adl5304 Analog Devices, Inc., adl5304 Datasheet - Page 20

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

ADL5304
Filtering to Improve Noise and Dynamic Behavior
The noise at the output of a log amp, particularly at low current
levels, leads to uncertainty in the measurement. Noise amplitude
is limited by the finite bandwidth.
If measurement speed is not of primary concern, additional
filtering can reduce noise. Figure 48 shows the recommended
locations for additional external filtering. Note the
consists of a current-to-voltage (transimpedance) conversion
(I
conversion (V
current-to-voltage conversion (I
Typically, capacitors are not used on the numerator side (I
to keep the speed of the device as high as possible. On the
denominator side (I
noise. In applications where I
logarithmic equation and I
reverse logarithmic slope is desired, filtering can be done on the
numerator side (I
NUM
or I
SHIELDED ENCLOSURE
DEN
BUFFER +9V
BUFFER –9V
AD8597
to V
NUM
10kΩ LINEAR 15 TURN
NUM
NUM
CONDUCTIVE PLASTIC
− V
4kΩ METAL FILM
DEN
).
− V
DEN
), additional filtering is useful to reduce
1MΩ log
DEN
to I
DEN
), followed by a voltage-to-current
LOG
NUM
DUT 5V
is a variable, for example, where a
), and then followed by another
LOG
BUFFER +9V
is used as the reference to the
Figure 47. Setup for Measuring Noise for I
+
–
to V
VSM1
VSM2
VSM3
VSM4
INUM
9V
IDEN
IREF
LOG
).
BUFFER –9V
2
3
4
5
6
7
8
IMON
1.5V
1.5V
–
+
MONITOR AND
30
ADL5304
(1.1× I
9V
PD BIAS
NUM
VNUM
NUM
VDEN
+
–
)
Rev. 0 | Page 20 of 32
)
32
9
9V
0.1µF
INNM
INDN
7805
NUM
10
31
COMM NMFS VNEG DNFS
V
V
1P5V
= 100 pA, I
DUT 5V
100nA
NUM
DEN
11
COMPENSATION
TEMPERATURE
C1D
1.5V
26
ADL5304
C1N
BSDC
12
IREF
BIAS AND
SHIELD BIAS
VSM1
VSM2
VSM3
VSM4
INUM
28
IDEN
1kΩ
NUM
V
REF
V
2VLT
NEG
= 1 nA, and I
13
2V
17
I
1kΩ LINEAR 15 TURN
LOG
Figure 48. Possible External Filtering Locations
2
3
4
5
6
7
8
DUT 5V
1.5V
1.5V
14
1kΩ
29
7.5kΩ
SPECTRUM ANALYZER
MONITOR AND
VPOS
5kΩ
ACOM
(1.1× I
PD BIAS
NUM
15
R&S FSEA30
C2D
= 10 nA
NUM
5kΩ
HFCP
VDEN
27
C2N
16
VNUM
DCBI
)
0.1µF
24
23
22
21
20
19
18
20MHz TO
INPUT
9
3.5GHz
32
INPS
INMS
VLOG
SCL1
SCL2
SCL3
ACOM
V
RD
NUM
RN
10
31
V
INDN
DEN
INNM
C3D
100nA
C3N
COAX SHIELD BIASED TO
VLOG DC LEVEL
BIAS
COAX SHIELD
BROKEN
1.5V
26
SHIELD
1P5V
I
BIAS
LOG
Data Sheet
7.5kΩ
5kΩ
27
DCBI
5kΩ
24
23
22
21
20
19
18
INMS
VLOG
SCL1
SCL2
SCL3
ACOM
INPS
CFB