AD8304 Analog Devices, AD8304 Datasheet - Page 17

• Manufacturer Part Number: AD8304
• Description: 160 DB Logarithmic Amplifier With Photo-diode Interface
• Manufacturer: Analog Devices

Characterization Setups and Methods
During the primary characterization of the AD8304, the device
was treated as a high precision current-in logarithmic amplifier
(converter). Rather than attempting to accurately generate
photocurrents by illuminating a photodiode, precision current
sources, like the Keithley 236, were used as input sources. Great
care was taken when applying the low-level input currents. The
triax output of the current source was used with the guard con-
nected to VSUM at the characterization board. On the board
the input trace was guarded by connecting adjacent traces and a
portion of an internal copper layer to the VSUM pins. One
obvious reason for the care was leakage current. With 0.5 V as
the nominal bias on the INPT pin, a resistance of 50 GΩ to
ground would cause 10 pA of leakage, or about one decibel of
error at the low end of the measurement range. Additionally,
the high output resistance of the current source and the long
signal cable lengths commonly needed in characterization make
a good receiver for 60 Hz emissions. Good guarding techniques
help to reduce the pickup of unwanted signals.
The primary characterization setup shown in Figure 18 is used
to measure the static performance, logarithmic conformance,
slope and intercept, buffer offset and V
ture, and the performance of the VPDB pin functions. For the
dynamic tests, such as noise and bandwidth, more specialized
setups are used.
+IN
EVALUATION
SIGNAL: INPT;
GUARD: VSUM;
SHIELD: GROUND
KEITHLEY 236
AD8138
BOARD
CONNECTOR
B
A
OUTPUT INPUT INPUTA
TRIAX
SPLITTER
POWER
ANALYZER
NETWORK
HP 3577A
INPT
PWDN VNEG
VSUM
CHARACTERIZATION
DC MATRIX, DC SUPPLIES, DMM
INPUTB
VPDB
1
2
3
4
5
6
7
AD8304
BOARD
VNEG
PWDN
VSUM
INPT
VSUM
VPDB
VREF
VPOS
VREF
REF
AD8304
ACOM
BFNG
VLOG
VOUT
VPS1
VPS2
drift with tempera-
BFIN
VLOG
VOUT
BFIN
14
13
12
11
10
9
8
49.9
RIBBON
CABLE
0.1 F
+V
S
Figure 19 shows the configuration used to measure the buffer
amplifier bandwidth. The AD8138 Evaluation Board provides a dc
offset at the buffer input, allowing measurement in single-supply
mode. The network analyzer input impedance was set to 1 MΩ.
The setup shown in Figure 20 was used for frequency response
measurements of the logarithmic amplifier section. In this con-
figuration, the AD8138 output was offset to 1.5 V and R1 was
adjusted to provide the appropriate operating current. The
buffer amplifier was then used; still any capacitance added at
the VLOG pin during measurement would form a filter with the
on-chip 5 kΩ resistor.
The configuration illustrated in Figure 21 measures the device
noise. Batteries provide both the supply and the input signal to
remove the supplies as a possible noise source and to reduce
ground loop effects. The AD8304 Evaluation Board and the
current setting resistors are mounted in closed aluminum enclo-
sures to provide additional shielding to external noise sources.
OUTPUT INPUT INPUTA
ALKALINE
D CELL
SPLITTER
POWER
+IN
NETWORK
ANALYZER
HP 3577A
EVALUATION
AD8138
BOARD
R1
750
1nF
INPUTB
B
A
R1
750
1nF
1
2
3
4
5
6
7
VNEG
PWDN
VSUM
INPT
VSUM
VPDB
VREF
AD8304
SOURCE TRIGGER
1
2
3
4
5
6
7
VNEG
PWDN
VSUM
INPT
VSUM
VPDB
VREF
ACOM
BFNG
VLOG
VOUT
AD8304
VPS1
VPS2
BFIN
HP 89410A
14
13
12
11
10
9
8
ACOM
BFNG
VLOG
VOUT
VPS1
VPS2
BFIN
CHANNEL
AD8304
1
14
13
12
11
10
9
8
ALKALINE
D CELL
CHANNEL
2
0.1 F
+V
S