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 INCREASING ADC603 INPUT RANGE By R. Mark Stitt, (602) 746-7445 The ADC603 is a 10MHz, 12-bit analog-to-digital converter with a ±1.25V input range. Many applications call for a higher input range such as ±2.5V. A resistor divider can be used as an input attenuator to increase the input range. The OPA620 can be used to buffer the input attenuator for highsource-impedance applications. Suggested component values and measured performance results are shown in this bulletin. VIN R2 100Ω 45 R3 66.5Ω R1 100Ω Since the ADC603 has a high-impedance input, a simple voltage divider as shown in Figure 1 can be used to increase its voltage input range. The source impedance of the divider as seen by the ADC603 is R 1 || R2 (the parallel combination of R1 and R2). A divider source impedance of 50Ω is recommended since it has been shown to give consistently good results. If a higher divider input impedance is needed and adding a buffer is not viable, source impedances up to 500Ω should give satisfactory results. If hardware gain trim is needed, select the next higher 1% resistor value for R 1 and use a 10kΩ multi-turn trim pot in parallel with R 1 for gain trim. FIGURE 2. ADC603 12-Bit ADC with Three-Resistor 2/1 Input Attenuator to Provide ±2.5V Input Range and 50Ω Termination Impedance. 2 VIN 3 +5V 7 OPA620 4 (±2.5V) 6 R2 100Ω VIN 3 45 R2 100Ω R1 100Ω 45 R1 100Ω 46 ADC603 12 Bits Out ADC603 46 ADC603 46 12 Bits Out 6 12 Bits Out –5.2V FIGURE 3. ADC603 12-Bit ADC with 2/1 Input Attenuator to Provide High Input Impedance ±2.5V Input Range. FIGURE 1. ADC603 12-Bit ADC with 2/1 Input Attenuator to Provide ±2.5V Input Range. If an input impedance of 50Ω to the circuit is needed as a termination, add a third resistor as shown in Figure 2. The three-resistor approach improves accuracy by placing the majority of the termination power dissipation in the third resistor. This minimizes error-producing self heating in the precision divider network. Pay attention to the power rating for R3. For a ±10V input, R3 must be rated 2W. Equations for determining recommended resistor values are: If a high input impedance is needed, drive the divider with a unity-gain-connected OPA620 buffer amp as shown in Figure 3. The OPA620 can be used for inputs as high as ±3V. N = input divider ratio 1990 Burr-Brown Corporation R1 = 50Ω • N/(N – 1) R2 = (N – 1) • R1 R3 = 50Ω • (R1 + R2)/(R1 + R2 – 50Ω) Where: R1, R2, R3 are in Ω AB-013 Printed in U.S.A. July, 1990 The table below shows recommend resistor values for selected input ranges. INPUT RANGE (V) DIVIDER (1/N) R1 (Ω) R2 (Ω) R3 (Ω) ±2 ±2.5 ±3 ±5 ±10 1/1.6 1/2 1/2.4 1/4 1/8 133 100 86.6 66.5 56.2 80.6 100 121 200 397 64.9 66.5 66.5 61.9 56.2 TABLE I. Resistor Values for Selected Input Attenuators. The spectral plots compare a standard ±1.25V input ADC603 to a ±2.5V input, OPA620 buffered ADC603 per Figure 3. In both cases, the circuit is sampling a 2.5MHz signal at 10MHz. The results show that the spurious-free dynamic range of the boosted circuit is as good as for the standard circuit. If anything, the boosted circuit has better performance (77dB vs 76dB). The ADC603 seems to perform slightly better when driven by the purely resistive 50Ω divider impedance instead of the complex impedance of the cable and signal generator. THD: SNR: SINAD: SFDR: Spur F: 0 1.25 2.5 MAGNITUDE SPECTRUM –73.44dBC 68.07dB 66.96dB 75.62dB FSR 2.66MHz 3.75 Magnitude (dB FSR) Magnitude (dB FSR) MAGNITUDE SPECTRUM 0 –10 –20 –30 –40 –50 –60 –70 –80 –90 –100 –110 –120 5 Frequency (MHz) Plot 1. Spectral plot of standard ADC603 sampling a ±1.25V, 2.5MHz input signal at 10MHz showing 75.6dB SFDR. 0 –10 –20 –30 –40 –50 –60 –70 –80 –90 –100 –110 –120 THD: SNR: SINAD: SFDR: Spur F: 0 1.25 2.5 –73.49dBC 68.58dB 67.36dB 77.14dB FSR 2.66MHz 3.75 5 Frequency (MHz) Plot 2. Spectral plot of increased input range (per Figure 3) ADC603 sampling a ±2.5V, 2.5MHz input signal at 10MHz showing 77.1dB SFDR. FIGURE 4. Spectral Plots. 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.