AD8305 Analog Devices, AD8305 Datasheet - Page 14

• Manufacturer Part Number: AD8305
• Description: 100 Db-range (10nA-1mA) Logarithmic Converter
• Manufacturer: Analog Devices

AD8305
provisions to offset V
ments over the full range of I
analyzer input impedances were set to 1 MW.
The setup shown in Figure 11 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 GW) to provide I
from the measurement system.
ALKALINE
“D” CELL
+IN
EVALUATION
Figure 11. Configuration for Logarithmic
Amplifier Bandwidth Measurement
Figure 12. Configuration for Noise Spectral
Density Measurement
AD8138
BOARD A
+
SOURCE
–
SPLITTER
1nF
OUTPUT
POWER
1nF
1k
R1
B
200k
1k
TRIGGER
1nF
NETWORK ANALYZER
1nF
1k
R1
INPUT R
PD
1k
LOG
R2
HP 89410A
. The buffer was used to deload VLOG
1
2
3
4
HP 3577A
at the buffer input, allowing measure-
IREF
INPT
VRDZ
CHANNEL 1 CHANNEL 2
VREF
PD
1
2
3
4
COMM COMM COMM COMM
VSUM VNEG VNEG VPOS
INPUT A
16
IREF
INPT
VRDZ
VREF
5
using a single supply. The network
COMM COMM COMM COMM
VSUM VNEG VNEG VPOS
16
5
AD8305
15
6
INPUT B
AD8305
15
6
14
7
14
7
13
8
VOUT
SCAL
VLOG
BFIN
13
8
VOUT
SCAL
VLOG
0.1 F
BFIN
12
11
10
9
0.1 F
ALKALINE
12
11
10
“D” CELL
9
+V
+
+ –
+
S
–
–
–14–
The configuration in Figure 12 is used to measure the noise
performance. Batteries provide both the supply voltage and the
input current in order 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.
Figure 13 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 config-
ured for unity gain. The buffer’s output is connected through a
short cable to the TDS5104 scope with input impedance set to
1 MW. 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.
EVALUATION BOARD
An evaluation board is available for the AD8305, the schematic
for which is shown in Figure 16. It can be configured for a wide
variety of experiments. The buffer gain is factory-set to unity,
providing a slope of 200 mV/decade, and the intercept is set to 1 nA.
Table I describes the various configuration options.
LECROY 9210
Figure 13. Configuration for Logarithmic
Amplifier Pulse Response Measurement
CH A
9213
1nF 1k
1nF
200k
1k
R1
1
2
3
4
IREF
INPT
VRDZ
VREF
COMM COMM COMM COMM
VSUM VNEG VNEG VPOS
16
5
AD8305
15
6
14
7
TDS5104
13
8
VOUT
SCAL
VLOG
BFIN
0.1 F
12
11
10
9
+V
REV. A
CH1
S