EVAL-ADN2850-25EBZ Analog Devices Inc, EVAL-ADN2850-25EBZ Datasheet - Page 23

• Manufacturer Part Number: EVAL-ADN2850-25EBZ
• Description: BOARD EVALUATION FOR ADN2850-25
• Manufacturer: Analog Devices Inc

Specifications of EVAL-ADN2850-25EBZ

• Main Purpose: Digital Potentiometer
• Utilized Ic / Part: ADN2850-35
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Secondary Attributes: -
• Embedded: -
• Primary Attributes: -
• Lead Free Status / Rohs Status: Supplier Unconfirmed

where:
I
V
transistors.
V
k is the Boltzmann’s constant, 1.38e-23 Joules/Kelvin.
q is the electron charge, 1.6e-19 coulomb.
T is the temperature in Kelvin.
I
I
Figure 38 shows a conceptual circuit.
The output voltage represents the average incoming optical
power. The output voltage of the log stage does not have to be
accurate from device to device, as the responsivity of the
photodiode will change between devices. An op amp stage is
shown after the log amp stage, which compensates for V
variation over temperature.
Equation 19 is ideal. If the reference current is 1 mA at room
temperature, characterization shows that there is an additional
30 mV offset between V
yields
The offset is caused by the transistors self-heating and the
thermal gradient effect. As seen in Figure 39, the error between
an approximation and the actual performance ranges is less
than 0% to –4% from 0.1 mA to 0.1 μA.
S1
PD
REF
1
T
, V
(V
and I
is the thermal voltage, which is equal to k × T/q
is the photodiode current.
is the reference current.
Figure 38. Conceptual Incoming Optical Power Monitoring Circuit
V
T
2
ADN2850
= 26 mV @ 25°C)
V
V
are V
2
DD
SS
S2
−
B
V
are saturation current.
1
1
BE
W
=
1
, base-emitted voltages of the diode connector
. 0
B
026
2
W
2
ln
–5V
Q
1
. 0
V
GND
2
I
1
I
10nF
PD
001
and V
PD
Q
TIA
2
R
V
LOG AMP
+
I
G
2
REF
1
. 0
. A curve fit approximation
AD623
0.75 BIT RATE
03
IN AMP
LPF
(1 + 100k/R
VT COMPENSATION
G
) × (V
POST
AMP
°C
PRC
THERMISTOR
1
– V
CDR
2
)
DATA
CLOCK
LOG
AVERAGE
POWER
T
(19)
Rev. C | Page 23 of 28
RESISTANCE SCALING
The ADN2850 offers 25 kΩ or 250 kΩ nominal resistance.
When users need lower resistance but must maintain the
number of adjustment steps, they can parallel multiple devices. For
example, Figure 40 shows a simple scheme of paralleling two
channels of RDACs. To adjust half the resistance linearly per
step, program both RDACs concurrently with the same settings.
Figure 40 shows that the digital rheostat change steps linearly.
Alternatively, pseudo log taper adjustment is usually preferred in
applications such as audio control. Figure 41 shows another type
of resistance scaling. In this configuration, the smaller the R2
with respect to R
of the circuit behaves.
The equation is approximated as
Users should also be aware of the need for tolerance matching
as well as for temperature coefficient matching of the components.
Figure 40. Reduce Resistance by Half with Linear Adjustment Characteristics
Figure 41. Resistor Scaling with Pseudo Log Adjustment Characteristics
0.30
0.25
0.20
0.15
0.10
0.05
0
0.1µ
R
E
QUIVALENT
Figure 39. V
AB
=
1µ
, the more the pseudo log taper characteristic
R
B1
WB
DEVICE 1
DEVICE 2
DEVICE 3
CURVE FIT
2
B1
– V
+
R
51
WB
1
Error Versus Input Current.
,
I
PD
W1
200
10µ
+
W1
(A)
51
B2
+
R
,
200
1024
I
T
REF
×
A
W2
100µ
= 25°C
R
= 1mA
ERROR
ADN2850
1m
12
9
6
3
0
–3
–6
(17)