鈥?/div>
OPA111
+
R
1
I
OUT
= 鈥?0V/R
1
LOAD
Can be connected
to ground or +V
S
.
5
FIGURE 2. Precision Current Sink.
Compliance of the circuit depends on input and output range
of the op amp used and the 11.4V minimum supply range of
the REF102. The application guide goes into more detail.
Keep in mind that the accuracy of a voltage-reference-based
current source depends on the absolute accuracy of the
current scaling resistor (R
1
). The absolute TCR and stability
of the resistor directly affect the current source temperature
drift and stability. If you use a 50ppm/擄C resistor (common
for 1% metal film resistors), the precision current source will
have approximately 50ppm/擄C drift with temperature鈥攚orse
than the 25ppm/擄C drift of a REF200.
The performance of circuits using current source references
depends only on the ratio accuracy of the scaling resistors.
It is much easier to get good resistor ratio accuracy than to
get good absolute accuracy, especially when using resistor
networks.
Burr-Brown offers a wide variety of support components
which are excellent choices for generation or conversion of
current. Application Guide (AN-165) has proven valuable in
selecting these components.
6
7
8
9
Application Bulletin Number 2
R
1
I
OUT
= 鈥?0V/R
1
LOAD
Can be connected
to ground or 鈥揤
S
.
OPA111
+
10
11
FIGURE 1. Precision Current Source.
The current sink is shown in Figure 2. The op amp drives
both the voltage reference ground connection and the cur-
rent-scaling resistor, R
1
, so that the voltage reference output
is equal to the load voltage. This forces 鈥?0.0V across R
1
so
that the current sink output is 鈥?0V/R
1
. The R
2
, C
1
network
provides local feedback around the op amp to assure loop
stability. It also provides noise filtering. With the values
shown, the reference noise is filtered by a single pole with
f
鈥?dB
= 1/(2 鈥?/div>
蟺
鈥?R
2
鈥?C
1
) = 16kHz.
12
13
14
15
16
漏
1990 Burr-Brown Corporation
1
AB-002C
Printed in U.S.A. February, 1994
1
2
