connected to the output. When using a dynamic microphone, terminals 3 and 4 of jumper
J3 should be shorted, while terminals 1 and 2 remain open.
4.4 DC Blocking Capacitors
Capacitors C4 and C9 are utilized as DC blocking capacitors. They provide AC-coupling to the microphone input,
as well as blocking the phantom voltage from reaching the PGA2500 input terminals when using a condenser
microphone. The blocking capacitors are selected to not degrade the dynamic performance of the PGA2500.
The capacitors shown in the
) are installed by default at the factory. If using an
alternative capacitor, use components rated for 50 V minimum, with 63 V or higher recommended for long-term
reliability.
4.5 Protection Network
Resistors R4 and R14, along with Schottky diodes D5, D6, D7 and 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 350 mV (the approximate turn-on voltage of the Schottky diodes).
A common fault condition is for either the hot (+) or cold (−) input of the preamplifier to be shorted to ground.
With phantom voltage applied, this causes 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 amperes in peak magnitude.
The Schottky diodes are forced into conduction during 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 voltage
swing present when the preamplifier is set to gains between 10 dB and 65 dB, the effect on the THD+N of the
PGA2500 is small or negligible. For unity-gain applications, where the voltage swing may become large enough
in magnitude to transition over a greater portion of the diodes nonlinear capacitance, the THD+N ratio may
degrade by as much as 3 dB from the published typical performance specifications.
Setup Guide
SBOU257 – JUNE 2021
PGA2500EVMV2 Evaluation Module
9
Copyright © 2021 Texas Instruments Incorporated
