Datasheet

AD5263
Rev. A | Page 23 of 28
APPLICATIONS
BIPOLAR DC OR AC OPERATION FROM DUAL
SUPPLIES
The AD5263 can be operated from dual supplies, enabling
control of ground referenced ac signals or bipolar operation.
The ac signal, as high as V
DD
/V
SS
, can be applied directly across
Terminal A to Terminal B, with the output taken from Terminal W.
03142-056
GND
SCLK
V
DD
AD5263
µC
GND
MCSI
SDA
SCL
V
DD
V
SS
+5.0
V
±5V p-p
±2.5V p-p
_
5.0V
D = 0x90
A1
W1
B1
A2
W2
B2
Figure 57. 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 58.
R1
47kΩ
25pF
C1
VI
B
A
R2
V
O
U1
W
200kΩ
C2
4.7pF
03142-057
Figure 58. Typical Noninverting Gain Amplifier
Notice the RDAC B terminal parasitic capacitance is connected
to the op amp noninverting node. It introduces a zero for the
1/β
o
term with +20 dB/dec, whereas 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.
Thus, 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 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
may use the relationship described 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, it avoids the gain peaking, ringing, or
oscillation in the worst case. For critical applications, C2 should
be found empirically to suit the need. In general, C2 in the
range of a few 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 disregarded in
most cases.
PROGRAMMABLE VOLTAGE REFERENCE
For voltage divider mode operation (Figure 59), it is common to
buffer the output of the digital potentiometer unless the load is
much larger than R
WB
. Not only does the buffer serve the
purpose of impedance conversion, but it also allows a heavier
load to be driven.
03142-058
U1
VIN
W
B
A
GND
V
O
AD8601
1
A1
5
V
VOUT
3
5V
AD5263
AD1582
Figure 59. Programmable Voltage Reference