AD8304 Analog Devices, AD8304 Datasheet - Page 16

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

AD8304
Programmable Multidecade Current Source
The AD8304 supports a wide variety of general (nonoptical)
applications. For example, the need frequently arises in test
equipment to provide an accurate current that can be varied over
many decades. This can be achieved using logarithmic amplifier
as the measuring device in an inverse function loop, as illustrated
in Figure 16. This circuit generates the current:
The principle is as follows. The current in QA is forced to sup-
ply a certain I
and V
formed by the internal op amp and capacitor C1, with a time
constant formed with the internal 5 kΩ resistor. The choice of
C1 in this example ensures loop stability over the full eight
decade range of output currents; C2 reduces phase lag. The
system is completed with a 10-bit MDAC using V
I
SRC
LOG
, and nulling this error by integration. This is per-
=
100
PD
by measuring the error between a setpoint V
pA
×
10
(
V
25k
SPT
C2
1nF
/ .
0 2
NC = NO CONNECT
1nF
750
)
10nF
I
PD
NC
1k
I
SRC
NC = NO CONNECT
NC
6
3
4
5
VSUM
VSUM
VPDB
INPT
6
3
4
5
VSUM
VSUM
VPDB
INPT
VPS2
V
VNEG
N
(–0.5V TO –5V)
1
VPS2
PDB
VNEG
10
REF
1
PDB
10
as its
PWDN
~10k
COMPENSATION
ACOM
PWDN
~10k
TEMPERATURE
BIAS
(17)
2
ACOM
SPT
COMPENSATION
TEMPERATURE
BIAS
14
2
14
VPS1
0.5V
reference, whose output is scaled to 1.6 V FS by R1 and R2
(whose parallel sum is also 5 kΩ).
Transistor QA may be a single bipolar device, which will result
in a small alpha error in I
emitter branch), or a Darlington pair or an MOS device, either
of which ensure a negligible difference between I
this example, the bipolar pair is used. The output voltage com-
pliance is determined by the collector breakdown voltage of
these transistors, while the minimum voltage depends on where
VSUM is placed. Optional components could be added to put
this node and VNEG at a low enough bias to allow the voltage to
go slightly below ground.
Many variations of this basic circuit are possible. For example,
the current can be continuously controlled by a simple voltage,
or by a second current. Larger output currents can be controlled
by setting V
VREF
VOUT
0.5V
VPS1
12
5k
VREF
VOUT
12
11
5k
VLOG
BFNG
BFIN
11
SUM
BFNG
BFIN
13
9
7
8
V
V
to zero and using a current shunt divider.
P
RH
VREF
SPT
13
9
7
8
V
10nF
P
VLOG
VREF
50M
V
C1
OUT
49.9k
100k
VOUT
SRC
VOUT
100k
(the current is monitored in the
MDAC
VREF
MDAC
VREF
PD
and I
SRC
. In