廬
APPLICATION BULLETIN
IMPROVED VOLTAGE REFERENCE
FILTER HAS SEVERAL ADVANTAGES
by R. Mark Stitt, (602) 746-7445
Mailing Address: PO Box 11400 鈥?Tucson, AZ 85734 鈥?Street Address: 6730 S. Tucson Blvd. 鈥?Tucson, AZ 85706
Tel: (602) 746-1111 鈥?Twx: 910-952-111 鈥?Telex: 066-6491 鈥?FAX (602) 889-1510 鈥?Immediate Product Info: (800) 548-6132
The Burr-Brown REF102 is a buried-zener-based precision
10V reference. It has better stability and about five times
lower output noise than band-gap-based voltage references
such as the PMI REF-10. Still, its output noise is about
600碌Vp-p at a noise bandwidth of 1MHz (the output noise
of the PMI REF-10 is about 3,000碌Vp-p at 1MHz).
You can reduce voltage reference noise by filtering its
output. Reduce broadband noise by the square-root of the
reduction in noise bandwidth. Filtering the output of the
reference to reduce the noise bandwidth by 100/1 (from
1MHz to 10kHz, for example) can reduce the noise by
10/1 (from 600碌Vp-p to 60碌Vp-p).
The conventional circuit, shown in Figure 1, uses a single-
pole RC filter and a buffer amplifier. One problem with this
circuit is that leakage current through the filter capacitor, C
1
,
flows through R
1
, resulting in DC error. Furthermore, changes
in leakage with temperature result in drift. The relatively low
RC time constants often needed dictate large capacitor
values prone to this problem.
R
2
2k鈩?/div>
+15V
2
V+
鈥?/div>
C
2
0.05碌F
OPA27
+
REF102
10V
Out
R
1
50鈩?/div>
+10V
OUT
+
C
1
1碌F
Tantalum
6
Com
4
88
FIGURE 2. Voltage Reference with Improved Filter.
The improved filter places the RC filter at the output of the
buffer amplifier. Reference noise is filtered by a single pole
of f
鈥?dB
= 2 鈥?/div>
蟺
鈥?R
1
鈥?C
1
. The R
2
, C
2
network assures amplifier
loop stability. Set R
2
鈥?C
2
= 2 鈥?R
1
鈥?C
1
to minimize amplifier
noise gain peaking. Since buffer amplifier bias current flows
through R
2
, keep the value of R
2
low enough to minimize
both DC error and noise due to op amp bias-current noise.
Also, load current flows in R
1
. The resulting voltage drop
adds to the required swing at the output of the buffer
amplifier. Keep the voltage drop across R
1
low鈥攍ess than
1V at full load for example鈥攖o prevent the amplifier output
from swinging too close to its power-supply rail.
+15V
2
V+
鈥?/div>
REF102
10V
Out
6
R
1
OPA27
+
+10V
OUT
C
1
Com
4
FIGURE 1. Voltage Reference with Conventional Filter.
Another problem with the conventional filter is the added
noise of the buffer amplifier. The noise acts at its full unity-
gain bandwidth, adding to the circuit output noise. Even if
the noise at the output of the RC filter is zero, the noise
added by the buffer can be intolerable in many applications.
The improved filter, shown in Figure 2, solves both prob-
lems.
At high frequency, the output impedance of the improved
filter is low due to C
1
. The reactance of a 1碌F capacitor is
0.16鈩?at 1MHz. For an A/D converter reference, connect C
1
as close to the reference input pin as possible.
漏
1994 Burr-Brown Corporation
AB-088
Printed in U.S.A. January, 1994
Application Bulletin Number 88
With the RC filter at the buffer output, the noise of both the
voltage reference and the buffer is filtered. Since the filter is
in the feedback loop of the buffer amplifier, C
1
leakage
current errors reacting with R
1
are divided down to an
insignificant level by the loop gain of the buffer amp. The
feedback also keeps the DC output impedance of the im-
proved filter near zero. Also, leakage through C
2
is negli-
gible because the voltage across it is nearly zero.
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