Setting Up The Evaluation Board; Input Configuration; Operation Mode Configuration; Output Configuration - Analog Devices SSM2317-EVALZ User Manual

Evaluation Board User Guide

SETTING UP THE EVALUATION BOARD

INPUT CONFIGURATION

A 4-pin header (J1) on the middle left side of the board feeds the
audio signal into the board (see Figure 1). If the input audio signal
is differential (IN+ and IN−), three pins of J1 are used for IN+,
IN−, and signal ground. For a single-ended audio input, only two
pins of J1 are used. One is for the signal ground and the other is
for either IN+ or IN−. If IN+ is used, place a jumper between
Pin 3 and Pin 4 of J1, shorting IN− to ground. If IN− is used,
place the jumper between Pin 1 and Pin 2 of J1, connecting IN+
to ground.

OPERATION MODE CONFIGURATION

The 6-pin header, J2, is used to turn on/off the SSM2317 amp-
lifier and configure the ALC operation modes. Placing a jumper
across Pin 1 and Pin 2 of J2 shuts down the
a minimum current (about 20 nA) is drawn from the power supply
(when R3 is shorted). Removing the jumper puts the SSM2317 in
normal operation. Placing a jumper across Pin 3 and Pin 4 of J2
disables ALC mode, while removing the jumper activates ALC
mode. When ALC is disabled, the SSM2317 behaves like a tradi-
tional amplifier with a fixed 18 dB. Placing a jumper across Pin 5
and Pin 6 of J2 shorts across the ALC threshold resistor (on
board) and sets a maximum limiter level of 90% of V
ALC is activated).

OUTPUT CONFIGURATION

The output connector, J4, is located on the right side of the board
(see Figure 1). J4 drives a loudspeaker whose impedance should
be no less than 3 Ω.
Although the SSM2317 does not require any external LC output
filters due to a low noise modulation scheme, if the speaker length
is >10 cm, it is recommended to put a ferrite bead (L1 and L2) near
each output pin of the SSM2317 to reduce electromagnetic interfer-
ence (EMI), as shown in the schematic in Figure 3. Some users may
want to replace the ferrite beads with these inductors to evaluate
applications with specific EMI vs. audio performance constraints. As
an aid, a properly tuned ferrite bead-based EMI filter is assembled at
the output terminals of the device.
For optimal performance, as specified in the SSM2317 data sheet
(in particular, for THD and SNR), remove the entire EMI filter,
short across the ferrite bead terminals, and open the capacitor
terminals.

POWER SUPPLY CONFIGURATION

The evaluation board schematic is shown in Figure 3. The 2-pin
header, J3, must be used to power the board. Care must be taken
to connect the dc power with correct polarity and voltage. The
positive voltage terminal of J3 (VDD) is indicated with an arrow
in Figure 1.
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Polarity and Voltage
The wrong power supply polarity or an overvoltage may damage
the board permanently. The maximum peak current is approxi-
mately 0.33 A when driving an 8 Ω load and when the input
voltage is 5 V.

COMPONENT SELECTION

Selecting the proper components is the key to achieving the
performance required at the cost budgeted.
ALC Threshold Setting Resistor—R5
When ALC mode is active, the maximum output amplitude
threshold (V ) during the limiting operation can be adjusted
TH
from 90% to 45% of V by inserting an external resistor, R
DD
between the VTH pin and GND. Shorting the VTH pin to GND
sets V to 90% of V . Leaving the VTH pin unconnected sets
TH DD
V to 45% of V . The relation of R
TH DD
following equation:
+
50 kΩ R
= ×
TH
V 0 9 .
+ ×
TH
50 kΩ 2 R
TH
Maximum output power is derived from V
equation:
2
⎛ ⎞
V
⎜ ⎟
TH
⎝ ⎠
2
=
P
OUT
R
SP
where R is the speaker impedance.
SP
To tune a variety of V levels, a potentiometer, R5, is mounted on
TH
the evaluation board. To measure the potentiometer resistance set-
ting, insert an ohmmeter across Pin 5 and Pin 6 of J2.
Note that measuring the resistance across the potentiometer is not
adequate to determine the actual R
internal input resistance of 50 kΩ at the VTH pin. The user must
take into account the internal resistance while evaluating actual
R . For example, after tuning R5 to a desirable level, the user
TH
measures the resistance from Pin 5 and Pin 6 of J2. Then, to infer
the actual R , use this simple calculation:
TH
1 1 1
= −
R R 50
Threshold Measured
where:
R is the desired external resistor value from the VTH pin
Threshold
to GND.
R is the measured resistance from the VTH pin to GND.
Measured
UG-192
,
TH
to V is shown by the
TH TH
×
V
DD
by the following
TH
value. This is due to the
TH
kΩ