ADL5310-EVAL AD [Analog Devices], ADL5310-EVAL Datasheet - Page 11

• Manufacturer Part Number: ADL5310-EVAL
• Description: 120 dB Range (3 nA to 3 mA) Dual Logarithmic Converter
• Manufacturer: AD [Analog Devices]

GENERAL STRUCTURE
The ADL5310 addresses a wide variety of interfacing conditions
to meet the needs of fiber optic supervisory systems and is
useful in many nonoptical applications. These notes explain the
structure of this unique style of translinear log amp. Figure 33
shows the key elements of one of the two identical on-board
log amps.
The photodiode current I
Pin INP2. The voltages at these nodes are approximately equal
to the voltage on the adjacent guard pins, VSUM, as well as
reference inputs IRF1 and IRF2, due to the low offset voltage
of the JFET operational amplifiers. Transistor Q1 converts I
to a corresponding logarithmic voltage, as shown in Equation 1.
A finite positive value of V
Q1 for the usual case of a single-supply voltage. This is inter-
nally set to 0.5 V, one-fifth of the 2.5 V reference voltage that
appears on Pin VREF. Both VREF pins are internally shorted,
as are both VSUM pins. The resistance at the VSUM pin is
nominally 16 kΩ; this voltage is not intended as a general bias
source.
The ADL5310 also supports the use of an optional negative
supply voltage, V
negative, VSUM may be connected to ground; thus, INP1, INP2,
IRF1, and IRF2 assume this potential. This allows operation as a
voltage-input logarithmic converter by the inclusion of a series
resistor at either or both inputs. Note that the resistor setting I
for each channel needs to be adjusted to maintain the intercept
value. Also note that the collector-emitter voltages of Q1 and Q2
are the full V
large input currents.
The input-dependent V
V
ated externally to a recommended value of 3 µA. However, other
values over a several-decade range can be used with a slight
degradation in law conformance.
BE2
(INP2)
PHOTODIODE
I
INP1
PD
CURRENT
of a second transistor, Q2, operating at I
INPUT
Q1
0.5V
VNEG (NORMALLY GROUNDED)
Figure 33. Simplified Schematic of Single Log Amp
VSUM
GENERATOR
80kΩ
N
2.5V
0.5V
and effects due to self-heating cause errors at
BIAS
N
, at Pin VNEG. When V
20kΩ
V
VREF
BE1
COMM
BE1
PD
of Q1 is compared with the reference
SUM
IREF
Q2
is received at either Pin INP1 or
0.5V
is needed to bias the collector of
I
REF
V
V
V
BE2
BE1
BE2
N
VRDZ
is 0.5 V or more
REF
COMM
(SUBTRACT AND
COMPENSATION
TEMPERATURE
6.69kΩ
DIVIDE BY T°K)
14.2kΩ
. I
REF
is gener-
44µA/dec
451Ω
VLOG
PD
Rev. A | Page 11 of 20
REF
THEORY
The base-emitter voltage of a bipolar junction transistor (BJT)
can be expressed by Equation 1, which immediately shows its
basic logarithmic nature:
where:
I
I
kT/q is the thermal voltage, proportional to absolute
temperature (PTAT), and is 25.85 mV at 300 K.
I
perature dependence, varying by a factor of roughly a billion
between −35°C and +85°C. Thus, to make use of the BJT as an
accurate logarithmic element, both of these temperature
dependencies must be eliminated.
The difference between the base-emitter voltages of a matched
pair of BJTs, one operating at the photodiode current I
other operating at a reference current I
The uncertain, temperature-dependent saturation current, I
that appears in Equation 1 has therefore been eliminated. To
eliminate the temperature variation of kT/q, this difference
voltage is processed by what is essentially an analog divider.
Effectively, it puts a variable under Equation 2. The output of
this process, which also involves a conversion from voltage
mode to current mode, is an intermediate, temperature-
corrected current:
where I
determines the slope of the function (change in current per
decade). For the ADL5310, I
temperature-independent slope of 44 µA/decade for all values
of I
voltage-mode output, V
It is apparent that this output should be 0 for I
would need to swing negative for smaller values of input
current. To avoid this, I
smallest value of I
V
connected to VREF. This moves the intercept to the left by four
decades (at 200 mV/decade), from 3 μA to 300 pA:
where I
Because values of I
supply of sufficient value is required to accommodate this
situation.
C
S
S
LOG
is a scaling current, typically only 10
is never precisely defined and exhibits an even stronger tem-
is the collector current.
PD
V
= ln(10) kT/q log
= 59.5 mV log
V
I
I
to shift it upward by 0.8 V when VRDZ is directly
LOG
LOG
and I
BE1
BE
Y
INTC
= kT/q ln(I
is an accurate, temperature-stable scaling current that
= I
= I
– V
REF
is the operational value of the intercept current.
Y
Y
BE2
. This current is subsequently converted back to a
log
log
= kT/q ln(I
10
10
(I
PD
(I
PD
10
PD
PD
C
. Accordingly, an offset voltage is added to
(I
/I
< I
/I
/I
10
PD
S
)
REF
REF
INTC
(I
LOG
/I
INTC
PD
REF
)
would need to be as small as the
, scaled 200 mV/decade.
)
PD
/I
) (T = 300 K)
result in a negative V
Y
REF
/I
is 44 µA, resulting in a
S
)
) – kT/q ln(I
–17
REF
A.
, can be written as
REF
/I
PD
S
)
= I
LOG
ADL5310
REF
, a negative
and
PD
and the
S
,
(2)
(1)
(3)
(4)