APPLICATION 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 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). R2 2kΩ +15V 2 – REF102 10V Out +10VOUT 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, C1 leakage current errors reacting with R1 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 improved filter near zero. Also, leakage through C2 is negligible because the voltage across it is nearly zero. 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 unitygain 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 problems. 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 Application Bulletin Number 88 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, C2 network assures amplifier loop stability. Set R2 • C2 = 2 • R1 • C1 to minimize amplifier noise gain peaking. Since buffer amplifier bias current flows through R2, keep the value of R2 low 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 R1 low—less than 1V at full load for example—to prevent the amplifier output from swinging too close to its power-supply rail. – 1994 Burr-Brown Corporation C1 1µF Tantalum 88 2 © +10VOUT + FIGURE 2. Voltage Reference with Improved Filter. V+ OPA27 + OPA27 + 4 +15V 6 6 R1 50Ω Com The conventional circuit, shown in Figure 1, uses a singlepole 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. R1 0.05µF V+ 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 C2 The improved filter can drive large capacitive loads without stability problems. Just keep (CLOAD + C1) • R1 < 0.5 • R2 • C2. R2, C2 network is responsible. A peak output impedance of about 0.7 • R1 occurs near the filter pole frequency. If lower midband output impedance is required, R1 must be reduced and C1 increased accordingly. (Condensed from Application Bulletin AB-003. Request PDS-466 for OPA27 and PDS900 for REF102. There is one caution with the improved filter. Although the output impedance is low at both high frequencies and DC, it peaks at midband frequencies. Reduced loop gain due to the The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant any BURR-BROWN product for use in life support devices and/or systems. 2