AD5280BRU200 查看數據表（PDF） - Analog Devices
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AD5280BRU200 Datasheet PDF : 20 Pages
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AD5280/AD5282
Gain Control Compensation
The digital potentiometer is commonly used in gain control
such as the noninverting gain amplifier shown in Figure 14.
B
47k⍀
R1 C1
25pF
Vi
200k⍀ A
W
C2
4.7pF
U1
VO
Figure 14. 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/
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 com-
pensation 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 above and scale C2 as if R2 is at its maximum
value. Doing so may overcompensate and compromise the per-
formance slightly when R2 is set at low values. However, it will
avoid the gain peaking, 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 pF to no more
than a few tenths of pF is usually adequate for the compensa-
tion.
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, Figure 15, it is common
to buffer the output of the digital potentiometer unless the load
is much larger than RWB. Not only does the buffer serve the
purpose of impedance conversion, it also allows a heavier load
to be driven.
5V
1 U1
VIN
AD5280 5V
VOUT 3
A
W
V+
GND
2 AD1582 B
AD8601
VO
V–
A1
Figure 15. Programmable Voltage Reference
8-Bit Bipolar DAC
Figure 16 shows a low cost, 8-bit, bipolar DAC. It offers the same
number of adjustable steps but not the precision as compared to
the conventional DACs. The linearity and temperature coeffi-
cients, especially at low value codes, are skewed by the effects of the
digital potentiometer wiper resistance. The output of this circuit is:
VO
=
2D
256
– 1
×VREF
(5)
+15V
Vi
U1
VIN
U2
W
B
A
OP2177
VO
A2
VOUT
TRIM
GND
R
+5VREF
R
+15V
–15V
؊5VREF
ADR425
OP2177
A1
–15V
U2 = AD5280
Figure 16. 8-Bit Bipolar DAC
Bipolar Programmable Gain Amplifier
For applications that require bipolar gain, Figure 17 shows one
implementation similar to the previous circuit. The digital po-
tentiometer, U1, sets the adjustment range. The wiper voltage at
W2 can therefore be programmed between Vi and –KVi at a
given U2 setting. Configuring A2 in the Noninverting Mode
allows linear gain and attentuation. The transfer function is:
VO
Vi
=
1
+
R2
R1
×
D2
256
× (1 +
K
)–
K
(6)
where K is the ratio of RWB1/RWA1 set by U1.
VDD
U2
W2
AD5282 A2 B2
Vi
A1 B1
W1 VDD
U1
AD5282
V+
OP2177
V–
V+
OP2177
V–
C1
A2 VSS
–kVi
VO
R2
R1
A1 VSS
Figure 17. Bipolar Programmable Gain Amplifier
Similar to the previous example, in the simpler (and much more
usual) case, where K = 1, a single digital potentiometer AD5280
is used and U1 is replaced by a matched pair of resistors to
apply Vi and –Vi at the ends of the digital potentiometer. The
relationship becomes:
VO
=
1
+
R2 2D2
R1 256
– 1
×Vi
(7)
If R2 is large, a few pF compensation capacitor may be needed
to avoid any gain peaking.
REV. 0
–15–
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