Datasheet

AD8305
Rev. B | Page 20 of 24
The setup shown in Figure 44 was used for frequency response
measurements of the logarithmic amplifier section. The
AD8138 output is offset to 1.5 V dc and modulated to a depth
of 5% at frequency. R1 is chosen (over a wide range of values up
to 1.0 GΩ) to provide I
PD
. The buffer was used to deload VLOG
from the measurement system.
0.1µF
SOURCE
CHANNEL 2
HP 89410A
1nF
1kΩ
1kΩ
1nF
R1
200kΩ
A
LKALINE
“D” CELL
ALKALINE
“D” CELL
+
–
+
–
+
–
+
–
AD8305
12
11
10
9
1
2
3
4
578
COMM COMM COMM COMM
16 131415
VSUM
VRDZ
VREF
IREF
INPT
VOUT
SCAL
BFIN
VLOG
VNEG VNEG VPOS
6
TRIGGER CHANNEL 1
03053-044
Figure 45. Configuration for Noise Spectral Density Measurement
The configuration in Figure 45 is used to measure the noise
performance. Batteries provide both the supply voltage and the
input current to minimize the introduction of spurious noise
and ground loop effects. The entire evaluation system,
including the current setting resistors, is mounted in a closed
aluminum enclosure to provide additional shielding to external
noise sources.
0.1µF
1nF
1kΩ
1kΩ
1nF
R1
200kΩ
CH A
9213
LECROY 9210
CH1
TDS5104
+V
S
AD8305
12
11
10
9
1
2
3
4
578
COMM COMM COMM COMM
16 131415
VSUM
VRDZ
VREF
IREF
INPT
VOUT
SCAL
BFIN
VLOG
VNEG VNEG VPOS
6
0
3053-045
Figure 46. Configuration for Logarithmic Amplifier Pulse Response
Measurement
Figure 46 shows the setup used to make the pulse response
measurements. As with the bandwidth measurement, the
VLOG is connected directly to BFIN and the buffer amplifier is
configured for unity gain. The output of the buffer is connected
through a short cable to the TDS5104 scope with input
impedance set to 1 MΩ. The LeCroy’s output is offset to create
the initial pedestal current for a given value of R1, the pulse
then creates one-decade current step.