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AD5122 Datasheet(PDF) 28 Page - Analog Devices

Part No. AD5122
Description  Dual Channel, 128-/256-Position, SPI, Nonvolatile Digital Potentiometer
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Manufacturer  AD [Analog Devices]
Direct Link  http://www.analog.com
Logo AD - Analog Devices

AD5122 Datasheet(HTML) 28 Page - Analog Devices

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AD5122/AD5142
Data Sheet
Rev. C | Page 28 of 32
In the bottom scale condition or top scale condition, a finite
total wiper resistance of 40 Ω is present. Regardless of which
setting the device is operating in, limit the current between
Terminal A to Terminal B, Terminal W to Terminal A, and
Terminal W to Terminal B, to the maximum continuous current
of ±6 mA or to the pulse current specified in Table 6. Otherwise,
degradation or possible destruction of the internal switch
contact can occur.
PROGRAMMING THE POTENTIOMETER DIVIDER
Voltage Output Operation
The digital potentiometer easily generates a voltage divider at
wiper to B and wiper to A that is proportional to the input voltage
at A to B, as shown in Figure 44.
W
A
B
VA
VOUT
VB
Figure 44. Potentiometer Mode Configuration
Connecting Terminal A to 5 V and Terminal B to ground
produces an output voltage at the Wiper W to Terminal B
ranging from 0 V to 5 V. The general equation defining the
output voltage at VW with respect to ground for any valid
input voltage applied to Terminal A and Terminal B is
B
AB
AW
A
AB
WB
W
V
R
D
R
V
R
D
R
D
V
)
(
)
(
)
(
(7)
where:
RWB(D) can be obtained from Equation 1 and Equation 2.
RAW(D) can be obtained from Equation 3 and Equation 4.
Operation of the digital potentiometer in the divider mode
results in a more accurate operation over temperature. Unlike
the rheostat mode, the output voltage is dependent mainly on
the ratio of the internal resistors, RAW and RWB, and not the
absolute values. Therefore, the temperature drift reduces to
5 ppm/°C.
TERMINAL VOLTAGE OPERATING RANGE
The AD5122/AD5142 are designed with internal ESD diodes
for protection. These diodes also set the voltage boundary of the
terminal operating voltages. Positive signals present on
Terminal A, Terminal B, or Terminal W that exceed VDD are
clamped by the forward-biased diode. There is no polarity
constraint between VA, VW, and VB, but they cannot be higher
than VDD or lower than VSS.
VDD
A
W
B
VSS
Figure 45. Maximum Terminal Voltages Set by VDD and VSS
POWER-UP SEQUENCE
Because there are diodes to limit the voltage compliance at
Terminal A, Terminal B, and Terminal W (see Figure 45), it is
important to power up VDD first before applying any voltage to
Terminal A, Terminal B, and Terminal W. Otherwise, the diode
is forward-biased such that VDD is powered unintentionally. The
ideal power-up sequence is VSS, VDD, VLOGIC, digital inputs, and
VA, VB, and VW. The order of powering VA, VB, VW, and digital
inputs is not important as long as they are powered after VSS,
VDD, and VLOGIC. Regardless of the power-up sequence and the
ramp rates of the power supplies, once VLOGIC is powered, the
power-on preset activates, which restores EEPROM values to
the RDAC registers.
LAYOUT AND POWER SUPPLY BIASING
It is always a good practice to use a compact, minimum lead
length layout design. Ensure that the leads to the input are as
direct as possible with a minimum conductor length. Ground
paths must have low resistance and low inductance. It is also
good practice to bypass the power supplies with quality capacitors.
Apply low equivalent series resistance (ESR) 1 μF to 10 μF
tantalum or electrolytic capacitors at the supplies to minimize
any transient disturbance and to filter low frequency ripple.
Figure 46 illustrates the basic supply bypassing configuration
for the AD5122/AD5142.
VDD VLOGIC
VDD
+
VSS
C1
0.1µF
C3
10µF
+
C2
0.1µF
C4
10µF
VSS
VLOGIC
+
C5
0.1µF
C6
10µF
AD5122/
AD5142
GND
Figure 46. Power Supply Bypassing


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