Datasheet > AB-088

AB-088

更新时间： 2024-11-18 23:26:55

品牌	Logo	应用领域
其他 - ETC		LTE
页数	文件大小	规格书
2页	25K
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AB-088 - IMPROVED VOLTAGE REFERENCE FILTER HAS SEVERAL ADVANTAGES

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AB-088 数据手册

AP P LICATION BULLETIN

Mailing Address: PO Box 11400 • Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706

Tel: (602) 746-1111 • Twx: 910-952-111 • Telex: 066-6491 • FAX (602) 889-1510 • Immediate Product Info: (800) 548-6132

IMPROVED VOLTAGE REFERENCE

FILTER HAS SEVERAL ADVANTAGES

by R. Mark Stitt, (602) 746-7445

The Burr-Brown REF102 is a buried-zener-based precision

R₂

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

600µVp-p at a noise bandwidth of 1MHz (the output noise

of the PMI REF-10 is about 3,000µVp-p at 1MHz).

2kΩ

+15V

C₂

0.05µF

V+

R₁

–

50Ω

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 600µVp-p to 60µVp-p).

REF102

+10V_OUT

C₁

OPA27

10V

Out

1µF

Tantalum

Com

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, C₁,

flows through R₁, 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.

8 8

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_–3dB= 2 • π • R₁• C₁. The R₂, C₂network assures amplifier

loop stability. Set R₂• C₂= 2 • R₁• C₁to minimize amplifier

noise gain peaking. Since buffer amplifier bias current flows

through R₂, keep the value of R₂low enough to minimize

both DC error and noise due to op amp bias-current noise.

Also, load current flows in R₁. The resulting voltage drop

adds to the required swing at the output of the buffer

amplifier. Keep the voltage drop across R₁low—less than

1V at full load for example—to prevent the amplifier output

from swinging too close to its power-supply rail.

+15V

V+

–

OPA27

REF102

R₁

+10V_OUT

10V

Out

C₁

Com

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, C₁leakage

current errors reacting with R₁are 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 C₂is negli-

gible because the voltage across it is nearly zero.

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 C₁. The reactance of a 1µF capacitor is

0.16Ω at 1MHz. For an A/D converter reference, connect C₁

as close to the reference input pin as possible.

AB-088

Printed in U.S.A. January, 1994

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