Datasheet

Rev. A | Page 64 of 80 | July 2011
ADSP-BF504/ADSP-BF504F/ADSP-BF506F
supply using the 0 to V
REF
range or 2 × V
REF
range, respectively.
The common mode must be in this range to guarantee the func-
tionality of the ADC.
When a conversion takes place, the common mode is rejected,
resulting in a virtually noise free signal of amplitude –V
REF
to
+V
REF
corresponding to the digital codes of 0 to 4096. If the 2 ×
V
REF
range is used, then the input signal amplitude extends from
– 2 V
REF
to +2 V
REF
after conversion.
Driving Differential Inputs
Differential operation requires that V
IN+
and V
IN–
be simultane-
ously driven with two equal signals that are 180° out of phase.
The common mode must be set up externally. The common-
mode range is determined by V
REF
, the power supply, and the
particular amplifier used to drive the analog inputs. Differential
modes of operation with either an ac or dc input provide the
best THD performance over a wide frequency range. Because
not all applications have a signal preconditioned for differential
operation, there is often a need to perform single-ended-to-dif-
ferential conversion.
Using an Op Amp Pair
An op amp pair can be used to directly couple a differential sig-
nal to one of the analog input pairs of the ADC. The circuit
configurations illustrated in Figure 72 (Dual Op Amp Circuit to
Convert a Single-Ended Unipolar Signal Into a Differential Sig-
nal) and Figure 73 (Dual Op Amp Circuit to Convert a Single-
Ended Bipolar Signal into a Differential Unipolar Signal) show
how a dual op amp can be used to convert a single-ended signal
into a differential signal for both a bipolar and unipolar input
signal, respectively.
The voltage applied to Point A sets up the common-mode volt-
age. In both diagrams, it is connected in some way to the
reference, but any value in the common-mode range can be
input here to set up the common mode. The AD8022 is a suit-
able dual op amp that can be used in this configuration to
provide differential drive to the ADC.
Take care when choosing the op amp; the selection depends on
the required power supply and system performance objectives.
The driver circuits in Figure 72 (Dual Op Amp Circuit to Con-
vert a Single-Ended Unipolar Signal Into a Differential Signal)
and Figure 73 (Dual Op Amp Circuit to Convert a Single-Ended
Bipolar Signal into a Differential Unipolar Signal) are optimized
for dc coupling applications requiring best distortion
performance.
The circuit configuration shown in Figure 72 (Dual Op Amp
Circuit to Convert a Single-Ended Unipolar Signal Into a Differ-
ential Signal) converts a unipolar, single-ended signal into a
differential signal.
The differential op amp driver circuit shown in Figure 73 (Dual
Op Amp Circuit to Convert a Single-Ended Bipolar Signal into a
Differential Unipolar Signal) is configured to convert and level
shift a single-ended, ground-referenced (bipolar) signal to a dif-
ferential signal centered at the V
REF
level of the ADC.
Pseudo Differential Mode
The ADC can have a total of six pseudo differential pairs. In this
mode, V
IN+
is connected to the signal source that must have an
amplitude of V
REF
(or 2 × V
REF
, depending on the range chosen)
Figure 70. Input Common-Mode Range vs. V
REF
(0 to V
REF
Range, V
DD
= 5 V)
Figure 71. Input Common-Mode Range vs. V
REF
(2 × V
REF
Range, V
DD
= 5 V)
V
REF
(V)
5.00 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
COMMON-MODE RANGE (V)
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
T
A
= 25°C
V
REF
(V)
2.50 0.5 1.0 1.5 2.0
COMMON-MODE RANGE (V)
5.0
4.0
4.5
3.0
3.5
2.0
2.5
0.5
1.0
1.5
0
T
A
= 25°C
Figure 72. Dual Op Amp Circuit to Convert a Single-Ended Unipolar Signal
Into a Differential Signal
GND
2 × V
REF
p–p
27
27
V+
V–
V+
V–
V
REF
2.5V
3.75V
1.25V
2.5V
3.75V
1.25V
V
REF
(D
CAP
A/D
CAP
B)
V
IN+
ADC
1
V
IN–
440
220
0.47μF
1
ADDITIONAL PINS OMITTED FOR CLARITY.
220
220
10k
A