Datasheet

Rev. A | Page 63 of 80 | July 2011
ADSP-BF504/ADSP-BF504F/ADSP-BF506F
Analog Inputs
The ADC has a total of 12 analog inputs. Each on-board ADC
has six analog inputs that can be configured as six single-ended
channels, three pseudo differential channels, or three fully dif-
ferential channels. These may be selected as described in the
Analog Input Selection section.
Single-Ended Mode
The ADC can have a total of 12 single-ended analog input chan-
nels. In applications where the signal source has high
impedance, it is recommended to buffer the analog input
before applying it to the ADC. The analog input range can be
programmed to be either 0 to V
REF
or 0 to 2 × V
REF
.
If the analog input signal to be sampled is bipolar, the internal
reference of the ADC can be used to externally bias up this sig-
nal to make it correctly formatted for the ADC. Figure 68 shows
a typical connection diagram when operating the ADC in sin-
gle-ended mode.
Differential Mode
The ADC can have a total of six differential analog input pairs.
Differential signals have some benefits over single-ended sig-
nals, including noise immunity based on the device’s common-
mode rejection and improvements in distortion performance.
Figure 69 (Differential Input Definition) defines the fully differ-
ential analog input of the ADC.
The amplitude of the differential signal is the difference between
the signals applied to the V
IN+
and V
IN–
pins in each differential
pair (V
IN+
V
IN–
). V
IN+
and V
IN–
should be simultaneously driven
by two signals each of amplitude V
REF
(or 2 × V
REF
, depending
on the range chosen) that are 180° out of phase. The amplitude
of the differential signal is, therefore (assuming the 0 to V
REF
range is selected) –V
REF
to +V
REF
peak-to-peak (2 × V
REF
),
regardless of the common mode (CM).
The common mode is the average of the two signals
(V
IN+
+ V
IN–
)/2
and is, therefore, the voltage on which the two inputs are
centered.
This results in the span of each input being CM ± V
REF
/2. This
voltage has to be set up externally and its range varies with the
reference value, V
REF
. As the value of V
REF
increases, the com-
mon-mode range decreases. When driving the inputs with an
amplifier, the actual common-mode range is determined by the
amplifier’s output voltage swing.
Figure 70 (Input Common-Mode Range vs. VREF (0 to VREF
Range, VDD = 5 V)) and Figure 71 (Input Common-Mode
Range vs. VREF (2 × VREF Range, VDD = 5 V)) show how the
common-mode range typically varies with V
REF
for a 5 V power
Figure 66. THD vs. Analog Input Frequency for
Various Source Impedances, Differential Mode
Figure 67. THD vs. Analog Input Frequency for Various Supply Voltages
INPUT FREQUENCY (kHz)
600 700 800 900 10000 200100 400300 500
THD (dB)
–60
–65
–70
–75
–80
–85
–90
F
SAMPLE
= 1.5MSPS
V
DD
= 3V
RANGE = 0V TO V
REF
R
SOURCE
= 300
R
SOURCE
= 0
R
SOURCE
= 10
R
SOURCE
= 47
R
SOURCE
= 100
INPUT FREQUENCY (kHz)
600 700 800 900 10000 200100 400300 500
THD (dB)
–50
–60
–55
–65
–70
–75
–80
–85
–90
V
DD
= 3V
SINGLE-ENDED MODE
V
DD
= 5V
SINGLE-ENDED MODE
V
DD
= 3V
DIFFERENTIAL MODE
V
DD
= 5V
DIFFERENTIAL MODE
F
SAMPLE
= 1.5MSPS/2MSPS
V
DD
= 3V/5V
RANGE = 0 TO V
REF
Figure 68. Single-Ended Mode Connection Diagram
Figure 69. Differential Input Definition
V
IN
0V
+1.25V
–1.25V
V
REF
(D
CAP
A/D
CAP
B)
V
A1
ADC
1
V
B6
R
R
3R
R
0V
+2.5V
0.47μF
1
ADDITIONAL PINS OMITTED FOR CLARITY.
V
IN+
ADC
1
V
IN–
V
REF
p-p
V
REF
p-p
COMMON
MODE
VOLTAGE
1
ADDITIONAL PINS OMITTED FOR CLARITY.