AD8305ACPZ-RL7 Analog Devices Inc, AD8305ACPZ-RL7 Datasheet - Page 16

• Manufacturer Part Number: AD8305ACPZ-RL7
• Description: IC, LOGARITHMIC AMP, 20mV, LFCSP-16
• Manufacturer: Analog Devices Inc
• Type: Logarithmic Converterr

Specifications of AD8305ACPZ-RL7

• No. Of Amplifiers: 1
• Dynamic Range, Decades: 5
• Scale Factor V / Decade: 0.2
• Supply Voltage Range: 3V To 12V
• Amplifier Case Style: LFCSP
• Supply Current: 5.4mA
• Input Offset Voltage: 20mV
• Design Resources: Interfacing ADL5315 to Translinear Logarithmic Amplifier (CN0056) Interfacing ADL5317 High Side Current Mirror to a Translinear Logarithmic Amplifier in an Avalanche Photodiode Power Detector
• Applications: Fiber Optics
• Mounting Type: Surface Mount
• Package / Case: 16-LFCSP
• Rohs Compliant: Yes
• Number Of Channels: 1
• Number Of Elements: 3
• Power Supply Requirement: Single/Dual
• Single Supply Voltage (typ): 5V
• Dual Supply Voltage (typ): ±3/±5V
• Power Dissipation: 500mW
• Rail/rail I/o Type: Rail to Rail Output
• Single Supply Voltage (min): 3V
• Single Supply Voltage (max): 12V
• Dual Supply Voltage (min): ±1.5V
• Dual Supply Voltage (max): ±6V
• Operating Temp Range: -40C to 85C
• Operating Temperature Classification: Industrial
• Mounting: Surface Mount
• Pin Count: 16
• Package Type: LFCSP EP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Lead free / RoHS Compliant
• Other names: AD8305ACPZ-RL7

AD8305
USING A NEGATIVE SUPPLY
Most applications of the AD8305 require only a single supply of
3.0 V to 5.5 V. However, to provide further versatility, dual
supplies may be employed, as illustrated in Figure 37.
The use of a negative supply, V
be placed at ground level whenever the input transistor (Q1 in
Figure 33) has a sufficiently negative bias on its emitter. When
V
default case when VSUM is grounded. This bias does not need
to be accurate, and a poorly defined source can be used. The
NEG
1nF
1k
V
RREF
200k
BIAS
Ω
= −0.5 V, the V
Ω
I
PD
V
R
+
–
VSUM
VRDZ
F
S
VREF
1k
1nF
IREF
V
INPT
Ω
N
SIGNAL
DETECTOR
0.5V
REFERENCE
DETECTOR
I
SIG
Figure 37. Negative Supply Application
0.5V
I
q
= I
+ I
5V
PD
SIG
20k
CE
+ I
P
Ω
COMM
of Q1 and Q2 is the same as for the
SIG
VNEG
REF
P
REF
Q1
Q2
1kΩ
80k
1nF
1kΩ
Ω
V
–
+
V
C1
V
NEG
VPOS
2.5V
1nF
BE2
N
VSUM
BE1
VRDZ
VREF
COMPENSATION
, allows the summing node to
INPT
IREF
TEMPERATURE
I
REF
≤ –0.5V
I
PD
5V
GENERATOR
0.5V
BIAS
0.5V
COMM
R
S
6.69kΩ
14.2k
≤
20kΩ
I
LOG
I
q
Ω
V
+ I
COMM
0.5 log
VNEG
N
SIGMAX
451
– V
COMM
Q2
Q1
VOUT
SCAL
80kΩ
BFIN
Figure 38. Optical Absorbance Measurement
Ω
F
10
2.5V
–
+
V
1nA
I
V
PD
BE2
VPOS
BE1
COMPENSATION
12k
GENERATOR
TEMPERATURE
VLOG
8k
C
10nF
FLT
Ω
Ω
5V
Rev. B | Page 16 of 24
BIAS
COMM
14.2kΩ
6.69kΩ
I
LOG
source does, however, need to be able to support the quiescent
current as well as the INPT and IREF signal current. For
example, it may be convenient to utilize a forward-biased
junction voltage of about 0.7 V or a Schottky barrier voltage of a
little over 0.5 V. The effect of supply on the dynamic range and
accuracy can be seen in Figure 10.
With the summing node at ground, the AD8305 may now be
used as a voltage-input log amp at either the numerator input,
INPT, or the denominator input, IREF, by inserting a suitably
scaled resistor from the voltage source to the relevant pin. The
overall accuracy for small input voltages is limited by the
voltage offset at the inputs of the JFET op amps.
The use of a negative supply also allows the output to swing
below ground, thereby allowing the intercept to correspond to a
midrange value of I
referenced to the ACOM pin, and while it does not swing
negative for default operating conditions, it is free to do so,
thus, adding a resistor from VLOG to the negative supply
lowers all values of VLOG, which raises the intercept. The
disadvantage of this method is that the slope is reduced by the
shunting of the external resistor, and the poorly defined ratio of
on-chip and off-chip resistances causes errors in both the slope
and the intercept.
451Ω
COMM
VOUT
VLOG
SCAL
BFIN
12.1kΩ
44.2kΩ
28.0kΩ
33nF
PD
. However, the voltage, V
18nF
0.5 log
10
I
I
REF
PD
+ 2
LOG
, remains