EUA2005
Input Resistors (R
I
)
The input resistors (R
I
) set the gain of the amplifier
according to equation (1).
2
×
150k
Ω
V
---------------------------------(1)
V
Gain
=
R
I
Resistor matching is very important in fully
differential amplifiers.
The balance of the output on the
reference voltage depends on matched ratios of the
resistors. CMRR, PSRR, and cancellation of the second
harmonic distortion diminish if resistor mismatch occurs.
Therefore, it is recommended to use 1% tolerance
resistors or better to keep the performance optimized.
Matching is more important than overall tolerance.
Resistor arrays with 1% matching can be used with a
tolerance greater than 1%.
Place the input resistors very close to the EUA2005 to
limit noise injection on the high-impedance nodes.
For optimal performance the gain should be set to 2 V/V
or lower. Lower gain allows the EUA2005 to operate at its
best, and keeps a high voltage at the input making the
inputs less susceptible to noise.
Decoupling Capacitor (C
S
)
Input Capacitors (C
I
)
The EUA2005 does not require input coupling capacitors
if the design uses a differential source that is biased from
0.5 V to V
DD
– 0.8 V (shown in Figure 23). If the input
signal is not biased within the recommended common
−mode
input range, if needing to use the input as a high
pass filter (shown in Figure 24), or if using a single-ended
source (shown in Figure 25), input coupling capacitors are
required.
The input capacitors and input resistors form a high-pass
filter with the corner frequency, f
C
, determined in equation
(2).
fc
=
1
(
2
π
R
I
C
I
)
--------------------------------------------(2)
The value of the input capacitor is important to consider
as it directly affects the bass (low frequency) performance
of the circuit. Speakers in wireless phones cannot usually
respond well to low frequencies, so the corner frequency
can be set to block low frequencies in this application.
Equation (3) is reconfigured to solve for the input
coupling capacitance.
1
--------------------------------------------(3)
(
2
π
R
I
f
c
)
If the corner frequency is within the audio band, the
capacitors should have a tolerance of
±
10% or better,
because any mismatch in capacitance causes an
impedance mismatch at the corner frequency and below.
For a flat low-frequency response, use large input
coupling capacitors (1 µF). However, in a GSM phone the
ground signal is fluctuating at 217 Hz, but the signal from
the codec does not have the same 217 Hz fluctuation. The
difference between the two signals is amplified, sent to
the speaker, and heard as a 217 Hz hum.
CI
=
The EUA2005 is a high-performance class-D audio
amplifier that requires adequate power supply decoupling
to ensure the efficiency is high and total harmonic
distortion (THD) is low. For higher frequency transients,
spikes, or digital hash on the line, a good low
equivalent-series-resistance (ESR) ceramic capacitor,
typically1 µF, placed as close as possible to the device
V
DD
lead works best. Placing this decoupling capacitor
close to the EUA2005 is very important for the efficiency
of the class-D amplifier, because any resistance or
inductance in the trace between the device and the
capacitor can cause a loss in efficiency. For filtering
lower-frequency noise signals, a 10µF or greater capacitor
placed near the audio power amplifier would also help,
but it is not required in most applications because of the
high PSRR of this device.
Summing Input Signals
Most wireless phones or PDAs need to sum signals at the
audio power amplifier or just have two signal sources that
need separate gain. The EUA2005 makes it easy to sum
signals or use separate signal sources with different gains.
Many phones now use the same speaker for the earpiece
and ringer, where the wireless phone would require a
much lower gain for the phone earpiece than for the ringer.
PDAs and phones that have stereo headphones require
summing of the right and left channels to output the stereo
signal to the mono speaker.
DS2005 Ver 1.1 Nov. 2006
11
ETC [ ETC ]
