Resistance Scaling; Resistance Tolerance, Drift, And Temperature Coefficient Mismatch Considerations - Analog Devices AD5235 Manual

AD5235
control calibrated by the AD5235's dual RDACs, the internal
driver controls the bias current, I
average power. It also regulates the modulation current, I
by changing the modulation current linearly with slope
efficiency. Any changes in the laser threshold current or slope
efficiency are, therefore, compensated. As a result, this optical
supervisory system minimizes the laser characterization efforts
and, therefore, enables designers to apply comparable lasers
from multiple sources.

RESISTANCE SCALING

The AD5235 offers 25 kΩ or 250 kΩ nominal resistance. For
users who need lower resistance but must still maintain the
number of adjustment steps, they can parallel multiple devices.
For example, Figure 57 shows a simple scheme of paralleling
two channels of RDACs. To adjust half the resistance linearly
per step, users need to program both RDACs concurrently with
the same settings.
A1
W1
B1
Figure 57. Reduce Resistance by Half with Linear Adjustment Characteristics
In voltage divider mode, by paralleling a discrete resistor as
shown in Figure 58, a proportionately lower voltage appears at
Terminal A-to-B. This translates into a finer degree of precision,
because the step size at Terminal W is smaller. The voltage can
be found as follows:
( R // R )
= 2 ×
AB
V ( D )
W
+
R R // R
3 AB 2
R2
Figure 58. Lowering the Nominal Resistance
Figure 57 and Figure 58 show that the digital potentiometers
change steps linearly. On the other hand, pseudolog taper
adjustment is usually preferred in applications such as audio
control. Figure 59 shows another type of resistance scaling. In
this configuration, the smaller the R2 with respect to R
more the pseudolog taper characteristic of the circuit behaves.
, and consequently the
BIAS
MODP
A2
W2
B2
D
×
V
DD
1024
V
DD
R3
A
R1
W
B
0
, the
AB
,
Figure 59. Resistor Scaling with Pseudo Log Adjustment Characteristics
The equation is approximated as
=
R
eq
Users should also be aware of the need for tolerance matching
as well as for temperature coefficient matching of the
components.
RESISTANCE TOLERANCE, DRIFT, AND
TEMPERATURE COEFFICIENT MISMATCH
CONSIDERATIONS
In a rheostat mode operation such as gain control (see
Figure 60), the tolerance mismatch between the digital potenti-
ometer and the discrete resistor can cause repeatability issues
among various systems. Because of the inherent matching of the
silicon process, it is practical to apply the dual-channel device in
this type of application. As such, R1 can be replaced by one of
the channels of the digital potentiometer and programmed to a
specific value. R2 can be used for the adjustable gain. Although
it adds cost, this approach minimizes the tolerance and
temperature coefficient mismatch between R1 and R2. This
approach also tracks the resistance drift over time. As a result,
these less than ideal parameters become less sensitive to system
variations.
(17)
Figure 60. Linear Gain Control with Tracking Resistance Tolerance, Drift,
Note that the circuit in Figure 61 can track tolerance,
temperature coefficient, and drift in this particular application.
The characteristic of the transfer function is, however, a pseudo-
logarithmic rather than a linear gain function.
Rev. B | Page 26 of 28
A1
W1
B1
R
× +
D R 51200
AB
× + + ×
D R 51200 1024 R
AB
R2
B A
W
C1
R1*
–
AD8601
+
V
i
U1
* REPLACED WITH ANOTHER
CHANNEL OF RDAC
and Temperature Coefficient
(18)
V
O