Datasheet

Hardware

2.2.7 PWM Low-Pass Filter (U3:A,B)

The PWM signal from the Output Compare PWM module on the dsPIC33F or PIC24H

device on the Explorer 16 board is demodulated by the PWM low-pass filter (Ref 6).

This fourth order filter uses two of the four op amps (U3:A and U3:B) on the MCP6024

quad op amp IC.

The input to the filter is selected by the Filter Input Select Jumper (J7). The output of

the filter is routed to the headphone and line output amplifiers via the Output Select

Jumper (J4).

The complete schematic of the low-pass filter, the input select jumper and the output

select jumper is shown in Figure A-4.

2.2.8 Filter Input Select Jumper (J7)

The input to the PWM Low-Pass Filter is selected by the FLT I/P SEL Jumper (Ref 10).

This jumper allows the following inputs to be selected:

• OCPWM channel 1 output (OC1) of the dsPIC33F or PIC24H device on the

Explorer 16 board

• OCPWM channel 2 output (OC2) of the dsPIC33F or PIC24H device on the

Explorer 16 board

• Output of the Line/Microphone Pre-Amplifier (MIC_SIG)

2.2.9 Output Select Jumper (J4)

The Output Select Jumper (Ref 5) determines whether the input signal for the Line

Output and Headphone Amplifiers comes from the PWM Filter, the codec or the

differential amplifier that converts a digital audio signal from the dsPIC33F device.

See Figure A-4 for circuit details.

2.2.10 Line Output Amplifier (U4:B)

The Line Output Amplifier is implemented using one of the four op amps on the

MCP6024 quad op amp IC (Ref 9). The output of this inverting AC amplifier is intended

to drive an external audio amplifier or recording device. The signal is AC-coupled to the

Line Output Jack (J9).

The gain of the amplifier is controlled by the Line Adjust Potentiometer (R24), as given

by Equation 2-2.

EQUATION 2-2: LINE OUTPUT AMPLIFIER GAIN

See Figure A-4 for a complete schematic of the Line Output Amplifier.

2.2.11 Line Output Gain Control Potentiometer (R24)

LINE ADJ potentiometer R24 (Ref 11) controls the gain of the Line Output Amplifier

(U4:B).

See Figure A-4 for a complete schematic of the Line Output Amplifier.

Gain

R24 R19+()

R21

-------------------------------=

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

and clip.