ad1990xcpzrl Analog Devices, Inc., ad1990xcpzrl Datasheet - Page 12

• Manufacturer Part Number: ad1990xcpzrl
• Description: Audio Switching Amplifier
• Manufacturer: Analog Devices, Inc.

AD1990
Note that the amplifier is capable of dc-coupled operation if the
circuit includes some means to account for this bias voltage.
Setting the Modulator Gain
The AD1990 modulator uses a combination of the input signal
and feedback from the power output stage to calculate its two-
state output pattern. The feedback input nodes are part of the
internal analog circuit that operates from the AV
5 V) power supply. Because the voltage measured at the power
outputs is nominally between 0 V and PV
the 0 V to AV
feedback to an appropriate level.
Resistor voltage dividers should sense the voltage on each side
of the differential output and provide these feedback signals to
the modulator, as shown in Figure 23.
The resistor values should satisfy the following equation to
maintain modulator stability.
Selecting a gain that meets this criterion ensures that the
modulator remains in a stable operating condition.
The ratio of the resistances sets the gain rather than the absolute
values. However, the dividers provide a path from the high
voltage supply to ground; therefore, the values should be large
enough to produce negligible loss due to quiescent current.
The chip contains a calibration circuit to minimize voltage
offsets at the speaker, which helps to minimize clicks and pops
when muting or unmuting. Optimal performance is achieved
for the offset calibration circuit when the feedback divider resistors
sum to 6 kΩ, that is, (R1 + R2) = 6 kΩ, and (R3 + R4) = 6 kΩ.
OUTx+
PGND
NFx+
PV
Gain
DD
0V
=
D1
D2
R
DD
1
R
+
range, a voltage divider is required to scale the
2
R
Figure 22. AC-Coupled Input Signal
Figure 23. H-Bridge Configuration
R1
R2
2
EXTERNAL COMPONENTS
=
L
R
C
3
R
+
4
R
4
R
L
=
+
PV
. 3
635
C
DD
L
AINL/
AINR
DD
R3
R4
, and thus beyond
D3
D4
DD
(nominal
PV
DD
NFx–
PGND
OUTx–
Rev. 0 | Page 12 of 16
This fixed total resistance to ground eliminates the last free
variable and gives the following equations for the resistors:
Note that the gain previously mentioned applies to each side of
the differential output pair. Therefore, the total forward gain for
the modulator and output stage is twice that value. Recommended
resistor values for some common supply voltages are shown
in Table 10.
Table 10. Recommended Feedback Resistor Values
PV
8
10
12
Programmable Gain Amplifier (PGA)
The Σ-Δ modulator itself requires a fixed gain for a given value
of PV
but many applications require more gain to account for low
source signal levels. The AD1990 includes a programmable gain
amplifier (PGA) to boost the overall amplifier gain. The total
gain for the amplifier is the product of the modulator gain and
the PGA gain. PGA1 (Pin 31) and PGA0 (Pin 32) select one of
four PGA gain values, as shown in Table 11.
Table 11. PGA Gain Settings
PGA1
0
0
1
1
The AD1990 incorporates a single-ended-to-differential
converter for each channel in the analog front-end section.
The PGA is also part of this analog front-end, and it affects the
analog input signal before it enters the Σ-Δ modulator. The
PGA1 and PGA0 pins are continuously monitored and allow
the gain to be changed at any time.
DD
R1 = R3 = 6000 − R2
R2
(V)
DD
to maintain optimal stability. This gain can be appropriate,
=
R4
R1 (kΩ)
3.27
3.82
4.18
=
21810
PV
PGA0
0
1
0
1
DD
R2 (kΩ)
2.73
2.18
1.82
PGA Gain
1 (0 dB)
2 (6 dB)
4 (12 dB)
8 (18 dB)
Voltage
Divider Gain
2.2
2.8
3.3
Differential
System Gain
4.4 (13.8 dB)
5.6 (17.6 dB)
6.6 (20.8 dB)