Datasheet
REV. A–14–
AD8304
ACOM Pin, and does not normally go negative with regard to this
pin, but is free to do so. Therefore, a resistor from VLOG to the
negative supply can lower V
LOG
, thus raising the intercept. A more
accurate method for repositioning the intercept is described below.
INPUT – A
100p 10m1n 10n 100n 1 10 100 1m
2.0
–2.0
ERROR – dB (10mV/dB)
1.5
0
–0.5
–1.0
–1.5
1.0
0.5
WITHOUT INTERCEPT ADJUST
WITH INTERCEPT ADJUST
V
NEG
= 0
V
NEG
= –0.5
V
NEG
= –3
Figure 11. Log Conformance (Linearity) vs. I
PD
for
Various Negative Supplies
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 macro-
model 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 Figure 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 currents. There-
fore, it is advisable to set these to zero. While this will not model
the AD8304 precisely, it is safer than using possibly 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.
V
E4
V2
R1
R2
VLOG
7
5
C2
V
V1
+
Q3
I1
IPD
IN
1
2
3
4
I1
I2
E3
E2
C1
6
RL
I1
C1
E1
V1
Q1
I2
Q2
I3
Q3
.MODEL
E2
E3
E4
V2
R1
C2
R2
RL
0
IN
2
1
IN
0
3
0
4
NPN
5
6
7
8
8
9
9
VLOG
IN
0
0
0
2
3
3
4
4
NPN
0
0
0
7
9
0
VLOG
0
DC
1.0N
IN
0.5
0
1
0
316.2
0
POLY (2)
POLY (2)
6 5 100K
0.8
100
163P
4.9K
1000K
1A
1
NPN
NPN
NPN
2 3 1 0 0, 0, 0, 0, 1
4 3 7 0 0, 0, 0, 0, 1
3K
Q2
Q1
100k
3k
+
Figure 12. Basic Macromodel