Datasheet

OPERATION WITH V
COM
IN = +2.5V
R
R
2R C
PGA
PGA2505
C
C1
C
C2
V +
OUT
V -
OUT
V IN
COM
ADC
A/DConverter
(1)
+
+
SerialDataOutput
PCMorDSD
Coupling
Capacitors
Attenuationand
AntialiasingFilter
PGA2505
SBOS396B – MARCH 2009 – REVISED JUNE 2009 ......................................................................................................................................................... www.ti.com
As a suggested alternative, the PGA2505 analog
outputs may be ac-coupled to the ADC inputs,
When interfacing the analog outputs of the PGA2505
allowing the PGA2505 to operate with V
COM
IN = 0V in
with audio ADC inputs, the converter may frequently
order to achieve best performance. The ac-coupling
have a common-mode dc output pin. This pin may be
capacitors affect the overall low-frequency response
connected to the V
COM
IN pin of the PGA2505 in order
of the preamplifier and converter combination, and
to facilitate a dc-coupled interface between the two
the user is advised to choose a value that best suits
devices. The common-mode dc voltage level is
the application requirements.
typically +2.5V, although some converters may have
a slightly lower value, usually between +2.1V and Figure 15 illustrates a typical PGA2505 to audio ADC
+2.5V. There are several issues that must be interface using ac-coupling. In addition to the coupling
considered when operating the PGA2505 in this capacitors, a passive RC filter is required as an
fashion. antialiasing filter for the converter. The vast majority
of audio ADCs are of the oversampling delta-sigma
Both the analog input and output pins of the
variety, with a simple single-pole filter meeting the
PGA2505 are level-shifted by the V
COM
IN voltage.
anti-aliasing requirements for this type of converter.
The analog outputs are shifted to the V
COM
IN level,
Providing at least 6dB of attenuation also allows the
while the analog inputs are shifted to approximately
PGA2505 to operate near full signal swing without
V
COM
IN – 0.65V, as a result of the offset that normally
overdriving the ADC inputs.
exists on the input pins. The level-shifting limits the
input and output swing of the PGA2505, reducing the Figure 16 illustrates an application where the V
COM
IN
overall signal-to-noise ratio and degrading the pin of the PGA2505 is connected to the
THD+N performance. common-mode dc output of the audio ADC, with a
dc-coupled interface between the PGA2505 analog
Given V
COM
IN = +2.5V and gains of 0dB through
outputs and the ADC analog inputs.
60dB, the output swing is limited to less than one-half
that specified in the Electrical Characteristics table. To ensure optimal performance, an output buffer to
The output hard-clips at approximately a diode drop the PGA2505 is recommended. Figure 17 illustrates
below the VA+ supply rail and a diode drop above the use of an OPA1632 as the buffer. Additionally,
analog ground. the feedback circuitry functions as the antialiasing
filter shown in Figure 15 and Figure 16 . Having a
Given V
COM
IN = +2.5V and a gain of 0dB, the
differential buffer with attenuation of 6dB or greater
practical maximum input or output voltage swing is
also allows for the PGA2505 to maximize the output
approximately 1.0V
RMS
differential. Increasing the
signal swing, while ensuring that the input swing does
signal level much beyond this point results in a
not exceed the full-scale input range of the ADC. An
substantial increase in distortion.
OPA227 is used to drive the output common-mode of
the OPA1632.
Plots of THD+N vs Frequency are shown in the
Typical Characteristics section of this data sheet for
both V
COM
IN = 0V and +2.5V. The performance
difference can be seen when comparing the plots.
The user must consider whether the difference is
acceptable for the end application.
(1) Recommended devices are the PCM1804 , PCM4202 , PCM4204 , PCM4220 , or PCM4222 .
Figure 15. PGA2505 Analog Output to ADC Analog Input Interface, AC-Coupled
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