EVAL-AD5262EBZ Analog Devices Inc, EVAL-AD5262EBZ Datasheet - Page 19

• Manufacturer Part Number: EVAL-AD5262EBZ
• Description: EVALUATION BOARD
• Manufacturer: Analog Devices Inc

Specifications of EVAL-AD5262EBZ

• Main Purpose: Digital Potentiometer
• Utilized Ic / Part: AD5262
• Primary Attributes: Dual Channel, 256 Positions
• Secondary Attributes: SPI Interface
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Embedded: -
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIONS INFORMATION
BIPOLAR DC OR AC OPERATION FROM DUAL
SUPPLIES
The AD5260/AD5262 can be operated from dual supplies
enabling control of ground referenced ac signals or bipolar
operation. The ac signal, as high as V
directly across Terminal A and Terminal B with output taken
from Terminal W. See Figure 58 for a typical circuit connection.
GAIN CONTROL COMPENSATION
Digital potentiometers are commonly used in gain control as in
the noninverting gain amplifier shown in Figure 59.
Note that when the RDAC B terminal parasitic capacitance is
connected to the op amp noninverting node, it introduces a zero
for the 1/β
gain bandwidth product (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 has 0 phase margin
at the crossover frequency. The output may ring or oscillate if
the input is a rectangular pulse or step function. Similarly, it is
also likely to ring when switching between two gain values
because this is equivalent to a step change at the input.
Depending on the op amp GBP, reducing the feedback resistor
may extend the zero’s frequency far enough to overcome the
problem. A better approach, however, is to include a compensa-
tion 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, the R1 × C1 =
R2 × C2 relationship can be used, and scale C2 as if R2 is at its
maximum value. Doing so may overcompensate and compromise
the performance slightly when R2 is set at low values. However,
MICROCONTROLLER
GND
V
DD
O
Figure 58. Bipolar Operation from Dual Supplies
term with +20 dB/dec, whereas a typical op amp
Figure 59. Typical Noninvertng Gain Amplifier
SCLK
MOSI
SS
47kΩ
R1
25pF
C1
CLK
SDI
GND
CS
V
i
V
V
DD
SS
4.7pF
U1
C2
200kΩ
B
W
R2
DD
A
/V
SS
, can be applied
D = 0x80
V
±2.5V p-p
O
+5.0V
–5.0V
±5V p-p
Rev. A | Page 19 of 24
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 a few picofarads (pF) to no more
than a few tenths of pF is usually adequate for the compensation.
Similarly, there are W and A terminal capacitances connected to
the output (not shown). Fortunately, their effect at this node is less
significant, and the compensation can be avoided in most cases.
PROGRAMMABLE VOLTAGE REFERENCE
For voltage divider mode operation, shown in Figure 60, it is
common to buffer the output of the digital potentiometer unless
the load is much larger than R
the purpose of impedance conversion, but it also allows a
heavier load to be driven.
8-BIT BIPOLAR DAC
Figure 61 shows a low cost 8-bit bipolar DAC. It offers the same
number of adjustable steps but not the precision of conventional
DACs. The linearity and temperature coefficients, especially at low
values codes, are skewed by the effects of the digital potentiometer
wiper resistance. The output of this circuit is
GND
V
V
V
IN
i
O
TRIM
V
=
U1
OUT
ADR425
⎛
⎜
⎝
256
2
Figure 60. Programmable Voltage Reference
1
2
D
AD5260
GND
V
5V
+5V
IN
−
AD1582
REF
1
V
U1
OUT
⎞
× ⎟
⎠
Figure 61. 8-Bit Bipolar DAC
R
V
3
REF
U2
AD5260
W1
B
W
A
B
A
A1
OP2177
R
WB
W
+5V
. Not only does the buffer serve
A1
–5V
AD8601
5V
AD5260/AD5262
–5V
A2
REF
OP2177
V
+5V
–5V
O
V
O
(4)