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| 型號： | EVAL-AD5233SDZ |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 16/32頁 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL FOR AD5233 |
| 標準包裝： | 1 |
| 系列： | * |
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AD5233
Rev. B | Page 23 of 32
APPLICATIONS INFORMATION
BIPOLAR OPERATION FROM DUAL SUPPLIES
The AD5233 can be operated from dual supplies ±2.5 V, which
enables control of ground-referenced ac signals or bipolar opera-
tion. AC signals as high as VDD/VSS can be applied directly
across Terminal A and Terminal B with output taken from
Terminal W. See Figure 46 for a typical circuit connection.
±2.5V p-p
AD5233
VSS
GND
SDI
CLK
SS
SCLK
MOSI
GND
MICRO-
CONVERTER
±1.25V p-p
VDD
+2.5V
–2.5V
CS
D = MIDSCALE
A
W
B
02
79
4
-04
7
Figure 46. Bipolar Operation from Dual Supplies
GAIN CONTROL COMPENSATION
A digital potentiometer is commonly used in gain control such
as the noninverting gain amplifier shown in Figure 47.
U1
VO
R2
100k
Vi
R1
33.2k
C1
35pF
W
BA
C2
10pF
02
79
4-
04
8
Figure 47. Typical Noninverting Gain Amplifier
When RDAC B terminal parasitic capacitance is connected
to the op amp noninverting node, it introduces a 0 for the
1/bO term with 20 dB/dec, while a typical op amp GBP has
20 dB/dec characteristics. A large R2 and finite C1 can cause
this zero’s frequency to fall well below the crossover frequency.
Therefore, the rate of closure becomes 40 dB/dec, and the
system as a 0° phase margin at the crossover frequency. The
output can ring or oscillate if an input is a rectangular pulse or
step function. Similarly, it is also likely to ring when switching
between two gain values; this is equivalent to a stop change at
the input.
Depending on the op amp GBP, reducing the feedback
resistor might extend the zero’s frequency far enough to
overcome the problem. A better approach is to include a
compensation capacitor, C2, to cancel the effect caused by
C1. Optimum compensation occurs when R1 × C1 = R2 ×
C2. This is not an option because of the variation of R2.
As a result, one can use the previous relationship and scale C2
as if R2 were at its maximum value. Doing this might over-
compensate and compromise the performance when R2 is
set at low values. On the other hand, it avoids the ringing or
oscillation at the worst case. For critical applications, C2 should
be found empirically to suit the need. In general, C2 in the
range of picofarads is usually adequate for the compensation.
Similarly, W and A terminal capacitances are connected to the
output (not shown); their effect at this node is less significant
and the compensation can be avoided in most cases.
HIGH VOLTAGE OPERATION
The digital potentiometer can be placed directly in the feedback
or input path of an op amp for gain control, provided that the
voltage across Terminal A and Terminal B, Terminal W and
Terminal A, or Terminal W and Terminal B does not exceed
|5 V|. When high voltage gain is needed, users should set a
fixed gain in an op amp operated at high voltage and let the
digital potentiometer control the adjustable input. Figure 48
shows a simple implementation.
R2R
5V
AD5233
A
W
B
15V
V+
V–
VO
0 TO 15V
A2
–
+
C
0
27
94
-0
49
Figure 48. 5 V Voltage Span Control
Similarly, a compensation capacitor, C, might be needed to
dampen the potential ringing when the digital potentiometer
changes steps. This effect is prominent when stray capacitance
at the inverted node is augmented by a large feedback resistor.
Usually, a capacitor (C) of a few picofarads, is adequate to combat
the problem.
DAC
Figure 49 shows a unipolar 8-bit DAC using the AD5233. The
buffer is needed to drive various loads.
AD5233
V+
V–
AD8601
W
A1
VIN VOUT
GND
AD1582
5V
U1
3
A
B
VO
1
2
02
79
4-
05
0
Figure 49. Unipolar 8-Bit DAC
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