6.
Circuit Description
This section describes the individual parts of the circuit and give design information about
the components, to allow you to adapt the circuitry of the codec board for your own
implementation.
6.1
Audio Amplifier
The Winbond codec is capable of driving a 32
Ω
load directly if the gain of the output amplifier
is reduced by a factor of 4. This is done by Setting R1 to 39k
Ω
.
Of the stereo headsets tested, it was found that 32
Ω
was a common impedance for each
earpiece. For a stereo headset where two speakers are being driven in parallel this would be
equivalent to driving a 16
Ω
load. This is out of the codec’s specification so a small
headphone amplifier, U3, has been used on the evaluation board. This is not required if the
impedance of the earpiece is equal or greater than 32
Ω
.
The large 100
μ
F decoupling capacitors have been used so that the codec could be tested in
its “high pass filter mode disabled” configuration. If you do not require a frequency response
to go down below 300 Hz, then these capacitors can be reduced to small values. The main
design consideration is the impedance should not be significant compared to the impedance
of the headphone selected at frequencies of interest.
E.g. if using a 32
Ω
headphone and expecting a 3dB point at 300 Hz, then the decoupling
capacitor impedance could be 32
Ω
at 300Hz i.e. 10
μ
F. This requires a much smaller
footprint than the 100
μ
F used in the reference design.
6.2
Driving the Headset Directly from the Codec
This will achieve the most cost effective design but care must be taken to ensure that the
32
Ω
specification of load is met by selecting an appropriate headset.
Remove R10, R13 and R12. Fit R11, R9, R38 as zero ohm links. Fit 39k
Ω
in place of R1 to
reduce the gain by 4.
In-house testing showed that with a 32
Ω
load and with R1 set to 39k
Ω
, that there was some
distortion at zero cross-over but that it was not easily perceptible.
Even though the output signal level had been reduced by a factor of 4, on the headsets
tested, the volume levels sounded loud enough for most applications. It is important to
check this with the target headset for your application.
6.3
Microphone Circuit
The microphone circuit is designed for an electret microphone (which is commonly used in PC
applications). Typically this would be powered by 5V via a 2.2k
Ω
series resistor. In the
reference design, it is powered by 3.3V to ensure a clean supply regardless of the power
supply used to power the Dev kit. This reduces the sensitivity of the microphone - you
should test your application with the microphone and voltage you intend to use in order to
determine your component values.
The gain of the microphone is set by R22 and R24, with gain being equal to R22/R24. The
current values are 62K and 3.9K, giving a gain of approximately 16. When changing to a
different gain, R27 and R25 should be set to the new values as well. This ensures that the
load seen by common mode noise on the microphone is identical and prevents it from being
amplified.
R31 is a no fit resistor. It’s purpose is to facilitate test modes where a user wants to loop
audio output directly back to the audio input to conduct an over the air audio test.
APN_06002_1v03 Winbond Audio Board Application Note
9