Datasheet

Audio PICtail

™

Plus Daughter Board User’s Guide

2.2.1 Line/Microphone Input Phone Jack (J1)

The Line/Microphone Input (Ref 4) is a 3.5 mm mono input phone jack (MJ3502). This

connection accepts either a condenser microphone or a line level signal (see

schematic in Figure A-2).

2.2.2 Line/Microphone Input Select Jumper (J8)

The Line/Microphone Input Select Jumper (Ref 13) determines if the Microphone/Line

Pre-Amplifier (U4:A) operates as a line amplifier or a microphone amplifier. If the MIC

option is selected, a bias voltage of +5V is applied to the Line/Microphone Input Phone

Jack (J1), as shown in the schematic in Figure A-2.

2.2.3 Line/Microphone Pre-Amplifier (U4:A)

The Microphone/Line Pre-Amplifier (Ref 9) is implemented using one of the four

op amps on the MCP6024 quad op amp IC (U4). The output of this non-inverting AC

amplifier is biased at 1.65V. The gain of the amplifier is controlled by Potentiometer

R29, as given by Equation 2-1.

EQUATION 2-1: INPUT PRE-AMPLIFIER GAIN

See Figure A-2 for the complete schematic of the Line/Microphone Pre-Amplifier.

2.2.4 Anti-Aliasing Low-Pass Filter (U4:C)

The Anti-Aliasing Low-Pass Filter uses one of the four operational amplifiers on the

MCP6024 quad op amp IC (U4). The output of the Line/Microphone Pre-Amplifier

(Ref 22) uses an anti-aliasing low-pass second-order Sallen-Key structure to filter the

signal and provide a cut-off frequency of 3300 Hz.

See Figure A-3 for a schematic of the Anti-Aliasing Low-Pass Filter.

2.2.5 Microphone Gain Control (R29)

MIC ADJ Potentiometer R29 (Ref 12) controls the gain of the Line/Microphone

Pre-Amplifier (U4:A).

See Figure A-2 for the complete schematic of the Line/Microphone Pre-Amplifier.

2.2.6 ADC Channel Select Jumper (J5)

The ADC Channel Select Jumper (Ref 3) is used to select the analog-to-digital input

channel (AN0 or AN3) on the dsPIC33F or PIC24H device on the Explorer 16

Development Board.

To use channel AN0, place the jumper between pins 2 and 3 on the ADC CH SEL

header (see Figure A-2). To use channel AN3, place the jumper between pins 1 and 2.

Gain 1

R29 R17+()

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Note: Setting the gain too high can cause the output of the amplifier to saturate

and clip.