Datasheet

MAX9742
Single-/Dual-Supply, Stereo 16W,
Class D Amplifier with Differential Inputs
______________________________________________________________________________________ 17
to-peak output voltage that causes 10% THD+N for a
given load.
where P
OUT_10%
is the output power that causes 10%
THD+N, R
L
is the load resistance, and V
OUT_P-P
is the
peak-to-peak output voltage. Determine the voltage
gain (A
V
) necessary to attain this output voltage based
on the maximum peak-to-peak input voltage (V
IN_P-P
):
Set the closed-loop voltage gain of the MAX9742 less
than or equal to A
V
to prevent clipping of the output,
unless audible clipping is acceptable for the application.
Input Amplifier
The external feedback networks of the MAX9742 input
amplifiers allow custom gain settings while maximizing
dynamic range. The input amplifiers also accommodate
a variety of standard amplifier configurations including
differential input, single-ended input, and summing
amplifiers. Due to the output current limitations of the
internal input amplifiers, always select feedback resistors
(R
F1
, see the Typical Application Circuits/Functional
Diagrams ) with values greater than or equal to 400kΩ. To
preserve gain accuracy, avoid using feedback resistors
with values greater than 1MΩ. For proper operation, limit
common-mode input voltages to ±3V.
Differential Input Configuration
The Typical Application Circuits/Functional Diagrams
show each channel of the MAX9742 configured as dif-
ferential input amplifiers. A differential input offers
improved noise immunity over a single-ended input. In
systems that include high-speed digital circuitry, high-
frequency noise can couple into the amplifier’s input
traces. The signals appear at the amplifier’s inputs as
common-mode noise. A differential input amplifier
amplifies the difference of the two inputs, and signals
common to both inputs are subtracted out. When con-
figured for differential inputs, the voltage gain of the
MAX9742 is set by:
where A
V
is the desired voltage gain in V/V. R
IN1
should be equal to R
IN2
, and R
F1
should be equal to
R
F2
.
When using the differential input configuration, the
common-mode rejection ratio (CMRR) is primarily limit-
ed by the external resistor tolerances. Ideally, to
achieve the highest possible CMRR, the resistors
should be perfectly matched and the following condi-
tion should be met:
To ensure the MAX9742 input amplifiers operate as
fully differential integrators, connect a capacitor
between IN_+ and MID whose value is equal to C
F
(see
the
Feedback Capacitor (CFB_) section).
Single-Ended Input
Each channel of the MAX9742 can be configured as a
single-ended input amplifier by connecting IN_+ to MID
(through an external resistor, R
OS
) and driving IN_- with
the input source (see Figure 5). In this configuration,
the MAX9742 is configured as a single-ended amplifier
whose voltage gain is equal to:
where A
V
is the desired voltage gain in V/V.
To minimize output offset voltages due to input bias cur-
rents, connect a resistor, R
OS
, (see Figure 5) between
IN_+ and MID. Select the value of R
OS
so that the DC
resistances looking out of inputs of the amplifier (IN_+
and IN_-) are equal. For example, when using the dual-
supply configuration with a DC-coupled input source, the
value of R
OS
should be equal to R
F
||R
IN
.
A
R
R
V
F
IN
(/)= − VV
R
R
R
R
F1
IN1
F2
IN2
=
A
R
R
V
F1
IN1
= ( / )VV
A
V
V
V
OUT_P P
IN_P P
(/)=
−
−
VV
V2 2PR
OUT_P P OUT_10% L−
=×
()
()V
OUT_
V
IN
C
IN
R
IN
R
OS
MAX9742
MID
TO CLASS D
MODULATOR
R
F
IN_-
IN_+
C
FB_
FB_
Figure 5. Single-Ended Input Configuration