Datasheet

Data Sheet ADN2850
Rev. E | Page 23 of 28
where:
I
S1
and I
S2
are saturation current.
V
1
, V
2
are V
BE
, base-emitted voltages of the diode connector
transistors.
V
T
is the thermal voltage, which is equal to k × T/q
(V
T
= 26 mV @ 25°C)
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
PD
is the photodiode current.
I
REF
is the reference current.
Figure 38 shows a conceptual circuit.
LPF
0.75 BIT RATE
TIA
AD623
IN AMP
10nF
CDR
POST
AMP
VT COMPENSATION
ADN2850
LOG
AVERAGE
POWER
W
1
B
1
B
2
V
DD
V
SS
I
PD
I
REF
R
G
W
2
V
1
V
2
Q
1
Q
2
°C
PRC
THERMISTOR
DATA
CLOCK
LOG AMP
GND
–5V
(1 + 100k/R
G
)×(V
1
–V
2
)
02660-137
Figure 38. Conceptual Incoming Optical Power Monitoring 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
T
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
2
and V
1
. A curve fit approximation
yields
03.0
001.0
ln026.0
12

PD
I
VV
(19)
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.
0.30
0.25
0.20
0.15
0.10
0.05
0
12
9
ERROR
6
3
0
–3
–6
0.1µ 1µ 10µ 100µ 1m
02660-138
V
2
–V
1
(V)
APPROXIMATING ERROR (%)
I
PD
(A)
I
REF
=1mA
T
A
= 25°C
DEVICE 1
DEVICE 2
DEVICE 3
CURVE FIT
Figure 39. V
2
– V
1
Error Versus Input Current.
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.
B1
W1
W2
B2
02660-058
Figure 40. Reduce Resistance by Half with Linear Adjustment Characteristics
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
AB
, the more the pseudo log taper characteristic
of the circuit behaves.
B1
W1
R
02660-060
Figure 41. Resistor Scaling with Pseudo Log Adjustment Characteristics
The equation is approximated as
RR
R
R
WB
WB
QUIVALENT



1024200,51
200,51
E
(17)
Users should also be aware of the need for tolerance matching
as well as for temperature coefficient matching of the components.