adl5304 Analog Devices, Inc., adl5304 Datasheet - Page 22

• Manufacturer Part Number: adl5304
• Description: High Speed, 200 Db Range, Logarithmic Converter
• Manufacturer: Analog Devices, Inc.

ADL5304
Reference Outputs
The
voltages, and one current (I
2.000 V at the 1P5V and 2VLT pins, respectively. The 1P5V
reference is intended to provide the bias to the VSM1 to VSM4,
DCBI, and INPS pins; it can sink up to 10 mA and source a
maximum of about 5 mA.
The 2VLT reference can source up to 20 mA of current, but it
cannot sink any current. The primary use of the 2.0 V reference
is for photodiode bias, or to generate reference currents other
than the 100 nA provided by I
the 1.5 V and 2.0 V references can reliably generate any current
up to approximately 5 mA.
The I
is primarily used as an input to the IDEN pin to provide the
denominator current, I
middle of the 1 pA to 10 mA range. I
input to the INUM pin and thereby invert the basic log response of
the ADL5304. If I
Whereas if I
V
Buffer Amplifier
A buffer amplifier completes the signal chain that takes the I
current from the temperature compensation block and converts
it to a voltage at the VLOG pin. The buffer amplifier gain and
offset can be configured to provide different logarithmic slope and
intercept at the V
scale factors and intercepts via the SCL1, SCL2, and SCL3 pins.
For example, in Figure 51, the default setup provides a scale of
0.2 V/decade and an intercept of 3.162 fA. V
when the internal I
varies from −400 μA to +400 μA with a scale of 80 μA/decade over
the full 200 dB input current range. In the default configuration,
I
input current is applied to the IDEN pin and the reference current
(I
is inverted, that is, V
for I
LOG
LOG
REF
) to the INUM pin, the slope of V
is negative for I
DEN
ADL5304
REF
decreases with increasing I
= 10 mA.
current, nominally 100 nA, flows out of the IREF pin and
NUM
has three trimmed precision references, two
= I
LOG
DEN
LOG
REF
LOG
output. On-chip resistors provide optimized
NUM
= I
= 0 A, which corresponds to I
and the input current is applied to IDEN,
is 2.5 V for I
DEN
> I
REF
. The choice of 100 nA places it in the
, V
DEN
REF
REF
LOG
). The voltages are 1.500 V and
and positive for I
. Together with a precision resistor,
DEN
increases with increasing I
DEN
.
LOG
REF
= 1 pA, and V
is negative and the range
can also be used as the
OFS
NUM
= V
NUM
< I
LOG
LOG
= I
DEN
is 0.5 V
= 1.5 V
DEN
. If the
. I
NUM
LOG
Rev. 0 | Page 22 of 32
LOG
.
The buffer amplifier is a voltage feedback op amp with supplies
between VPOS and VNEG. For single-supply operation, the
VNEG pin is tied to ground, and the INPS pin, the positive
input of the op amp, to the 1P5V pin.
If a ground referenced input is desired at the INUM or IDEN
pins, then the INPS and DCBI pins together with the VSMx
pins must be tied to ground, and VNEG needs to be less than
−2 V. If larger slopes are required, VPOS can increase to +5 V,
and VNEG can increase to −5 V. For example, if the SCL3 pin is
connected to VLOG, and SCL1 and SCL2 remain open, the
internal 7.5 kΩ resistor, together with the 80 μA/decade I
provides a slope of 0.6 V/decade at the VLOG pin. Implementation
of slopes of 0.2 V/decade to 0.8 V/decade is easily
accomplished.
Setting the Log Slope and Intercept
The choice of optimal slope and intercept depends on the
application and supply voltage(s). For example, when an input
current range of less than the full 200 dB is desired, a higher slope
can be chosen to better use the full voltage span available at
VLOG, and perhaps optimally position it to suit the input capacity
of a subsequent analog-to-digital converter (ADC). Very high
slopes, such as 0.8 V/decade, can be realized, allowing a smaller
range of I
Any other intercept and slope can be realized using external
resistors, but these do not, in general, form accurate ratios to
the on-chip resistors. Therefore, some inaccuracies should be
expected. If the SCL1, SCL2, and SCL3 pins are not connected
and a resistor is placed between the INMS and VLOG pins, the
I
has a log slope that is 80 μA/decade times R
the voltage applied to the INPS pin.
LOG
current is forced through the external resistor and thereby
PD
Figure 51. Buffer Amplifier in Default Configuration
to be measured at high sensitivity.
COMPENSATION
TEMPERATURE
BIAS
1.5V
26
1P5V
I
LOG
7.5kΩ
5kΩ
15
27
ACOM
DCBI
5kΩ
24
23
22
21
20
19
EXT
INPS
INMS
VLOG
SCL1
SCL2
SCL3
; V
Data Sheet
OFS
is equal to
LOG
,