AD5235BRU25-RL7 Analog Devices Inc, AD5235BRU25-RL7 Datasheet - Page 25

• Manufacturer Part Number: AD5235BRU25-RL7
• Description: IC DGTL POT DUAL 1024POS 16TSSOP
• Manufacturer: Analog Devices Inc

Specifications of AD5235BRU25-RL7

• Rohs Status: RoHS non-compliant
• Taps: 1024
• Resistance (ohms): 25K
• Number Of Circuits: 2
• Temperature Coefficient: 35 ppm/°C Typical
• Memory Type: Non-Volatile
• Interface: 4-Wire SPI Serial
• Voltage - Supply: 3 V ~ 5.5 V, ±2.25 V ~ 2.75 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP (0.173", 4.40mm Width)
• Number Of Elements: 2
• # Of Taps: 1024
• Resistance (max): 25KOhm
• Power Supply Requirement: Single/Dual
• Interface Type: Serial (4-Wire/SPI)
• Single Supply Voltage (typ): 3.3/5V
• Dual Supply Voltage (typ): ±2.5V
• Single Supply Voltage (min): 3V
• Single Supply Voltage (max): 5.5V
• Dual Supply Voltage (min): ±2.25V
• Dual Supply Voltage (max): ±2.75V
• Operating Temp Range: -40C to 85C
• Operating Temperature Classification: Industrial
• Mounting: Surface Mount
• Pin Count: 16
• Lead Free Status / RoHS Status: Not Compliant
• Other names: AD5235BRU25-RL7; AD5235BRU25-RL7TR

First, users should select convenient values for the capacitors.
To achieve maximally flat bandwidth, where Q = 0.707, let C1
be twice the size of C2 and let R1 equal R2. As a result, the user
can adjust R1 and R2 concurrently to the same setting to
achieve the desirable bandwidth.
PROGRAMMABLE OSCILLATOR
In a classic Wien bridge oscillator, the Wien network (R||C, R'C')
provides positive feedback, whereas R1 and R2 provide negative
feedback (see Figure 55).
At the resonant frequency, f
the positive feedback causes the circuit to oscillate. With R = R
C = C
where R is equal to R
At resonance, setting R2/R1 = 2 balances the bridge. In practice,
R2/R1 should be set slightly larger than 2 to ensure that the
oscillation can start. On the other hand, the alternate turn-on
of the diodes, D1 and D2, ensures that R2/R1 is smaller than 2,
momentarily stabilizing the oscillation.
When the frequency is set, the oscillation amplitude can be
turned by R2B because
V
selection of R2B, an equilibrium is reached such that V
converges. R2B can be in series with a discrete resistor to
increase the amplitude, but the total resistance cannot be too
large to saturate the output.
O
, I
D
ω
R
2
3
',
, and V
O
WA
V
Figure 55. Programmable Oscillator with Amplitude Control
and R2 = R2A /(R2B + R
O
=
(
=
D
RC
2.2nF
1
R = R' = AD5235
R2B = AD5231
D1 = D2 = 1N4148
)
I
D
D
=
R2B
C
are interdependent variables. With proper
or
1024
ADJUSTMENT
FREQUENCY
1024
f
+
25kΩ
O
WA
−
V
=
R
D
B
A
D
such that :
2
×
π
1
W
RC
R
O
, the overall phase shift is zero, and
AB
VP
DIODE
+
B
R1
1kΩ
2.2nF
10kΩ
W
R2B
R
+
–
OP1177
–2.5V
C'
+2.5V
W
V+
), the oscillation frequency is
V–
ADJUSTMENT
A
AMPLITUDE
25kΩ
U1
W
A
R'
2.1kΩ
R2A
B
D2
D1
V
O
O
(13)
(14)
(15)
Rev. C | Page 25 of 28
'
,
In Figure 54 and Figure 55, the frequency tuning requires that
both RDACs be adjusted concurrently to the same settings.
Because the two channels might be adjusted one at a time, an
intermediate state occurs that might not be acceptable for some
applications. Of course, the increment/decrement instructions
(Instruction 5, Instruction 7, Instruction 13, and Instruction 15)
can all be used. Different devices can also be used in daisy-chain
mode so that parts can be programmed to the same settings
simultaneously.
OPTICAL TRANSMITTER CALIBRATION WITH
ADN2841
The AD5235, together with the multirate 2.7 Gbps laser diode
driver, ADN2841, forms an optical supervisory system in which
the dual digital potentiometers can be used to set the laser average
optical power and extinction ratio (see Figure 56). The AD5235
is particularly suited for the optical parameter settings because
of its high resolution and superior temperature coefficient
characteristics.
The ADN2841 is a 2.7 Gbps laser diode driver that uses a
unique control algorithm to manage the average power and
extinction ratio of the laser after its initial factory calibration.
The ADN2841 stabilizes the data transmission of the laser by
continuously monitoring its optical power and correcting the
variations caused by temperature and the degradation of the
laser over time. In the ADN2841, the IMPD monitors the laser
diode current. Through its dual-loop power and extinction
ratio control calibrated by the dual RDACs of the AD5235, the
internal driver controls the bias current, IBIAS, and consequently
the average power. It also regulates the modulation current,
IMODP, by changing the modulation current linearly with slope
efficiency. Therefore, any changes in the laser threshold current or
slope efficiency are compensated for. As a result, the optical
supervisory system minimizes the laser characterization efforts
and, therefore, enables designers to apply comparable lasers
from multiple sources.
CLK
SDI
CS
CONTROL
AD5235
Figure 56. Optical Supervisory System
EEMEM
EEMEM
RDAC1
RDAC2
DATAP
DATAN
CLKN
CLKP
B2
W2
A1
W1
B1
A2
PSET
ERSET
ADN2841
IMODP
IMPD
IBIAS
V
CC
AD5235
V
CC