AD8304 Analog Devices, AD8304 Datasheet - Page 14

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

AD8304
referenced to the ACOM pin, and does not normally go nega-
tive with regard to this pin, but it is free to do so. Therefore, a
resistor from VLOG to the negative supply can lower V
raising the intercept. A more accurate method for repositioning
the intercept is described below.
APPLICATIONS
The AD8304 incorporates features that improve its usefulness
in both fiber optic supervisory applications and in more general
ones. To aid in the exploration of these possibilities, a SPICE
macromodel is provided and a versatile evaluation board is
available.
The macromodel is shown in generalized schematic form (and
thus is independent of variations in SPICE programs) in Fig-
ure 12. Q1, QM, and Q2 (here made equal in size) correspond
to the identical transistors in Figure 1. The model parameters
for these transistors are not critical; the default model provided
in SPICE libraries will be satisfactory. However, the AD8304
employs compensation techniques to reduce errors caused by
junction resistances (notably, RB and RE) at high input cur-
rents. Therefore, it is advisable to set these to zero. While this
will not model the AD8304 precisely, it is safer than using pos-
sibly high default values for these parameters. The low current
model parameters may also need consideration. Note that no
attempt is made to capture either dynamic behavior or the effects of
temperature in this simple macromodel; scaling is correct for 27°C.
–0.5
–1.0
–1.5
–2.0
2.0
1.5
1.0
0.5
0
100p
WITH INTERCEPT ADJUST
1n
WITHOUT INTERCEPT ADJUST
10n
100n
INPUT – A
1
10
V
NEG
100
V
NEG
= –0.5
= –3
V
1m
NEG
= 0
LOG
10m
, thus
Summing Node at Ground and Voltage Inputs
A negative supply may be used to reposition the input node at
ground potential. A voltage as small as –0.5 V is sufficient.
Figure 13 shows the use of this feature. An input current of up
to 10 mA is supported.
This connection mode will be useful in cases where the source is a
positive voltage V
photodiodes, or other “perfect” current sources. R
current and should be chosen to optimally position the range of I
or provide a very high input resistance, thus minimizing the loading
of the signal source. For example, assume a voltage source that spans
the four decade range from 100 mV to 1 kV and it is desired to
maximize R
1 mA. Using a value of 10 MΩ, the same four decades of input voltage
would span the central current range of 10 nA to 100 mA.
Smaller input voltages can be measured accurately when aided by
a small offset-nulling voltage applied to VSUM. The optional net-
work shown in Figure 13 provides more than ±20 mV for this purpose.
IPD
V
NC = NO CONNECT
I1
P
I1
C1
E1
V1
Q1
I2
Q2
I3
Q3
MODEL
E2
E3
E4
V2
R1
C2
R2
R3
IN
RIN
V
SIG
1k
I
PD
10k
NC
C1
0
IN
2
1
IN
0
3
0
4
NPN
5
6
7
8
8
9
9
VLOG
V
6
3
4
5
IN
LOW
VSUM
VSUM
VPDB
INPT
V1
. When set to 1 GΩ, I
IN
0
0
0
2
3
3
4
4
0
0
0
7
9
0
VLOG
0
V
Q1
VPS2
1
SIG
DC
1.0N
IN
0.5
0
0
0
POLY (2)
POLY (2)
6
0.8
100
163P
4.9K
1000K
1
PDB
referenced to ground, rather than for use with
VNEG
10
E1
V
NPN
NPN
316.2
NPN
2 3 1 0 0, 0, 0, 0, 1
4 3 7 0 0, 0, 0, 0, 1
N
1 A
1
1
3
PWDN
~10k
2
I1
3K
COMPENSATION
Q2
ACOM
TEMPERATURE
BIAS
3
2
PD
14
I2
spans the range 100 pA to
Q3
4
0.5V
VPS1
VREF
VOUT
12
5k
IN
11
BFNG
scales the input
BFIN
6
5
C2
13
7
8
9
V
E4
P
VLOG
VREF
RB
V2
VLOG
V
OUT
V
PD
RA
R2
R1
7
,