廬
APPLICATION BULLETIN
MAKE A PRECISION 鈥?0V REFERENCE
By R. Mark Stitt (520) 746-7445
Mailing Address: PO Box 11400 鈥?Tucson, AZ 85734 鈥?Street Address: 6730 S. Tucson Blvd. 鈥?Tucson, AZ 85706
Tel: (520) 746-1111 鈥?Twx: 910-952-111 鈥?Telex: 066-6491 鈥?FAX (520) 889-1510 鈥?Immediate Product Info: (800) 548-6132
1
The need for a precision 鈥?0.0V reference arises often. For
example, the best way to get a 0V to +10V output from a
CMOS MDAC is to use a 鈥?0V reference (see Figures 4-6).
ADI/PMI has the REF-08 鈥?0V reference, but it has limited
performance. Although Burr-Brown offers no 鈥?0V refer-
ence, the REF102 precision +10.0V reference can be accu-
rately converted to a precision 鈥?0.0V reference. The circuit
is simple and requires no precision components. The 2.5ppm/
擄C
temperature drift of the Burr-Brown REF102 is twenty
times better than the 50ppm/擄C best grade of the PMI REF-
08. (Even our lowest grade is five times better.)
The simplest approach for converting a REF102 into a
鈥?0.0V reference is shown in Figure 1. The only extra
component is a 1k鈩?resistor connected to 鈥揤
S
. This circuit
is useful, but has limitations. Maximum expected load cur-
rent plus maximum reference quiescent current must be
supplied by the resistor at minimum 鈥揤
S
. Changes in current
resulting from load and power supply variations must be
driven by the reference. The excess current through the
reference reduces its accuracy due to drift from self-heating
and thermal feedback. Changes in reference output current
due to power-supply variations translate into line regulation
error. Voltage reference load regulation is not usually as
good as line regulation. Finally, the output impedance due to
the resistor pull-down causes settling problems with dy-
namic loads.
2
+V
S
(1.4V to 26V)
2
R
1
2k鈩?/div>
+V
S
(1.4V to 26V)
2
3
4
V+
REF102
6 10V
Out
C
1
1000pF
Gnd
4
OPA27
鈥?0V Out
5
FIGURE 2. Improved 鈥?0V Reference.
To understand how the circuit works, notice that the refer-
ence is in the feedback loop of the op amp. The op amp
output forces the Gnd connection of the reference to exactly
鈥?0.0V so that the voltage at the op amp inverting input is
the same as at its noninverting input (ground). Since no
current flows into the op amp input, the reference output
current remains at zero, eliminating voltage reference ther-
mal feedback or load regulation errors. The R
1
, C
1
network
assures loop stability and provides noise filtering. Reference
noise is filtered by a single pole of f
鈥?dB
= 1/(2 鈥?/div>
蟺
鈥?R
1
鈥?C
1
).
Bias current flowing through R
1
can produce DC errors and
noise. If a lower filter pole is needed, keep R
1
= 2k鈩?and
increase C
1
to preserve accuracy.
+V
S
(1.4V to 26V)
6
7
8
9
10
11
12
V+
REF102
6 10V
Out
R
1
2k鈩?/div>
2
Gnd
4
R
S
1k鈩?/div>
I
L
鈥?5V
鈥?0V Out
5V
1.4 <
鈥?I < 5.4mA
R
S L
OPA27
C
1
0.05碌F
V+
REF102
6 10V
Out
5
20k鈩?/div>
R
2
50鈩?/div>
Trim
Gnd
4
C
2
1.0碌F
Tantalum
鈥?0V Out
FIGURE 1. Simple 鈥?0V Reference.
The circuit shown in Figure 2 solves these problems. As in
Figure 1, no precision resistors are needed. The error con-
tributed by the op amp is negligible (the OPA27 0.6碌V/擄C
V
OS
/dT adds only 0.06 ppm/擄C drift to the 鈥?0V reference).
As a bonus, the circuit incorporates noise filtering.
漏
13
14
15
FIGURE 3. Improved 鈥?0V Reference with Improved Filter,
with V
OUT
Trim.
Printed in U.S.A. June, 1995
1990 Burr-Brown Corporation
AB-004B
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